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AD-A284 076 



By 

Clark K. Ray 


A Thesis Submitted to the Graduate 
Faculty of Rensselaer Polytechnic Institute 
in Partial Fulfillment of the 


Requirements for the Degree of 
DOCTOR OF PHILOSOPHY 


Major Subject: Computer and Systems Engineering 



James M. Tien, Member 



distribution is uniiante_--- 


Rensselaer Polytechnic Institute 
Troy, New York 


April 1994 

(For Graduation May 1994) ^ 

94 9 01 


086 


DHL vi 








29 August, 1994 


Defense Technical Information Center 
ATTN: Selection Section (DTIC-FDAC) 
Building 5, Cameron Station 
Alexandria, VA 22304-6145 


Under the provisions of AR 621-1,1 recently finished Army fully-funded graduate study 
leading to a Ph.D., for which a thesis was required. Per Army requirements, I am 
furnishing two copies of this thesis (enclosed). 

Thesis Title: Representing Visibility for Siting Problems 
Name: Clark K. Ray 
Rank: Lieutenant Colonel 
Date: April 1994 

Should any additional information be required, I can be contacted at: 

LTC Clark K. Ray 

Department of Electrical Engineering and Computer Science 
US Military Academy 
West Point, NY 10996 

EMAIL: ray@eecsl.eecs.usma.edu 
Phone: DSN 688-3029 

Commercial 914-938-3029 


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CONTENTS 


LIST OF TABLES. vi 

LIST OF FIGURES.vii 

1. INTRODUCTION. 1 

1.1 Motivation. 1 

1.2 Nature of Computation Costs in the Visibility Problem Domain ... 2 

1.3 Methodology . 4 

1.3.1 Construct Methods to Access Data. 4 

1.3.2 Elevation Model and Problem Setting. 5 

1.3.3 Viewshed Algorithms. 5 

1.3.4 Visibility Index Algorithms. 5 

1.3.5 Accuracy and Performance Evaluation. 7 

1.3.6 Potential Approach. 7 

1.4 Issues and Goals . 7 

1.4.1 LOS Calculation Constraints. 7 

1.4.2 Performance Improvements over Existing Algorithms. 8 

1.4.3 Empirical Validation of Algorithms on Real Terrain. 8 

1.4.4 Identification of Candidate Point Sites. 8 

1.4.5 Exploit Parallel Processing to Compute Visibility Indexes ... 9 

1.4.6 Demonstration to User Community. 9 

1.5 Organization of Thesis.10 

2. LITERATURE REVIEW .12 

2.1 Background.12 

2.1.1 Visibility.12 

2.1.2 Parallelism.13 

2.1.3 Siting.13 

2.1.4 Terrain Elevation Data.14 

2.2 Characteristics of the Problem and its Domain.14 

2.3 Practical Considerations.15 


n 

































3. VISIBILITY PROBLEM TAXONOMY AND ELEVATION MODEL ... 17 

3.1 Taxonomy of Visibility Analv^s Characteristics.17 

3.1.1 Terrain Characteristi .18 

3.1.2 Elevation Data Characteristics.18 

3.1.3 Selection of Sample Observer Points.20 

3.1.4 Elevations for Non-Data Point Locations.22 

3.1.5 Uncertainty in the Data.22 

3.1.6 Observer, Target, and LOS Characteristics.23 

3.1.7 Calculation Characteristics.24 

3.1.8 Placement of this Research within Taxonomy .26 

3.2 Elevation Model. . .26 

3.3 Characteristics of an Elevation Dataset.32 

3.4 Considerations in LOS Calculation.33 

3.5 Summary.36 

4. AREA VISIBILITY DETERMINATION—THE R2 ALGORITHM .... 37 

4.1 Overview.37 

4.2 Algorithm Considerations .39 

4.3 R2 Algorithm Development.46 

4.4 Validity of R2 Algorithm.51 

4.5 Initial R2 Execution Time Results.55 

4.6 Summary.56 

5. DEVELOPMENT OF LOS RAY ESTIMATOR ALGORITHMS.60 

5.1 VL Algorithm.60 

5.2 Investigating Visibility Index Estimates.61 

5.3 Hypotheses and Characteristics.63 

5.4 Pseudo-Fixed Point Optimizations.67 

5.5 WeightF Algorithm.67 

5.6 Weight Algorithm.68 

5.7 Visibility Images.69 

5.7.1 Magnitude-Only Visibility Images.69 

5.7.2 Directional Visibility Images.70 


in 





































5.8 Parallelism Considerations 

5.9 Summary. 


70 

72 


6. ACCURACY OF VISIBILITY ESTIMATES.73 

6.1 Overview of Methodology .73 

6.2 Performance of the VL Algorithm.75 

6.3 Relationships to Elevation.79 

6.4 Improvements to VL Estimation Algorithm.83 

6.5 WeightF to R2 Correlation as a Function of Ray Density.88 

6.6 Analysis of Other Locales .90 

6.6.1 Dijon Area.90 

6.6.2 Pripyat’ Area.91 

6.6.3 Summary.99 

6.7 Confidence in Finding Highest Points.99 

6.8 Impact of Errors in Elevation Data.102 

6.9 Summary.103 

7. EXECUTION TIMES.105 

7.1 Overview.105 

7.2 R3 Execution Time Performance Evaluation.106 

7.3 R2 Execution Time Performance Evaluation.107 

7.4 WeightF Execution Time Performance Evaluation.115 

7.5 Summary.118 

8. SUMMARY AND CONCLUSIONS.119 

8.1 Summary.119 

8.2 Significant Contributions and Accomplishments.121 

8.3 Future Work.123 

LIST OF SYMBOLS.127 

GLOSSARY.129 

REFERENCES.132 

APPENDICES.137 

A. Computation and Storage Costs of Visibility Determinations.138 

iv 




































B. Formal Definition of the HAWK Siting Problem.142 

C. Investigation of Elevation-Visibility Relationships.146 

D. Demonstration to User Community.151 


v 









LIST OF TABLES 


Table 1.1 Comparative Costs of Increasing Grid Density. 3 

Table 3.1 Placement within Taxonomy.27 

Table 4.1 Properties of R2 LOS-Height Error.52 

Table 4.2 Count of Visible Points by R2 and R3.52 

Table 4.3 Sample Execution Times of R2 and R3.55 

Table 7.1 R3 Execution Times as a function of the Visibility Radius . . 106 

Table 7.2 R2 Execution Times as a function of the Visibility Radius; 

Separate Fitted Curves for each Group.110 

Table C.l Best Points Visibility—Elevation Correlations.147 

Table C.2 AH Points Visibility—Elevation Correlations.149 


vi 












LIST OF FIGURES 


Figure 1.1 Visibility Cells between Latitudes North 34-40 and Longi¬ 
tudes East 124-130. 6 

Figure 3.1 Grids of Varying Point Densities.19 

Figure 3.2 Elevation Histogram of DTED Cell N37E127.32 

Figure 3.3 Log Scale Elevation Histogram of DTED Cell N37E127 .... 33 

Figure 3.4 Example of a LOS Profile.34 

Figure 4.1 Octant Divisions.37 

Figure 4.2 Logical View of First Octant.38 

Figure 4.3 LOS Crossings of Grid Lines.40 

Figure 4.4 Progression of LOS Wave Front.42 

Figure 4.5 Erroneous Propagation in Wave Front Algorithms.43 

Figure 4.6 Propagation of Maximum Chunk Distortion LOS-Height Er¬ 
ror with Distance from the Observer.45 

Figure 4.7 Operation of the Basic R3 Algorithm.46 

Figure 4.8 Coverage Problems in the Initial R2 Algorithm.48 

Figure 4.9 Modified R2 Algorithm.50 

Figure 4.10 Distribution of R2 LOS-Height Differences (meters).51 

Figure 4.11 Comparing 4 Variants of R2.53 

Figure 4.12 Agreement of R2 and R3 Values for 1041 Points in N37E127 . 54 

Figure 4.13 Alternative Tessellation of R2 Observations.57 

Figure 4.14 De-Cluttered Alternative Tessellation from R3.57 

Figure 5.1 40-kilometer Visibility Index Values, 32 and 128 LOS Rays, 

N37E127.62 

Figure 5.2 Gray-Scale Elevation Image of DTED Cell N37E127.64 

vii 























Figure 5.3 Gray-Scale Visibility Image from DTED Cell N37E127.64 

Figure 5.4 Log Distribution of LOS Index Values from DTED Cell N37E127. 65 


Figure 5.5 Point-Matched Elevation and Visibility Values from DTED 

Cell N37E127.66 

Figure 5.6 Elevation and Visibility, 110 Best VL Visibility Points, N37E127. 66 

Figure 6.1 Spatial Distribution of 1024 Pseudorandom Points.75 


Figure 6.2 N37E127 - VL and R2 Visibility for 1024 Random Points. . . 76 

Figure 6.3 N37E127 - Fit Residuals, VL and R2 Visibility, for 1024 Ran¬ 
dom Points.77 

Figure 6.4 N37E127 - VL and R2 Visibility for Top 256 VL Points. ... 78 

Figure 6.5 N37E127 - Elevation and R2 Visibility for 1024 Random Points. 79 

Figure 6.6 N37E127 - Elevation and R2 Visibility for Top 256 VL Points. 80 

Figure 6.7 N37E127 - Elevation and VL Visibility for Top VL 256 Points. 81 

Figure 6.8 N37E127 - Elevation and VL Visibility for Top VL 100 Points. 82 

Figure 6.9 N37E127 - VL and WeightF Visibility for Top VL 256 Points. 83 

Figure 6.10 N37E127 - WeightF and R2 Visibility for Top VL 256 Points. 84 

Figure 6.11 N37E127 - WeightF and R2 Visibility for 1024 Random Points. 85 

Figure 6.12 N37E127 - WeightF (128 rays) and R2 Visibility for 1024 Ran¬ 


dom Points.86 

Figure 6.13 N37E127 - WeightF (128 rays) and R2 Visibility for Top 256 

VL Points.87 

Figure 6.14 N37E127 - Weight and R2 Visibility for 1924 Random Points. 88 

Figure 6.15 N37E127 - Correlation Variation of R2 to WeightF.89 

Figure 6.16 N37E127 - Correlation Variation of R2 to WeightF (8 rays). . 90 

Figure 6.17 N47E005 - Gray-Scale Elevation Image.92 

Figure 6.18 N47E005 - WeightF and R2 Visibility for 1024 Random Points. 93 

Figure 6.19 N52E028 - Gray-Scale Elevation Image.94 


vm 














Figure 6.20 N52E028 - Elevation Histogram for 1024 Random Points. ... 95 

Figure 6.21 N52E028 - WeightF and R2 Visibility for 1024 Random Points. 96 

Figure 6.22 N52E028 - WeightF and R2 with Alternate LOS Parameters 

for 1024 Random Points.97 

Figure 6.23 N52E028 - Histogram of R2 Visibility for 1024 Random Points 98 

Figure 6.24 N52E028 - Histogram of R2 Visibility with Alternate LOS 

Parameters for 1024 Random Points .99 

Figure 6.25 Center of N37E127 - WeightF Points Required to Find the 

Highest R2 Points.100 

Figure 6.26 Center of N37E127 - Histogram of R2 Visibility Values .... 101 

Figure 6.27 Center of N37E127 - Ratio of WeightF Points Required to 

Number of Top R2 Points.102 

Figure 6.28 N37E127 - R2 Visibility for Original and Perturbed Elevations 103 

Figure 7.1 N37E127 - 8 Random Points; Weighted Curve Fit.107 

Figure 7.2 N37E127 - Points within R2 Radius.108 

Figure 7.3 N37E127 - R2 Execution Times for 256 Random Points .... 109 

Figure 7.4 N37E127 - R2 Execution Times for 256 Random Points (lower 

range).112 

Figure 7.5 Comparison of R3 and R2 Execution Time for 256 Points. . . 112 

Figure 7.6 N37E127 - R2 Execution Times for 256 Random Points (upper 

range).113 

Figure 7.7 N37E127 - WeightF Execution Times for 16 runs of 256 Pseudo- 

Random Points (out to 60 kilometers) .116 

Figure 7.8 N37E127 - WeightF Execution Times for 16 runs of 256 Pseudo- 

Random Points (from 70 to 90 kilometers).117 

Figure 8.1 Elevation Vicinity n37el26y.124 

Figure 8.2 Visibility Vicinity n37el26y.124 

Figure D.l Screen from PC Demo Application.152 


IX 

















ACKNOWLEDGMENT 


I would like to thank my thesis advisor, Professor Wm. Randolph Franklin, 
and the members of my committee, Professor James M. Tien, Professor George 
Nagy, and Professor Susan H. Rodger for their patience and guidance during the 
course of my research. 

Partial support for this work was provided by the Directorate for Computer 
and Information Science and Engineering, National Science Foundation grant CDA- 
8805910, and by National Science Foundation grant CCR-9102553. 

Part of this work was supported by the U.S. Army Topographic Engineering 
Center (Mr. John R. Benton) under the auspices of the U.S. Army Research Office 
Scientific Services Program administered by Battelle (Delivery Order 2756), contract 
DA AL03-86-D-0001. 

I am deeply appreciative for the equipment and other support provided by 
the Department of Electrical Engineering and Computer Science, U.S. Military 
Academy. 

I am also very grateful for the support provided by the Fannie and John Hertz 
Foundation during significant portions of my graduate experience, and to the Free 
Software Foundation and other contributors to the body of freely available software 
tools. 

Finally, I can thank but never repay the members of my family for their un¬ 
derstanding and support, and their belief in me regardless of the circumstances. 


x 






ABSTRACT 


Many siting and other terrain analysis problems require the determination of 
the visibility from large numbers of observer locations over their surrounding ter¬ 
rain. The computational costs of existing visibility algorithms often require serious 
compromises on the fidelity, accuracy, or scope of analysis in real world settings. A 
highly accurate viewshed determina n algorithm, R2, is presented that executes 
in 0(R?) time (where R is proportional to the number of points along the radius 
of the potential viewshed) and does not suffer from numerical problems, complex 
special cases, or high constant facias. Since the size of the output in viewshed 
determination is also 0(R 2 ), the time complexity of R2 is within a constant factor 
of optimality. 

The R2 algorithm is used to verify a fast and effective visibility index estima¬ 
tion procedure, WeightF. WeightF has time complexity O(R) and produces visibility 
index estimates with a correlated variation to R2 in excess of 0.9 on a variety of 
terrain datasets and problem settings. These visibility indexes can be used to se¬ 
lect points with best aggregate visibility in an area. On real terrain, analysis using 
WeightF shows that points with significantly above average visibility index values 
are only a very small number of the total observer points. This property and the 
fast execution time of WeightF makes it useful for identifying high visibility points 
for use in siting algorithms, and to produce a viewable visibility index image that 
can display a large amount of visibility information in an intuitive fashion. 


xi 






CHAPTER 1 
INTRODUCTION 


1.1 Motivation 

The explosion of capabilities for collecting and distributing digital elevation 
data for terrain from around the world has led to a significant increase in the amount 
of computation required to exploit this information for problems where visibility is 
an important factor. Use of naive visibility determination algorithms results in much 
greater than linear increases in computation requirements as elevation dataset sizes 
increase. 

The increasing power of widely available workstations and personal computers 
invite exploitation of increasing volumes of terrain data. However, the straight¬ 
forward use of existing visibility analysis procedures either degrades a significant 
amount of the information present in the original elevation data or imposes compu¬ 
tational requirements far in excess of current or anticipated hardware capabilities 
when examining even modest real world problems. Various existing approximation 
methods reduce this complexity but can suppress the effects of relatively minor 
variations in terrain elevation that may have a major impact on visibility. Careful 
design and selection of visibility estimating algorithms and representations can make 
exploiting elevation data more computationally tractable. 

Intrinsic to visibility is the concept of a line-of-sight. The term line-of-sight 
(LOS) refers to a path connecting am observer and the point being observed (the 
target). To determine a LOS, the location of the observer and the target must be 
known in three dimensions - the two-dimensional geographic location and height 
above some common referent. A LOS may be an essentially straight line, as in the 
case of an optical LOS, or (for many electromagnetic signals) may ‘bend’ around 


1 






2 


irregularities m the terrain. If the LOS between two points is not obstructed by 
terrain or other interference, it is referred to as clear. If some obstruction prevents 
observation between the two endpoints of a LOS, it is considered blocked. 

One important consideration in the problem of siting certain kinds of facilities 
(such as radar and communication antennas) is the quantity and location of areas 
visible from candidate sites. Many practical problems require maximizing the total 
value of covered sub-regions without requiring complete coverage of the geographic 
area of interest. The details of a specific siting problem are described in Appendix B. 
In real world cases, such problems are characterized by insufficient facilities (e.g., air 
defense units) to provide complete coverage of all important facilities within the Area 
of Interest (AO!) and the need to select and evaluate potential siting combinations 
in an on-line fashion. 

Traditional methods of comparing visibility from different locations become 
difficult to use when more than a trivially small number of locations is being con¬ 
sidered. Non-traditional methods of portraying visibility information for on-line 
analysis might prove to be of great benefit to the process of interactive analysis and 
decision making [Ray94]. 

1.2 Nature of Computation Costs in the Visibility Problem Domain 

Storage requirements increase with the square of the linear density of data 
points per unit distance on actual terrain. The terrain model employed here, based 
on gridded elevation data, is described in Chapter 3. For a specific ground distance 
radius, the computation cost for determining the visibility to all surrounding points 
from a given observer point by naive methods increases with the cube of the linear 
density of data points. The computation cost for determining the visibility to all 
surrounding points from all potential observer points in a given ground area increases 
with the fifth power of the linear density of data points. 






An example 1 using representative problem parameters illustrates the impact of 
point density on computational cost. The costs for data storage and computation to 
calculate visibility from each point in an area approximately 100 kilometers square 
out to all points up to 40 kilometers distant are as shown in Table 1.1. These 
results assume elevation values are stored as two-byte integers and that visibility 
determination to each surrounding point from a given observer point takes on the 
order of 1 millisecond of CPU time executed on a Sun IPC workstation for the case 
of 80 meter spacing between points. 


Point Spacing 

Storage (MB) 

Time 

1000 meters 
300 meters 
80 meters 
30 meters 
10 meters 

0.06MB 

0.69MB 

9.68MB 

68.84MB 

619.52MB 

67 minutes 

19 days 

39 years 

52 centuries 
1,274 millennia 


Table 1.1: Comparative Costs of Increasing Grid Density 
Greater elevation point density is desirable to allow more faithful representa¬ 
tion of the terrain for which any given analysis is being conducted. However, as 
Table 1.1 shows, modest increases in the density of data lead to an unacceptable 
explosion in the cost of computation when attempting a thorough visibility analysis 
by traditional methods. Significant increases in point density for a variety of raster 
terrain data have occurred in the past decade, and given the ongoing increases in the 
ability to collect, store, and access large volumes of data this trend can be expected 
to continue. For example, digital satellite imagery from the former U.S.S.R. is now 
available on the commercial market at a resolution estimated at between 1.5 and 2 
meters. Low and decreasing prices for GPS (Global Positioning System) receivers 
and completion of the associated system of satellites will greatly reduce the cost 
of collecting accurate digital elevation measurements, potentially leading to large 
volumes of high density data. Sample 1 meter resolution elevation datasets have 
1 See Appendix A for details. 













4 


: 

already been produced at the U.S. Army Topographic Engineering Center (TEC) 
using automated digital stereo extraction. 

1.3 Methodology 

To find suitable methods for performing visibility analyses at reduced compu¬ 
tational cost, our general approach is to construct the necessary automated tools 
to facilitate investigations using real and simulated data; define a formal elevation 
model and problem setting for algorithm development; develop a computationally 
tractable and highly accurate algorithm for viewshed determination to serve as a 
benchmark for faster ordinal visibility estimation techniques; develop techniques to 
rapidly and accurately estimate a relative ordinal value of visibility in large sets of 
observer points; validate accuracy and predicted performance; and suggest potential 
applications. 

1.3.1 Construct Methods to Access Data 

Digital elevation data is available from several sources, such as the Defense 
Mapping Agency (DMA) and the U.S. Geological Survey (USGS). Datasets are pro¬ 
duced at several resolutions baaed on different underlying models [USG90], and are 
typically not organized in a form for efficient access and processing. A necessary 
preliminary step in researching approaches to extracting amd making use of visibil¬ 
ity information derived from elevation dataisets of significant size is the ability to 
transparently access severed existing formats of wide-coverage elevation data. Tools 
constructed to provide this access should facilitate construction of higher levels of 
abstraction ais appropriate adgorithms amd representations aire developed. 








5 


1.3.2 Elevation Model and Problem Setting 

In general, lines-of-sight between two locations will pass between points of 
known elevation, not just directly over them. This requires that a strategy must 
be selected for approximating the elevations of points not explicitly represented in 
the data. On a rectilinear grid where the use of higher order than linear interpola¬ 
tion methods is not warranted, we can establish data point selection guidelines for 
designing and evaluating line-of-sight determining algorithms. 

1.3.3 Viewshed Algorithms 

A fundamental objective of this research is to construct and validate algorithms 
that provide substantially improved levels of performance (required computation) 
for given levels of accuracy. 

Exhaustive, non-approximate determination (referred to as the R3 algorithm) 
of the visibility from each grid point within the radius of interest from an observer 
point varies with the cube of the linear density of elevation data points. An algorithm 
(referred to as the R2 algorithm) for convex areas of interest was developed whose 
complexity varies with only the square of the linear density of points while producing 
high agreement (more than 90 percent) with the results of the R3 algorithm in 
determining the visibility of target points from an observer point. Details of the R3 
and R2 algorithms are presented in Chapter 4. 

1.3.4 Visibility Index Algorithms 

To derive some measure of visibility for each point in an area of realistic size, 
the results summarized in Table 1.1 clearly demonstrate some form of estimation is 
required. Assigning a visibility index to each point in the grid of elevation data pro¬ 
duces a corresponding visibility grid. Relationships that exist between corresponding 
elevation grids and the visibility grids need to be identified and exploited. 





Figure 1.1: Visibility Cells between Latitudes North 34-40 and Lon¬ 
gitudes East 124-130. 





7 


The process of investigating and refining visibility index estimation procedures 
was conducted over a variety of terrain datasets in order to explore the broadest 
applicability and to obtain insights from as many detected special cases as possible. 
Digital Terrain Elevation Data (DTED) data cells (described in Appendix A) from 
DMA that have been processed to produce visibility information are shown as the 
outlined area in Figure 1.1 

1.3.5 Accuracy and Performance Evaluation 

The R2 algorithm, due to its reduced computational requirements, can be 
used as a means to evaluate the accuracy of visibility index estimation algorithms. 
Predicted variations in the computation costs with increasing problem size can be 
compared to timing results obtained from tests conducted on actual hardware. 

1.3.6 Potential Approach 

To make the calculation of visibility values for candidate facility sites more 
tractable, a hierarchical approach can be employed. First, an algorithm with greatly 
reduced computation load can be employed to examine each possible site to rank 
order them from worst estimated visibility to best estimated visibility by assigning 
a visibility index to each point. Once this has been accomplished, a more computa¬ 
tionally demanding procedure can be used to investigate a limited number of points 
(those with the highest estimated visibility). 

1.4 Issues and Goals 

1.4.1 LOS Calculation Constraints 

In keeping with the elevation model presented in Chapter 3, any LOS proce¬ 
dures employed should take into account ail relevant elevation data but prevent all 
other elevation data points from influencing visibility results. Stated more formally: 






8 


• Criterion for Adequacy Every elevation point which is 4-neighbor (east, 
west, north, or south) adjacent to a line-of-sight should contribute to deter¬ 
mining visibility and line-of-sight height for that line-of-sight. 

• Criterion for Appropriateness Any point which is not 4-neighbor ad¬ 
jacent to a line-of-sight should not contribute to determining visibility and 
line-of-sight height for that line-of-sight. 

1.4.2 Performance Improvements over Existing Algorithms 

The execution time of the R2 algorithm compares very favorably with the 
results from [Jun89] and [Sha90]. Since the total number of locations, N, is propor¬ 
tional to the square of the linear density of points for a convex area of interest, in 
the terms of [Jun89] and [Sha90] the R2 algorithm achieves a 0(N) running time 
with a low constant term, along with a lack of geometric special cases and floating 
point round-off errors. 

1.4.3 Empirical Validation of Algorithms on Real Terrain 

Since we will attempt to capitalize on the spatial correlation present in real 
terrain to reduce the computation required to determine useful estimates of visibility 
index values, the results of such algorithms are checked and evaluated on real terrain 
of various compositions and locations. Accuracy is evaluated based on demonstrated 
utility in finding best visible points or in showing residual visibility uncertainty equal 
to or less than that due to uncertainties in the elevation data. 

1.4.4 Identification of Candidate Point Sites 

Single point candidate sites can be selected based on their visibility index 
values. However, naive selection of the n best sites over an entire area will produce 
in some cases highly clustered results in potentially unsuitable areas. What is more 






9 


likely to produce useful results is to select the m best sites in k subareas of the total 
area, where n = k x m. Some mechanism for estimating the appropriate size of 
subareas, based on the characteristics of the terrain or the specific siting problem 
type, must be developed. 

A straightforward procedure for forcing the selection of the n best sites to 
spread over the entire area of interest by means of subarea histogramming has been 
developed and demonstrated. This could be extended to use overlapping subareas 
and employ local terrain characteristics to influence subarea size. 

1.4.5 Exploit Parallel Processing to Compute Visibility Indexes 

The independence of the computations involved in calculating the visibility 
index for a given point (e.g., the rays out from the observer) and for each point 
in the AOI suggests the applicability of parallel processing to reduce the real time 
requirements for producing a visibility image of a given area. This has been employed 
with success using a series of workstations of various capacities belonging to the 
Department of Electrical Engineering and Computer Science at the U.S. Military 
Academy at West Point. Disk storage requirements remained modest due to the 
use of networked file systems. Similar parallelism can employed in use of the R2 
algorithm to refine the estimated selection of best points produced by calculation 
of the visibility index. Parallel computation at severed levels of granularity was 
employed in conducting visibility analyses, resulting in an increase in throughput 
approximately linear in the number of processors employed. 

1.4.6 Demonstration to User Community 

At an interim point in this research, feedback from users with interests in ter¬ 
rain visibility was sought by distributing preliminary results and soliciting evaluation 
and comment on the most useful aspects and areas for further investigation. 




10 


A demonstration system that can execute on many common MS-DOS personal 
computers has been constructed and furnished to TEC for evaluation and possible 
distribution to a terrain user community within the U.S. Army (see Appendix D). 
This system provides side-by-side presentation of elevation and visibility gray-scale 
images with predetermined locations of high-visibility sites, along with data (reduced 
’etail 16-to-l) for 12 one-degree cells on the Korean Peninsula. An enhanced 

a of this prototype may as the basis for a component of the next release of the 
TLv, AirLand Battle Environment (ALBE) software suite. 

TEC is also using algorithms developed as part of this research as part of their 
prototype 1 meter spacing elevation dataset analysis project for the U.S. Marine 
Corps. Applications include on-line viewshed display and visibility image genera¬ 
tion. Plans are in progress to incorporate this work into TEC’s Terrain Elevation 
Module and in applications on the Department of Defense Simulation Network (SIM- 
NET). 

1.5 Organization of Thesis 

This chapter presented an introduction to the general research topic and the 
methodology, goals, and organization of the thesis. 

Chapter 2 provides a survey of previous work on the analysis of elevation data, 
methods for visibility determination, and approaches to making use of visibility 
results, such as in siting decisions. 

Chapter 3 presents a taxonomy of factors that affect the conduct of visibil¬ 
ity analyses and an explicit, detailed model of the elevation data upon which our 
research investigations will be conducted. 

The development of basic and improved algorithms for determining the view- 
shed from a given observer location is described in Chapter 4. 





11 


Chapter 5 describes the rationale, development, and refinement of a fast pro¬ 
cedure to determine accurate visibility index estimates for large numbers of observer 
locations, including considerations for parallel execution. 

In Chapter 6, the accuracy of the visibility algorithms developed during the 
course of this research is analyzed in a variety of problem and terrain settings. 

Chapter 7 presents the results of verifying the predicted execution time per¬ 
formance of the viewshed determination and visibility index estimation algorithms. 

Chapter 8 summarizes the results and contributions of this research and pro¬ 
poses several avenues for continued work. Among the most significant results are 
the development and validation of an accurate 0(R 2 ) viewshed determination al¬ 
gorithm, called R2, and a consistent visibility index estimation algorithm called 
WeightF that executes in O(R) time. 





CHAPTER 2 

LITERATURE REVIEW 


This chapter presents a summary of pertinent results regarding the visibility prob¬ 
lem area and its associated aspects. Where they relate to specific points in other 
chapters, certain references will be cited in that context. 

2.1 Background 

2.1.1 Visibility 

Approaches to computer analysis of visibility have been examined for decades. 
A method for determining visibility for air defense models, the Minimum Altitude 
Visibility Diagram, was presented in [SI71]. Several terrain visibility algorithms 
with worst case execution times ranging from 0(N 2 a(N 2 , N)) to 0(N 3 a(N 3 , N)) 
were evaluated 2 in [Jun89], with N proportional to the number of locations being 
tested for visibility from a given observer. Empirical evidence was presented that 
the method with the best worst case time, the Triangle Sort Visibility Algorithm, 
had an average case execution time of 0( N) but required substantial special condi¬ 
tion handling. Noted in [Sha90] was that the impact of the extremely high constant 
term in the expression for the execution time of the Triangle Sort Visibility Algo¬ 
rithm precluded its use on the datasets studied there (the quantity of elevation data 
involved corresponded to the size of a DTED Level I cell). As a result, a visibility 
algorithm with an empirically derived execution time of 0(N 124 ) was employed. 
Also presented in [Sha90] was a matrix of visibility counts based on visibility from 
a ‘scenic view’ path and corresponding to the grid of elevation values. 

Visibility calculations have been used in non-traditional ways, such as for use 

2 Where a (m, n) is the inverse of Ackermann’s function and for all ‘practical’ purposes has an 
upper limit of 4. 


12 





13 


in feature inference [DFFP + 86], [DFNN88]. [DFJN92] present the use of visibility 
counts for use in image interpretation by performing a visibility analysis on a gray- 
scaled image by treating the pixel values as elevations and using a standard 8(/2 3 ) 
per observer visibility algorithm. A current overview of visibility computation issues 
is presented in [Nag94]. 

2.1.2 Parallelism 

Calculating visibility lends itself well to parallelized computation. Medium and 
coarse grained parallelism in investigating the visibility properties of a large number 
of observer points can be exploited without special programming or hardware (see 
Section 5.8). A finer grained approach using hypercube hardware for parallelizing 
visibility determination from a single observer is presented in [TD92], along with a 
discussion of some of the common pitfalls possible in designing visibility algorithms. 
Relative visibility values are often a factor in facility siting problems. The use of 
a transputer array in a heterogeneous computing environment, including personal 
computers, to execute a hierarchical location selection algorithm is described in 
[Den93]. A strategy for integrating parallel computation hardware for use in spa¬ 
tial analysis problems into existing networks and GIS environments is described in 
[Li93]. [Mow93] presents a case study of implementing spatial analysis procedures 
on Thinking Machines CM-2 and CM-5 parallel computing hardware, observing 
that synchronous message passing operations common in fine grained parallelism 
can significantly reduce overall computational throughput. 

2.1.3 Siting 

Methods of siting a limited number of resources to cover spatially distributed 
demands have also been studied for some time. Optimal site selection under the 
relatively simple constraint of a fixed maximum distance or time between a site and 





14 


its serviced localities was considered in [TR72]. When each potential site can be 
associated with coverage of a specific sub-region of the total AOI, determining the 
minimum number of sites required to cover the entire region can be stated as the 
Minimum Cover Problem [GJ79] which is known to be NP-Complete unless each 
potential site covers two or fewer ‘points’ (distinct locations). The Maximal Covering 
Location Problem as formulated in [CR74] incorporates maximizing coverage of a 
population and presents several heuristics and a linear programming formulation of 
solutions. [Mil93] presents a three by three location problem typology of nine distinct 
combinations of point, line, and polygon natures of facility and client locations. It 
was shown in [DFFP + 86] that a standard set-covering algorithm can determine the 
minimal set of observation points from which an entire surface can be inspected, 
using a triangulated terrain model and determining the visibility regions of the 
vertices to provide the required inputs. 

2.1.4 Terrain Elevation Data 

DTED Level I cells of elevation data typically represent a geographic area 
measuring one degree of latitude by one degree of longitude. Such cells are often 
referred to by the geographic coordinate of their southwest corner. For example, 
‘N37E127’ refers to a cell of approximately 1.5 million elevation points in a 1201 
by 1201 matrix. This cell represents terrain between latitudes 37 and 38 north and 
longitudes 127 and 128 east. DTED Level I datasets (cells) have been produced for 
large portions of the earth’s total land area during the past 15 years. 

2.2 Characteristics of the Problem and its Domain 

While many approaches have been made to the problem of determining or using 
visibility characteristics, such as Triangulated Irregular Networks (TIN) [PDDT92], 
Blocking Edges [Nai88], and Visibility Dominance [Lee92], none seem to translate 



15 


well to the scope of current real world problems and datasets. Existing approaches 
tend to become too computationally costly as the size and density of data increase, 
or involve polygonal approximations of the available data resulting in loss of in¬ 
formation significant enough to obscure the true relative visibility merit of some 
potential sites. Results presented in [Kum92] contradict the notion of storage space 
advantages to TINs over DEMs, as do cases cited in [Lay93]. Advances in the capa¬ 
bilities and availability of on-line data compression and decompression technologies 
may further the relative space efficiencies of DEM representations. 

2.3 Practical Considerations 

The need to clearly describe the models and algorithm implementations used 
in visibility studies and GIS systems in general has been stated in [Fis93]. In many 
commercial GIS systems, the algorithms and assumptions employed in viewshed and 
other calculations are often not revealed due to commercial competitive or liability 
considerations. This amplifies the potential utility of a set of efficient, experimentally 
investigated, freely available visibility tools. 

The general data model used here will be based upon a raster representation of 
the elevation attributes of a terrain surface. Other representations exist and are in 
common use, such as Triangulated Irregular Networks (TIN) and elevation contours. 
An overview of TIN’s and the use of Delaunay triangulation to generate them from 
grid cell data is given in [Tsa93]. 

It is unavoidable that in attempting visibility determinations there will be 
errors introduced in attempting to represent physical terrain with a model and 
finite digital representation. [Goo92a] distinguishes between error descriptors and 
error models. Error descriptors are measures of the difference between the value 
of a model quantity and the ground-truth value. An error model forms the basis 
for a stochastic process that can generate a population of sample instances of true 







16 


terrain values that differ only in the distortions introduced by error. Generally, error 
descriptors can be thought of as pertaining to inputs and error models help gauge 
outputs. 

Errors in DEM values can be distributed randomly or systematically. The 
RMSE for many visibility analyses is often taken as about 7 meters [Fis92], based 
on a common USGS standard. [Lay93] mentions two common types of systematic 
DEM errors, spikes and strips, with the nature of the errors in a specific DEM 
influenced by the methods used in its creation; strip error magnitudes of 10 meters 
were reported. Aspect (directional, not gradient magnitude) measures were reported 
as highly sensitive to the influence of random errors in DEMs. 






CHAPTER 3 

VISIBILITY PROBLEM TAXONOMY AND ELEVATION MODEL 


In this chapter we first present a detailed taxonomy of the specific characteristics 
of the type of visibility analyses we will be addressing. We then present a model 
of the elevation values (based on raster elevation datasets) that will be used in 
designing the desired viewshed and visibility algorithms, along with a description of 
the general nature of LOS determination. 

3.1 Taxonomy of Visibility Analysis Characteristics 

Although a great deal of work has been done on various ways of examining 
the general problem of determining visibility information from digital terrain data, 
it is not always easy to directly compare the details and conclusions of various 
investigations. In many analyses of typical GIS operations, the specific assumptions 
and methods are not consistently, clearly, and completely stated [Kna92]. The degree 
of general applicability from any given analysis may be significantly impacted by 
the problem parameters used; for example, an analysis may have been conducted 
for a relatively short visibility range for only a few points in a heterogeneous terrain 
setting. 

To lay out a framework for developing and analyzing the various visibility al¬ 
gorithms presented in later chapters, as sin initial step it is necessary to explicitly list 
some of the important dimensions of visibility problems. There will be no attempt 
to claim that the specific results to be presented will span all of the characteristics 
described below, but by clearly placing the context in which this research resides it 
will be more straightforward to evaluate it, extend it, and identify contexts in which 
it is most and least applicable. 


17 










18 


3.1.1 Terrain Characteristics 

• Relative Complexity of terrain. The complexity of terrain, for the purposes 
of visibility determination, is related to the degree and frequency of slope 
changes. The significance of such changes is dependent on the assumed height 
of the observer and target above ground level and of the length of the radius 
of interest. 

• Heterogeneity in the kinds of surface configuration making up the area being 
investigated can complicate analysis; for example, the presence of large bodies 
of water or plains areas adjacent to complex terrain can influence the raw 
visibility of points on the boundary between regions. 

• Man-Made Features may have a significant impact on potential visibility due 
to built-up areas or other construction. 

• Vegetation can seriously impact visibility and is generally not represented in 
digital elevation data [Slo93]. Other terrain data can provide vegetation infor¬ 
mation (when available). Such data may be in a non-raster form, be affected 
by seasonal variations, and have error characteristics much different from spa¬ 
tially corresponding elevation data. 

3.1.2 Elevation Data Characteristics 

• Density of the elevation grid (how many data points are used to represent a 
given distance along the ground) affects the fidelity of the representation of 
the physical terrain, the complexity and practicality of the types of analysis 
which can be conducted, and even the nature of what is being measured. 
For very dense grids, such sis 1 meter between points, simply measuring the 
elevation at the given location may be adequate. For more sparse grids, such 













20 


as 100 meters or greater between points, the data collection agent may have 
deliberately chosen a ‘representative’ value to best reflect, according to some 
(often not explicitly stated) criteria, all of the terrain which is closer to the 
point being measured than any other grid point. This is an instance of the 
Modifiable Areal Unit Problem (MAUP) [Fot89]. To provide am intuitive feel 
for the effect of point density in representing terrain elevation, Figure 3.1 
depicts a grid of elevation values taken near cell coordinate (240,240) of DTED 
cell N37E127 at 1:5, 1:2, and 1:1 samplings. 

• Dataset Size. Many visibility analyses are performed on datasets of modest 
sizes, such as one to two hundred points on a side. This limits the amount 
of computation required and mitigates issues such as earth curvature. How¬ 
ever, it limits the ability to conduct large radius visibility analysis, the choice 
of observer locations (since any observer location with a complete potential 
viewshed must be no closer than the visibility radius to the edge of available 
data), and the heterogeneity that can be encountered on real terrain. 

• Earth Curvature is often ignored in many studies that work with small datasets 
that cover limited terrain areas [Fis93]. In attempting to work with larger 
scale problems, however, earth curvature must be accounted for. A close 
approximation for the variation in effective elevation drop with distance from 
the observer is 

A E c &Re~ V Re 2 - D 0 2 (3.1) 

where Do is the distance from the observer, Re is the effective radius of the 
earth, and A Ec is the change in elevation due to earth curvature. 

3.1.3 Selection of Sample Observer Points 

• Exhaustive selection simply chooses every point within an area of interest as 




21 


an observer location for visibility analysis. The obvious advantage is complete 
coverage of the available data (except for the points near the boundaries of the 
available data for which elevation values axe not available out to the radius of 
visibility in all directions), the relative freedom from the risk of biasing results 
because of sampling problems, and the large quantity of result values from 
which to draw inferences. The obvious disadvantages are the potentially large 
costs in computation time and, depending on the nature of the information 
collected from the analysis, storage requirements. 

• Arbitrary selection allows an analyst to investigate hypotheses and perform 
intuitive exploration, but provides no systematic basis for identifying points 
that best meet some particular criterion, such as having the highest visibility. 

• Regular Sampling of the total grid of data points by choosing a subset of points, 
where the points selected are equally ‘spaced’ according to some criterion, is 
an easy way of selecting a sample from a raster of elevation values for the 
purpose of reducing the amount of data to process when investigating the 
visibility characteristics of an area. This can produce good coverage spread 
over the entire area but is less desirable them random samples for the purposes 
of statistical analysis and may interact with systematic errors in the elevation 
dataset in unanticipated ways. 

• Pseudorandom selection of observer locations can meet the dual requirements 
of spatially distributing a sample of observer locations and preserving the 
statistical independence of the sample locations from underlying patterns in 
the elevation data, as long as the pseudorandom number generators are robust. 

• Extreme Value selection can be used to attempt to match the subset of observer 
points to particular requirements or investigations. Extreme (high or low) 
values can be selected or computed based on raw elevation values, visibility 




22 


estimates, local gradients, or any measure significantly less costly to calculate 
than the visibility analysis to be conducted on the sample points themselves. 

• Dispersed Extreme Value selection employs extreme value selection constrained 
in some way to ensure spatial dispersion. This can be accomplished by taking 
the extreme valued point(s) from uniformly distributed subcells, pseudoran¬ 
dom sampling that rejects points that are too close to ail existing samples, or 
other means. 

3.1.4 Elevations for Non-Data Point Locations 

• Locations of Defined Elevations. Possible choices of the set of locations where 
am elevation value is provided by the elevation model (based on the available 
data) cam include all areas, points on grid intersections and grid line segments 
(see Section 3.2), data value points only, or a mapping of the available data 
points to a less dense grid. The TerraBase system [Com88], for example, 
for some operations maps screen coordinates to geographic coordinates that 
generadly do not fall on grid lines. It uses the four adjacent grid point elevation 
values to interpolate a value for the desired coordinate. 

• Interpolation Method. Interpolations can include step functions such as min, 
max, and nearest; averaging the values of the nearest data points; simple linear 
interpolation; and higher-order interpolations based on various combinations 
of points that may use values beyond nearest neighbors. 

3.1.5 Uncertainty in the Data 

The importance of being aware and explicit about the inevitable data uncer¬ 
tainty in the digital elevation information used as the basis for visibility determina¬ 
tion has been cited in [WF93] and many other publications. One breakout of data 
uncertainty issues is: 







23 


• Quantization error is inherent due to the nature of digital data representation 
and the essentially continuous variation of elevation on real terrain. 

• Discrete Measurement error is the difference between the discrete data value 
closest to the true ground elevation and the value actually present in the digital 
data; see Equation 3.2 

• Magnitude of errors assumed present in the data, such as an estimated stan¬ 
dard deviation. For analysis in which no error impacts are dealt with, this is 
implicitly taken as zero. 

• Distribution of errors, including both a probability density form such as the 
Gaussian and the degree of spatial autocorrelation (generally taken as zero 
[Fis92]) presumed present in the errors. 

3.1.6 Observer, Target, and LOS Characteristics 

• Nature of Observer and Target Locations. Although commonly taken to be 
points, the observer and the target can each be represented as a point, a line, 
or an area. As an example, [TDD91] presents a region (area) to region parallel 
visibility algorithm. 

• Height of the Observer above the terrain. In many analyses the height of 
the observer is taken as zero. This may not be the best choice because it can 
make the analysis unduly sensitive to minor random, systematic, or discretiza¬ 
tion errors in the elevation of the observer point and its eight neighbors. In 
many potential applications of visibility analysis, the observer may be a person 
standing upright, a facility, or an antenna; none of these take their visibility 
from ground level. 





24 


• Height of the Target above the terrain. The height of the target is often taken 
as zero, although in many cases the purpose of the visibility analysis is to 
determine where objects such as antennas, buildings, aircraft, or eyesores can 
be seen from the observer. Tops of such objects are generally not taken to be 
at ground level. 

• Range from observer to target is often modest both in terms of number of 
elevation grid points (100-200 on a side) and in the actual ground distances 
(1-2 km) corresponding to the analysis. 

• LOS Curvature. The LOS is often assumed to be ‘straight’, in the sense of 
an optical LOS with negligible distortions. In problems involving antennas, 
the ability of an electromagnetic signal to ‘bend’ around terrain features and 
detect targets that are not visible to an optical LOS must be considered. Even 
in these problems, determination of an optical viewshed may be a first step. 

• Tangent Conditions. Due to the integer representations of elevation common in 
raster elevation datasets, situations where a computed LOS is exactly tangent 
to an elevation can occur relatively often. An explicit determination of how 
to treat such a LOS (clear or blocked at the point of tangency) is necessary. 

3.1.7 Calculation Characteristics 

• Integer. Use of integer arithmetic throughout visibility determination is at¬ 
tractive because of its simplicity, speed, relative stability, and match to the 
problem inputs of integer elevation values. Care must be taken on what inter¬ 
mediate values are represented with integers: for example, if the LOS height 
above ground at a given point is measured as an integer and carried forward as 
the basis for the visibility determination for a point further from the observer, 
accumulated error increases very rapidly. 





25 


• Floating Point calculations and representations of non-integer values such as 
slope provide a convenient means of implementing visibility algorithms, but 
can run substantially slower than integer or fixed point operations on many 
hardware architectures and introduce complications when testing for equality 
between values due to round-off error. 

• Fixed Point arithmetic can combine the speed and discrete comparison nature 
of integer operations with some of the range and flexibility of floating point 
operations when the range of values, intermediate results, and accuracy re¬ 
quirements are well understood. Some programming languages (such as Ada) 
provide built-in support for fixed point operations. 

• Continuous interpolations provide modeled elevation values for locations which 
do not have an exact corresponding data point in the elevation raster dataset 
by means of some continuous function, such as linear or quadratic interpola¬ 
tion. 

• Discrete approximations provide elevation values for non-data-value points 
using functions such as min, max, or nearest; the effect of such approximations 
is typically to propagate an existing elevation data value to some set of points 
in its vicinity. For example, [TD92] use the elevation data value of a point 
for visibility computations on any LOS passing through the rectangular cell 
centered on that point. 

• Number of Calculation Instances. When we explicitly model uncertainty in 
the data, we have the option of performing multiple analyses over several 
instances, or possible realizations, of the elevation data. 

• Representation and Operation Ordering. The selection of the quantities to 
be represented in visibility calculations and the order in which arithmetic 






26 


operations are applied can have an impact on the outcome of specific visibility 
results. Just the simple decision to carry forward the integer distance and 
height values of a visibility-determining elevation instead of a fractional slope 
value can impact visibility cases where ground elevation is close to the LOS 
height based on the previous visible point. 

3.1.8 Placement of this Research within Taxonomy 

The models and analyses employed here, unless otherwise noted, are positioned 
within the visibility taxonomy described in Section 3.1.1 according to the entries in 
Table 3.1. 

Although errors unrelated to quantization in the input elevation datasets are 
generally not modeled here, see section 6.8 for some preliminary results on the 
impact on visibility produced by assuming the presence of errors in the input data. 


3.2 Elevation Model 


Let a landscape L be the physical terrain surface of the AOI defined by a 
real-valued function zl = /i(xt,yt) where xl is the longitude, yi is the latitude, 
and zi is the height 3 of the surface above mean sea level (elevation) at coordinate 
(x L ,y L ). A tessellation T of integer values ex = Vt) at discrete coordinates 

{xtiVt) approximates the elevation at the corresponding location (xl> Vl) in L. 
Define discrete measurement error to be 


dr = 


! 


\er — zi — 0.5] if zi > ej 
0 if zi = ej 


[ [er - zl + 0.5J if zl < er 


(3.2) 


This definition of dr specifies that there is a non-zero discrete measurement error 
(in terms of whatever units axe being employed, e.g., meters) in the approximation 


3 Although the unit of elevation measurement is arbitrary, meters are used in DTED data. 




27 


Terrain Characteristics 

Relative Complexity 

varied (complex) 

Heterogeneity 

varied (high) 

Man-Made Features 

none 

Vegetation 

none 

Elevation Data Characteristics 

Density 

70-120 meters (DTED Level I) 

Dataset Size 

viewshed « 750,000 points 

Earth Curvature 

Equation 3.1 

Selection of Sample Observer Points 

Exhaustive 

yes 

Arbitrary 

no 

Regular Sampling 

no (tools developed) 

Pseudorandom 

yes 

Extreme Value 

yes 

Dispersed Extreme Value 

yes 

Elevations for Non-Data Point Locations 

Locations of Defined Elevations 

grid points & line segments 

Interpolation Method 

linear 

Uncertainty in the Data 

Quantization 

integer (meters) 

Discrete Measurement 

not modeled 

Magnitude 

not modeled 

Distribution 

not modeled 

Observer, Target, and LOS Characteristics 

Nature of Observer and Target 

points 

Height of Observer 

5 meters 

Height of Target 

25 meters 

Range 

40 kilometers 

LOS Curvature 

straight (optical) 

Tangent Conditions 

treat as visible 

Calculation Characteristics 

Integer 

yes (for LOS calculation) 

Floating Point 

(only for full weighting) 

Fixed Point 

yes (for LOS-height calculation) 

Continuous interpolation 

yes (linear) 

Discrete Approximations 

no 

Number of Calculation Instances 

one 

Representation & 

Operation Ordering 

32-bit integer intermediate; 
manually optimized 


Table 3.1: Placement within Taxonomy 






28 


of zl by ej if the absolute difference between zl and e? exceeds 0.5. 

Note that for any given ( x,y ) € T, in general ej ^ zl because ej is integer 
valued while zl is real valued, or because ej may have been chosen by the data 
collection agency to ‘represent’ the terrain in the vicinity of (i L,yt) rather than 
most closely approximate the precise value of /r,(*L>yL)- The discussion of covering 
triangles in this section provides an example of representative elevation selection. 

The coordinates (xT,yr) € T that correspond to coordinates (x£, yt) € L 
are typically discrete multiples of some fundamental geographic measurement, such 
as degrees, minutes, or seconds of longitudinal (xi) or latitudinal (yi) arc. For 
example, the typical DTED Level I data cell has longitudinal and latitudinal spacing 
of 3 arc-seconds between 4-neighbor adjacent coordinates. While this method of 
selecting discrete locations results in a constant latitudinal spacing (approximately 
92.6 meters for a 3 arc-second interval), the variation in the longitudinal spacing 
over such a cell may vary from about 0.02% near the equator to about 2% near north 
or south 50 degrees latitude. As in many other visibility investigations making use 
of real world elevation raster data, here the impact of the variation in longitudinal 
spacing at different latitudes is assumed to be negligible in comparison with other 
errors resulting from the measurement or sampling process. Variations due to the 
possible use of ellipsoidal (as opposed to perfectly spherical) surface spheroids in 
the compilation of geographic data are similarly ignored. 

In contrast, elevation variations due to earth curvature cannot be ignored over 
the distances encountered in many siting problems. For example, over a distance 
of 40 km the drop due to earth curvature is approximately 125 meters (see Equa¬ 
tion 3.1), a distance that is significant in the context of trying to track aerial targets 
as low as 25 meters above the terrain. Accounting for earth curvature here will usu¬ 
ally be accomplished by adjusting the elevation values for each point in the input 
geographic datasets (which are generally referenced to mean sea level) by the amount 




29 


of elevation drop corresponding to the distance between that point and some fixed 
reference location, such as the center or the southwest corner of the dataset. If other 
information, such as vegetation height, is available and is significant in determining 
visibility, it can be incorporated in this step. 

Consider a rectangular matrix grid of points M to be derived from T in ac¬ 
cordance with the assumptions stated above. Every point in T has a corresponding 
point in M . Therefore, if T is based on a latitude and longitude reference system, so 
is M. Since M is rectangular then for any (xm,xjm) in M without a corresponding 
(xt, yr) in T an arbitrary value well outside the range of known actual elevations is 
chosen as the value at M{xM,ym). Let M x be the number of distinct longitudinal 
coordinates and M y be the number of distinct latitudinal coordinates represented 
in M. The total number of points in M is r»Af = M x x M y . The terrain distance 
corresponding to all longitudinally adjacent points in M is a constant, m, x , and the 
terrain distance corresponding to all latitudinally adjacent points in M is a con¬ 
stant, m,„, for all points in M. Stated more formally, let GD[pi,pi] be the ground 
distance (for example, in meters) between points Pi, Pi € L, that corresponds to 
the two points pi,pi € M. Then 

= GD[{xi,yj),{x i+ i,yj)} 

for i € {1,2,..., M x — 1}, j € {1,2,..., M y } (3.3) 

m ay = GD[{xi,yj),{xi,y H1 )) 

fort € {1,2,...,A/*}, j € {1,2,...,M y - 1} (3.4) 

Some terrain models (e.g., [TD92]) consider each grid point to be at the center 
of a rectangular cell and use its value for the elevation of any location within the 
rectangle whose elevation is required. Other models make provisions for estimating 
elevation values of locations not coincident with a grid point as some function of 
two or more points in their neighborhood. In the model presented here the only 

1 







30 


locations for which elevation values are known or will be estimated occur either at 
grid intersection points M(x,, y,), i € 1... M x , j € 1... M v or on grid lines. Grid 
lines inside M are defined by the equations 


x = i for i € {1,2,...,M X } 

(3.5) 

y = j for j € {1,2 ,...,M„} 

(3.6) 


A grid line segment is a portion of a grid line whose endpoints are grid points 
in M that are 4-neighbor adjacent. Generally, the elevation value for a location 
corresponding to a point on a grid line segment but not on a grid intersection will 
be estimated by linear interpolation between the values at the endpoints. More 
sophisticated estimation schemes (splines, etc.) for points on and off the grid lines 
can be used where specific knowledge of the terrain or application warrant, but this 
model in its basic form attempts to minimize the assumptions and computation 
complexity in dealing with commonly available elevation raster datasets. 

For dealing with the elevations of points not on grid line segments, we intro¬ 
duce the concept of a covering triangle. Consider grid intersection points A , J3, and 
C in M, where B is longitudinally adjacent to A and C is latitudinally adjacent 
to A. The triangle ABC is a covering triangle; in this model we assume that the 
elevation of any point in L is no higher than the elevation of a corresponding point 
in a covering triangle. There can be at most two distinct covering triangle elevations 
corresponding to a particular (xr,yr), and we assume that /l( x l, Vl) is no higher 
than a projected point in any corresponding covering triangle. Given this assump¬ 
tion, we can make LOS visibility determinations by examining the intersections of 
LOS profiles with grid line segments and grid intersection points, and not have to 
explicitly estimate the elevation of other (non-grid) points in L. 

Using the previous definitions, consider the two-and-a-half-dimension surface 
S constructed from the grid points and grid lines in M. Every location (xl, Vl) in L 
can be mapped to a corresponding location in S by adding tin appropriate constant 








31 


offset (OLS s ,OLSy) and scaling by m„ and 

xs = (*l + OLS s )lm„ (3.7) 

ys = {vl + OLS y )/m„ (3.8) 


In general, xs and ys are real valued such that 0 < xis < A/* and 0 < yLS < M„. 
Points in S which are not on grid intersection points or grid line segments are, by 
assumption, not above their covering triangles and can have no effect on visibility 
not already modeled by grid segment and grid intersection points. Conceptually, 
questions about visibility between two points in L that correspond to points on grid 
lines or grid line intersections in M can be analyzed by identifying the correspond¬ 
ing points in S, performing the analysis using the model definition and available 
elevation data, and applying the results back to the original points in L. 

The elevation value, z$ = fs(xs,ys), where 0 < Xs < M x and 0 < ys < M„, 
is defined as 


where 


*s= < 


M(x s ,y s ) 

I NT ERPX(xs, ys) 
INTERPY(x s ,ys) 

undefined 


if x s = |xsJ>J/s = [ys\ 
if x s ± L*sJ,ys = [ys\ 
if x s = [x s \,ys / L2/sJ 
otherwise 


I NT ERPX (x,y) 


M([xl,y) x(x- |zj) + M(|xJ,y) x ([x] - x) 


INTERPY(x,y) 


M ( x , fyl) x (y - LyJ) + M(x , LyJ) x (fy] - y) 


where ‘undefined’ means irrelevant for LOS determination purposes (see the discus¬ 
sion of covering triangles, above). Let S xy be the set of points (x, y) for which zs is 
defined. 

Other estimates such as the maximum, minimum, or average instead of linear 
interpolation can be used to explore the tradeoffs between and sensitivity of various 
algorithms. 








32 


3.3 Characteristics of an Elevation Dataset 



200 400 €00 800 1000 1200 1400 

Elevation 


Figure 3.2: Elevation Histogram of DTED Cell N37E127 


As a specific example of an elevation dataset, we now consider the empirical 
properties of the approximately 1.4 million elevation values in the DTED Level I 
cell N37E127. Figure 3.2 provides a histogram of the elevations for this area, with 
values ranging from 0 to 1400 meters above mean sea level. In this particular region, 
clearly most locations are at the lower elevations. An interesting characteristic is 
revealed when this same histogram is plotted on a log scale as in Figure 3 3. 

With the exception of the very extreme values of elevation, the log plot of 
frequency of points shows a near linearly decreasing profile, suggesting (again, for 
this particular cell) an approximately exponential decline in the number of points at 
a given elevation as elevation increases. A similar decreasing exponential relationship 
for the number of points for given visibility index values is shown in Figure 5.4, 
although as illustrated in Figures 5.5 and 5.6, and in Chapter 6, there is no apparent 
strong relationship between high visibility and high elevation points. 












33 


T 



Elevation 

Figure 3.3: Log Scale Elevation Histogram of DTED Cell N37E127 

3.4 Considerations in LOS Calculation 

The details of calculating a LOS often receive relatively small mention in de¬ 
scriptions of visibility analyses but can have a significant impact on both results 
and execution times. Even given an elevation data structure as seemingly straight¬ 
forward as a DTED grid, many interpretations of where and how to calculate LOS 
information have been employed. In [TD92] calculations used the elevation and loca¬ 
tion of the nearest grid data point. Some military terrain analysis systems ([Com88]) 
have evaluated LOS information at points on the interior of an elevation grid cell, 
requiring an interpolation of elevation from the four surrounding data points. In 
accordance with the model of elevation presented in this chapter, the LOS methods 
employed here will only make use of elevation values or estimates on data grid points 
or grid segments. 

Figure 3.4 provides am example of a LOS profile. Shown are observer 0 located 
a distance Oh above the ground and three potential target locations, A, B, and C. 
There are several! significant locations, signified by points po through where certaun 







34 



Figure 3.4: Example of a LOS Profile. 


LOS’s intersect with or are tangent to the ground profile. Intuitively, observer 0 
will have a clear LOS to all points on or above the ground between po and pi, on 
or above line segment pipi, on or above the ground between pi and P 3 , and on or 
above the ray from O through p$. 

Key in determining visibility over terrain is the slope from the observer to 
points on the ground and to potential target points. Given that the elevation data 
has been pre-adjusted to compensate for earth curvature, a slope value S is deter¬ 
mined by the familiar 


5 = 


A/i 

Ad 


(3.9) 


where Ah is the difference in the height and Ad the horizontal distance between 
the endpoints of the line segment defining the slope. Using terminology from the 
elevation model presented in this chapter, the slope <%— from the observer O to 
point pi in Figure 3.4 is given by 

c__ h(x Pl ,y Pl ) ~ ( fL(xp,yo ) + Oh) 

° Pl GD[(x Pl ,y Pl ),(x 0 ,yo)] 


(3.10) 











35 


where Ok is the height of the observer above ground level and (xo, yo) is the coordi¬ 
nate of the observer projected into the xy-plane. In Figure 3.4 point po is at ground 
level directly beneath the observer, i.e., (xo, yo) = (**>> !/?*>)• Similarly, the slope 
Sqj from the observer to a target point A at height Ak above ground level would 


._ jfL(xA,yA) + Ak) - ( fL(xp,yo ) 4- Ok) 

°* GD[(xA,y A ),(xo,y o)] 


(3.11) 


Consider the set of all ground surface points from the observer out to a distance 
R along a fixed azimuth. Moving out from the observer, a given point on the ground 
will or will not be visible from the observer. It will not be visible if the slope to 
some point closer to the observer is greater than the slope to the given point. This 
implies that a point can only be visible if the slope to it from the observer is equal to 
or (more typically) greater than the slope from the observer to every ground point 
between it and the observer. It follows that any ground point further out from the 
given point can only be visible if the slope from the observer to that point is greater 
than or equal to the slope to the given point. Thus, any visible ground surface 
point P determines a controlling slope , equal to the slope of a line segment from the 
observer to P, such that if a line with that slope is projected out from the observer 
to a distance R then no point beyond P which falls below that line can be visible 
from O. 


In the context of Figure 3.4 the initial controlling slope is —oo, which is the 
slope from the observer 0 to point po- Moving further out (to the right in Figure 3.4), 
each subsequent ground surface point defines a new controlling slope until pi is 
reached. Point pi is visible from 0 so by definition the controlling slope S Pl at p\ 
is the slope of line segment 0p\. The ray pip? has slope Sn, so all surface points 
between pi and p? are not visible from 0, and is the controlling slope between 
Pi and pa. The surface points (xyt,yx) and (xb, J/b) beneath potential target points 
A and B are between pi and p 2 . Since $oa > point A is visible from O. 








36 


Similarly, because S$g <%T» point B is not visible from O. 

Since the entire surface line segment between pj and is visible, each suc¬ 
cessive point from pa to p$ defines a new (and progressively increasing) controlling 
slope. No points beyond P 3 in this example are visible, so remains the control¬ 
ling slope to the right of p$. Point C is beyond P 3 and r > so point C is 
visible from 0 . 

Although as abstractions the components of LOS and visibility determination 
appear straightforward, once we begin to address specific data models and realiz¬ 
able execution environments then correctness and efficiency considerations begin to 
make themselves felt. To some degree, common data models simplify the theoretical 
probl • restricting elevation values to integers of a known range. For most mod¬ 
els, however, the distance from the observer to various points along a LOS cannot 
always be represented by integer values. Referring to Figure 4.3, we can observe 
that although the ground distance between successive x-crossings or successive y- 
crossings along a given LOS is constant and therefore scalable to integer values, 
the ratio of all y-crossing to x-crossing interval distances is not. Slope values that 
are quotients of elevation differences and ground distances (regardless of whether 
in integer or real domains) are usually real valued, although when elevations and 
ground or grid distances can be represented as integers then slope values can, of 
course, be represented as the quotient of two integers. 

3.5 Summary 

This chapter has presented the problem setting taxonomy and elevation model 
that will be used to develop and validate the desired viewshed and visibility index 
algorithms, as well as to provide a specific basis to interpret the results of those 
algorithms. The general nature of LOS determination based on elevation profile 
data was also described. 





CHAPTER 4 

AREA VISIBILITY DETERMINATION—THE R2 ALGORITHM 


4.1 Overview 

The R3 algorithm provides a straightforward but relatively costly 0(n„ 3 ) 
method of determining for which points within a fixed radius R or an average radius 
Ran of some observer point p 0 a target at height h t would be visible to an observer 
at height h Q above p 0 . It operates by determining a LOS to each potentially visible 
point in S corresponding to a grid intersection point in M. Elevations are estimated 
as necessary to fulfill adequacy and appropriateness criteria by whatever means are 
assumed by the data model, such as linear interpolation (see Chapter 3). In this 
chapter we will develop and validate a suitable viewshed determination algorithm, 
R2, that executes in 8(n* 2 ) time. 



Figure 4.1: Octant Divisions. 


37 

















39 


To provide a frame of reference for discussing certain aspects of constructing 
practical algorithms for LOS determination, Figure 4.1 shows the numbering system 
to be used in referring to specific octants. The definition of an octant does not 
include points that are on the boundaries between octants, which fall on lines making 
angles of Arw/4 with the x-axis, where k is an integer. Chapter 3 presents some 
of the efficiency and correctness issues that can arise and must be addressed by 
algorithms employed to compute a single LOS between two points using the data 
model employed here. Figure 4.2 shows the views of just the first octant elevations 
for the same elevation data samplings shown in Figure 3.1 earlier. 

In the following discussion it is again assumed that the observer point is located 
within (not on the boundary of) a convex region of target points, which is often 
circular or elliptical. We consider the case of determining visibility from a single 
observer point, as opposed to a possibly related group of observer points. Under 
these conditions, in the absence of further information or assumptions about the 
terrain other than the gridded elevation data, it is not possible to determine visibility 
from the observer to the surrounding points in less than 0(n R 2 ) time because each 
of the surrounding points must be examined at least once and there are 0(n R 2 ) such 
points. 

4.2 Algorithm Considerations 

The R3 algorithm meets the Criterion for Adequacy and Criterion for Appro¬ 
priateness given in Section 1.4.1 but its cost of computation increases with the cube 
of the average number of points along a LOS. Figure 4.3 shows the operation of 
the R3 algorithm in the first octant. The algorithm evaluates a unique LOS from 
the observer location to each target location, as signified by the dashed arrows. 
In this case there axe 10 target points, with the observer located at the lower left 
comer. Also indicated are instances of an x-crossing point and a y-crossing point. 






40 



Figure 4.3: LOS Crossings of Grid Lines. 


An x-crossing point occurs when a LOS intersects with a grid line in M which is 
perpendicular to the x-axis, and y-crossing points occur when a LOS intersects a 
grid line in M which is perpendicular to the y-axis. The observer location is not 
considered a crossing point. 

In octants I, IV, V, and VIII there will be more x-crossing points than y- 
crossing points, with the reverse being true for the other four octants. Figure 4.3 
depicts 10 LOS’s (one for each target point), for which there are a total of 40 x- 
crossings and 20 y-crossings. In general, for a triangular first octant extending out 
x g x grid lines from the observer point, there will be n = * target points, 

+ 1) x-crossing points, and J^i" 1 *(*' + l)/2 y-crossing points. Note that 
the number of taxget points varies with x g 2 , that there are twice as many x-crossings 
as y-crossings, and the number of x-crossings and y-crossings varies with x g z . In 
octants I, IV, V, and VIII we observe that 0(n H ) ^ 0(x tf ) and the R3 algorithm 
must make an elevation determination at every x-crossing. By symmetry, a similar 
relation holds for n R and y g in octants II, III, VI, and VII. Therefore, the R3 

















41 


algorithm cannot run in less than 0(n* 3 ) 

It would be desirable to construct an algorithm which ran closer to the intu¬ 
itive limit of 0(n* 2 ) than R3 does. If an approximation more simple than linear 
interpolation is made for the elevation and LOS height for all crossing points on a 
given segment (or given half-segment), computation would be reduced, but only by 
a constant scale factor. This would produce less exact visibility determinations, but 
may be acceptable depending on the uncertainty of the elevation data and the in¬ 
tended application. Several obvious approximations would be minimum, maximum, 
or simple average of the elevations and LOS-heights. 

To achieve a running time faster than R3, an algorithm must either process 
LOS’s to fev/er target points (n* 2 for R3) or reduce the average cost of determining 
the LOS to a target point ( n K for R3). Figure 4.3 provides a visual clue to one as¬ 
pect of the R3 algorithm that is generating the extra factor of n R over our proposed 
minimum cost of computation. Especially on grid segments close to the observer, 
many elevation and LOS height determinations must be made for x-crossing points 
(other than grid intersections) which are only slightly apart. Note that the LOS 
height at a point, if accurately determined, completely represents all the necessary 
LOS information between that point and the observer that may be needed to de¬ 
termine the LOS height at points further out. A procedure to produce a suitable 
LOS height estimate in constant time for all LOS’s crossing a given line segment 
would eliminate the multiple determinations on a single segment occurring in the 
R3 algorithm, and provide the desired running time of 0(n R 2 ). 

This idea of consolidating elevation and LOS height information along a grid 
segment into an estimate represented at one or two points and using it as the basis 
for LOS determinations for crossing points on the next column (for the case of the 
first octant) of target points suggests the paradigm of an expanding wave front 
(see Figure 4.4). This has the apparent potential to improve running time over the 




42 



Figure 4.4: Progression of LOS Wave FYont. 

0(n R 3 ) of R3 by reducing the cost of computing the LOS height of a target point to a 
constant (instead of n R for R3). Several approaches have been developed to exploit 
this idea. [TD92] developed an algorithm, MB, which required 0(n R ) processors 
and 0(n R ) time. The fast LOS procedure for the Xdraw routine in [RFM92]) 
executes in a non-parallel environment in 0(n R 2 ). A similar implementation is 
described in [Bro80]. Unfortunately, all of these approaches suffer from what is 
referred to in [TD92] as chunk distortion. In the context used here, the Criterion for 
Appropriateness is violated because in the course of the progressive application of 
the approximation, elevation values that should have no influence on a given LOS 
propagate thei~ influence as shown in Figure 4.5. 

Consider first the triangle of points labeled a, b, and c. For points in the first 
octant, the angle 9 formed by the LOS and the x-axis must be between 0 and tt/4, 
so the LOS to a target point a must have an x-crossing on grid segment be that is 
not at b or c. Given the elevation data model being used, the elevation estimate of 
an x-crossing between b and c should be determined by the known elevation values 






Figure 4.5: Erroneous Propagation in Wave Front Algorithms. 

for b and c, so those points are proper contributing points in determining the LOS 
from the observer to a. 

Applying the criteria for appropriateness and adequacy from Section 1.4.1 
determines which points are proper contributing points in the determination of any 
given LOS. In Figure 4.5, the triangle encompassing a, b, and c indicates that there 
is a proper dependency of the current LOS on all three enclosed points. A shaded 
triangle indicates the existence of an improper dependency — that one or more of 
the enclosed points are contributing to the determination of the current LOS but 
should not be. 

With these terms in mind, the parallelogram region formed by the proper 
and improper dependency triangles encompassing a LOS from the observer to point 
7 illustrates the problem that arises from straightforward wave front algorithms. 
One of the values properly contributing to the LOS determination lor point 7 is 
the LOS height at point 6. This was calculated when the wave front algorithm was 


















































44 


determining results for points along the x grid line containing point 6, and one of the 
points influencing the value calculated for point 6 was point 5. Looking one column 
further back in the progression of the wave front reveals the improper influence of 
point 4 in determining the LOS for point 7. Point 4 properly contributes to the 
LOS determination for point 5, but by attempting to capitalize on point 5’s value 
(to avoid operations on values on grid lines closer to the observer and already passed 
by the wave front) to determine a LOS height value for point 6 and subsequently 
point 7, point 4 becomes an improper contributing point. 

As suggested by the arrangement of contributing points and dependency tri¬ 
angles, the number of proper contributing points npc will vary from one to two 
times the number of x grid lines crossed by the LOS, depending on 9. However, 
there are drastic fluctuations in the number of improper contributing points, n/c- 
Assume an observer at coordinate (0,0) and a LOS out to a grid point (x, y). Let 
8' — arctan For 0 -* 0 or 6 -* v/4, npc < 2x and n/c -V 0. For 0 < 8 < tt/4, 
npc < 2x but as 9 -* O' then n/c -> ^ — npc- In general, for 0 < 9 < tt/4 then 
n/c = 0(x 2 ) as x —» oo. This does not bode well for the wave front approach, since 
the ratio of improper contributing points to proper contributing points along a LOS 
with a given 8 will increase proportionally with x. 

Given this result, the likelihood that one or more relatively extreme elevation 
values (such as a hill top or ridge line) will occur in the set of improper contributing 
points increases as the LOS length increases, although its impact decreases with 
its distance from the observer. A single such extreme value could produce patently 
erroneous LOS height results for a large fraction of the total target points, raising 
serious questions on the usefulness of a direct wave front approach. 

Figure 4.6 shows the propagation of LOS-height error in an elevation dataset 
in which all elevation data values are zero except for the point (3,3), which has an 
elevation of one unit. The horizontal axis shows distance from the observer in x-axis 








45 


1 


Figure 4.6: Propagation of Maximum Chunk Distortion LOS-Height 
Error with Distance from the Observer. 

intervals, and the vertical axis shows the maximum LOS-height error (based on the 
criteria from Section 1.4.1) at that x-axis distance. Note that the maximum absolute 
error increases essentially in a linear fashion with distance from the observer. Errors 
introduced relatively close to the observer, even if small, can produce large changes 
in LOS-height at longer ranges. Since 75% of the points in a circular region around 
an observer will be found at the longer (greater than or equal to R/2) ranges, 
avoiding chunk distortion error is important for any accurate visibility algorithm. 

It is possible to modify the wave front approach to reduce or eliminate the 
influence of improper points. [TD92] developed a modified algorithm, PB, which 
runs in 0(n R ) time but requires 0(n* 2 ) processors and so leads to an 0(n R 3 ) pro¬ 
cedure for sequential machines. Modifying the Xdraw procedure in [RFM92] to 
incorporate careful control of the size and order of evaluation of subintervals of the 
wave front can eliminate either of the ‘wings’ of improper contributing points, but 
not both. 


3 

I 






46 


4.3 R2 Algorithm Development 

We turn now to Figure 4.7, another representation of selected aspects of the 
R3 algorithm in operation in octant I, for insights into developing a more efficient 
way of determining or adequately approximating visibility from an observer to all 
grid intersection target points within a convex region. Target points on x grid lines 
3, 4, and 5 are labeled a through t. The non-target point x-crossings of x grid lines 
3 and 4 by LOS’s to target points a through d are emphasized with small shaded 
square markers; x-crossings on lines 1 and 2 are not emphasized due to congestion. 


5 

4 

3 

2 

1 

0 

0 1 2 3 4 5 

Figure 4.7: Operation of the Basic R3 Algorithm. 



Since in the R3 algorithm the order in which LOS’s to target points are eval¬ 
uated is not important, let us presume that LOS’s to points a through d are de¬ 
termined before any other points in the first octant. As a LOS is run from the 
observer to each of points a through d, it encounters 4 x-crossings. Let fc, represent 





















47 


the x-crossing where a LOS from the observer to point k crosses x grid line t. While 
we know the visibility and LOS height at points a through d , we do not have any 
exact corresponding information about the first octant target points on x grid lines 
1 through 4 (such as points e through i). However, if during the operation of the 
algorithm we save the LOS height values at x-crossings, we do have potential esti¬ 
mates for the points e, /, and g in the form of a 4 , 64 , c 4 , and d 4 (the four shaded 
boxes on x grid line 4). This suggests a general approach of running LOS’s to the 
outermost points of the region and using their intermediate results as estimates for 
the visibility and LOS height information for interior target points. We achieve a 
faster running time in comparison with R3 by reducing the number of points to 
which complete LOS’s are calculated, instead of reducing the cost of running (or 
extending) a LOS to a specific point as in the wave front approach. 

By visual inspection we can see that, of the x-crossings available, a 4 is the 
best (in terms of spatial proximity) estimate for point e and d 4 is the best estimate 
for point g. While the best estimate for /, b 4 or c 4 , is not readily apparent to 
the unaided eye, it is an inexpensive determination to make during computation. 
Considering x grid line 3, we see that if we used R3 there would be 7 x-crossings, 
only the four emphasized crossings 03 , 63 , c 3 , and d 3 would be available here as 
estimates for points h and i because e 3 , / 3 , and g 3 would not have been computed. 
LOS’s would not be run from the observer to points e, /, g; their visibility and LOS 
height information would be estimated as described above. 

Note that as we work our way back towards the observer, the x-crossing points 
on LOS’s to the outermost points become more closely spaced and can generally pro¬ 
vide estimates closer to target points than at x grid lines further from the observer. 

An algorithm based on this idea of estimating interior point values from x- 
crossings of LOS’s to ‘boundary’ points would calculate a number of LOS’s propor¬ 
tional to 27rn H . The number of locations along each LOS at which operations (each 







48 


taking no more than constant time) would be performed would be proportioned to n R . 
Therefore, the expected running time of this method would be ©(27m* 2 ) = ©(n* 2 ). 
Since this is our targeted improvement over the ©(n* 3 ) time in the R3 algorithm, 
we refer to this approach for now as the initial R2 algorithm. 

To implement this algorithm it is necessary to define more formally what 
‘boundary’ points are. Initially, let B be the set of points on the boundary of a 
convex region containing an observer such that all points in B are target points 
(and therefore the distance between them and the observer is less than or equal to 
R) and they are 4-neighbor adjacent to at least one point that is not a target point 
(not in the convex region). An example of such points, labeled a through g, for the 
first octant is shown in Figure 4.8 



Figure 4.8: Coverage Problems in the Initial R2 Algorithm. 


The convex but non-circular outline of the region of target points in Figure 4.8 
illustrates two problems that can arise with the boundary definition for the initial 

























49 


R2 algorithm. A type 1 problem is illustrated by shaded region 1. Intuitively, we 
desire that any crossing point that is to provide a LOS height and visibility estimate 
for a target point be within one-half of a grid segment length from that point. The 
grid intersection point at the center of shaded region 1 has no x-crossing point 
that close. A type 2 problem is illustrated by shaded region 2. In programming 
an implementation of the R2 algorithm, it is possible to make the code simpler and 
more efficient when (again from the perspective of the first octant) it can be assumed 
that the grid segment beneath every target point contains at least one x-crossing. 
This is perhaps less serious than a type 1 problem, but it would be useful if any 
modification to the initial R2 algorithm to prevent occurrences of type 1 problems 
also eliminated type 2 problems. 

Both type 1 and type 2 problems derive from relatively wide angular separa¬ 
tions between adjacent LOS’s, which are in turn related to the distance between 
adjacent boundary points. These distances are greatest when 9 approaches 7t/4 
and the adjacent boundary points are not 4-neighbor adjacent (i.e., a line segment 
between them is a diagonal, not a grid segment). 

Figure 4.9 illustrates a modification to the R2 algorithm that increases total 
computation by no more than a factor of two and which prevents occurrences of type 
1 and type 2 problems. Here, the definition for B is changed so that all boundary 
points may be 8-neighbor (instead of 4-neighbor) adjacent to at least one point 
outside the region of target points. LOS’s running from the observer to points cq , 
do, e 0 , fo, and go have been added as a result of changing the definition of B. In 
particular, the LOS to do removes the type 1 problem and the LOS to f 0 removes 
the type 2 problem. The additional LOS rays also provide more spatially proximate 
estimates for many other interior points. 








































51 


4.4 Validity of R2 Algorithm 

The degree of agreement between the R2 and R3 algorithms can be evaluated 
in several ways, depending on the intended use of the visibility results. One direct 
way is to measure the difference in the LOS-height at each target point between the 
value calculated by R3 and the estimate provided by R2 and call this difference the 
R2 LOS-height error. 



Figure 4.10: Distribution of R2 LOS-Height Differences (meters). 

The degree of agreement between the R2 and R3 algorithms can be charac¬ 
terized by measuring the differences in the LOS-heights calculated by each method 
at each target point. A histogram of these LOS-height differences, computed using 
an observer-point with a high visibility index in DTED cell N37E127, is shown in 
Figure 4.10. In this figure the LOS-height produced by the R3 algorithm is taken 
as the reference height, with the R2 algorithm producing the estimated height. The 
difference between these two LOS-heights at a given point is the LOS-height error 
of the R2 algorithm. 

Figure 4.10 shows a histogram representation of the distribution of these errors 






52 


over a circular area of ground radius 24,500 meters in the southwest quadrant of 
N37E127, encompassing over 276,000 interior octant points. The mean difference 
between calculated LOS heights was less than one meter, with a standard deviation 
of less than four meters. The basic statistics of the LOS-height error for these points 
are given in Table 4.1. Further confirmation of the validity of using the results from 
the R2 algorithm as an accurate visibility measure is presented in Figure 4.12. 


LOS-height Error Statistics 

Mean error 

-0.82 

meters 

Min error 

-67 

meters 

Max error 

60 

meters 

Std deviation 

3.4 

meters 


Table 4.1: Properties of R2 LOS-Height Error. 

Both Figure 4.10 and Table 4.1 suggest a generally high degree of agreement 
between the LOS-heights produced by each algorithm. Further evidence of the 
utility of the estimates produced by the R2 algorithm is provided by considering a 
simpler question: is or is not a target at a given point visible from the observer, 
without regard to specific LOS-height? Table 4.2 shows for the same sample area 
mentioned above the total interior quadrant points considered, the number of points 
where the two algorithms agreed on whether or not the point was visible from the 
observer, and the total number of points predicted to be visible by each algorithm. 


Total 

Points 

Matching 

Points 

Visible 
by R2 

Visible 
by R3 

276,460 

273,144 

124,161 

122,627 


Table 4.2: Count of Visible Points by R2 and R3. 

The two algorithms agree on a point-by-point basis on over 98.8% of points, 
and on their respective counts of visible points they vary by less than 0.6% of the 
total points. In the visibility results over two test sites from 7 GIS systems presented 
in [Fis93], the reported number of visible points within the potential viewsheds 














53 


varied from 1780 points to 2610 points for one site and from 1465 points to 2270 
points for the other. The standard deviation of the reported number of visible points 
was 322.4 for the first site and 363.9 for the second. Compared with the reported 
variation in results from established GIS systems, the correspondence between R2 
and R3 is excellent. 

The preceding analysis was conducted using a version of the R2 algorithm that 
linearly interpolates elevations between 4-neighbor adjacent grid points as described 
in the model of the elevation data in Chapter 3. Some constant time savings can 
be achieved by employing other methods of estimating such elevation values, such 
as taking the maximum, minimum, or simple average of the two known elevations 
at the end of the pertinent grid segment. 



Figure 4.11: Comparing 4 Variants of R2 


The distribution of LOS-height errors resulting from these and from linear 
interpolation are shown in Figure 4.11. While all produce good agreement with 
R3, the original linear interpolation method provides the best match in terms of 
minimizing the average and standard deviation of the absolute error. The cost for 






54 


its additional floating point interpolation operations is minimal compared to the 
basic cost of the operation of the algorithm, especially on a superscalar or other 
pipelined architecture. 

A more systematic evaluation of the consistency of the agreement between the 
visibility fractions calculated by R3 and R2 was conducted by selecting 1041 points 
spatially distributed across the DTED cell N37E127. Figure 4.12 shows the high 
degree of agreement between the R2 and R3 results across the spectrum of visibility 
values for the points sampled. Each potential viewshed consists of 740,843 points. 
Approximately 25 days were required to compute the R3 values; R2 values were 
computed in less than 8 hours. 



Figure 4.12: Agreement of R2 and R3 Values for 1041 Points in 
N37E127 


Note that the form of the equation fitted to the observations is y = a + bx, 
with a very high 0.999776 degree of correlated variation between the R2 and R3 
visibility fractions. The y-intercept value of 0.00002945 is very close to the expected 
value of 0, and the proportional term of 0.99324 is very close to the expected value 
of 1. 













55 


4.5 Initial R2 Execution Time Results 

The entire purpose of devising the R2 algorithm is to create a procedure with 
acceptable, if approximate, results with a running time of 0(n* 2 ) instead of the 
0(n H 3 ) time for R3. Although it is clear from the design of R2 that it should 
achieve this goal, actual execution time information is in order for confirmation 
purposes. 


Radius 

Number of 

R2 Time 

R3 Time 

(meters) 

Points 

(seconds) 

(seconds) 

24500 

278K 

18 

896 

12500 

70K 

4-5 

131 


Table 4.3: Sample Execution Times of R2 and R3. 

Table 4.3 illustrates the variation of execution time with radius of interest 
for the same region in N37E127 used for Figures 4.10 and 4.11. As the radius of 
visibility increases by a factor of 1.96, the execution time of R2 increases by about 
a factor of 4. The time of R3 increases by close to a factor of 6.8, which is close 
to the factor of 7.5 predicted by a cube relation. Variations between expected and 
actual timings for different values of R are explained in part by the fact that the 
cost of operations at each crossing point will be different for visible and non-visible 
points. Visible points may require updates to the determining LOS data, whereas 
non-visible points do not. Since the ratio of visible to non-visible points for areas 
with different values of R will not in general be identical, their relative per-point 
execution times will vary within a bounded constant factor from direct quadratic 
or cubic relations. This is in addition to small variations due to virtual memory 
operations, caching patterns, etc., on the host workstation. 

It is worth noting that even for relatively modest distances, Table 4.3 shows 
the R2 algorithm is already between one and two orders of magnitude faster than 



56 


R3 (similar program coding techniques having been employed in implementing each 
method). The performance advantage of R2 over R3 can be expected to increase 
linearly with the increase in the radius of visibility. An interesting speculation at 
this point is that the radius of visibility (for this region) at which R3 becomes more 
efficient than R2, due to the internal bookkeeping and boundary sequence control 
required in R2, may be at about 3000 meters, which is between 35 and 45 grid 
points for the sample dataset, depending on azimuthal direction. More detailed 
information on R2 execution time performance is presented in Chapter 7. 

4.6 Summary 

Have our goals for the R2 algorithm been achieved? The modified R2 algo¬ 
rithm has run time 0(n R 2 ), and is therefore within a constant factor of optimality 
as discussed above. It provides exact results only for points in B, points on the 
boundary lines between octants, and for the relatively few interior points where a 
LOS to a point in B crosses a grid intersection point. For the majority of points for 
which only estimates are provided (in the form of exact results from spatially close 
crossing points), values for target points closer to the observer will on average have 
close? pproximating points than target points further out towards the boundary. 
Figure 4.12 provides further evidence of the accuracy of R2 based on a large number 
of test points. 

In a narrow sense the criteria for appropriateness and adequacy will not be 
satisfied in all cases, because the LOS from the observer to the estimating crossing 
point will be ‘near’ but not identical to a LOS from the observer to the target point. 
In a broader sense, however, these criteria are satisfied in that the problems with 
wave front algorithms illustrated in Figure 4.5 — the undue influence of extreme 
elevation points over large areas — have been eliminated. 

Depending on the use intended for the results of the R2 algorithm, it may 




Figure 4.13: Alternative Tessellation of R2 Observations 



Figure 4.14: De-Cluttered Alternative Tessellation from R3 











58 

be appropriate to view its operation as generating an alternative tessellation to 
one that corresponds to a normal rectangular grid. Figure 4.13 shows the pattern 
generated by the x-crossings of the modified R2 algorithm. For points in B, on the 
boundary between octants I and II, and certain interior locations, the points in this 
alternative tessellation correspond to those of the original elevation grid. Figure 4.14 
shows the same points with the LOS lines removed for greater clarity. The increasing 
divergence between the elevation data tessellation and the R2 tessellation can be 
noted as distance from the observer increases. 

Given that the original layout of elevation values is only one of many choices of 
patterns to represent the underlying terrain, it may be valid to consider the points 
produced by the R2 algorithm as an alternative pattern for representative visibility 
values, which closely follows but is not identical to the pattern of the elevation 
data. In this sense, the pattern produced by the R2 algorithm are not estimates for 
points in the elevation data pattern, but simply an alternative spatial sampling. For 
latitudes where the inequality of north-south and east-west ground spacing among 
elevation sample points in DTED data results in a pronounced rectangular (non¬ 
square) grid, the variations in the R2 pattern from the original grid may on average 
provide a more uniform sampling of the terrain. 

Tradeoffs between computational cost and fidelity to the original grid can be 
made by extending the value of R used in determining the boundary points by some 
constant scale factor C; for example, let R! = CR. In determining boundary points 
we follow a boundary that is a distance R! from the observer, but when actually 
running a LOS we only extend it to points within distance R from the observer. 
The cost of execution is now &((2nn R >R) = 0(27r Cn R ) = 0(n fl J ), which is within 
constant factor C of R2 when R! — R. 

A further generalization of R2 would be to employ an angular separation 
between adjacent LOS’s that is not determined by ending each LOS on the exact 









59 


location of a boundary point. Other criteria could be used, such as minimizing the 
number of LOS’s to avoid type 1 and type 2 errors, or providing the most uniform 
coverage (best match to the original grid) for a given number of LOS’s. Another 
question that could be investigated is determining the fraction of interior points for 
which R2 computes an exact LOS height as a function of R and convex region shape. 






CHAPTER 5 

DEVELOPMENT OF LOS RAY ESTIMATOR ALGORITHMS 


In this chapter we will propose several visibility index determination algorithms, 
all based on sampling potentia’ visibility from an observer by constructing a set of 
equally spaced LOS rays from an observer. The computational cost for all of these 
algorithms is 0(n*), compared with 0(n* 3 ) for R3 and 0(n* 2 ) for R2. 

The analysis of visibility information for a significant real world problem area 
(in excess of 23 million observer points representing approximately 150, OOOfcm 2 of 
terrain on the Korean Peninsula) was conducted to develop, refine, and validate 
visibility index estimation algorithms. Elevation data was adjusted according to 
Equation 3.1 prior to conducting visibility calculations. Selected terrain regions have 
been analyzed to examine changes in visibility estimates produced by variations in 
approximation parameters such as ray density and point-distance weighting. 

5.1 VL Algorithm 

The basic VL (Visibilty-LOS) algorithm to calculate an estimated visibility 
index from a specific observer location is shown below. It borrows from the Object- 
Oriented notation from the C++ programming language and some additional syntax 
from the Ada programming language. The fundamental object, grid, contains the 
adjusted elevation raster and various descriptive and visibility state information 
accessed and manipulated by method routines. 

The method Define_EI«vatioflJ > rofil« creates a one dimensional vector of elevation 
values for each location on the grid where the LOS ray crosses an x grid line in 
octants I, IV, V, and VIII and where it crosses a y grid line in octants II, III, VI, 
and VII. Where the LOS ray crosses a grid line segment that is not on a grid point, 
linear interpolation between the endpoint values of the segment crossed provides the 


60 








61 


# Basic VL algorithm. 

Ray.Gridpoints := 0; 

VkiMt-Gridooiats ■= 0* 

FOR i IN 1 ... Numb*r_of.LOS-Ray* 

grid.DefiM.Ekvatioa.Pro6le(ObMrver,LOS.Ray-Eitdpoint[i]); 
Ray.GridpoMts += grid. Valuesj*_Profilc; 
grid.CalculateJ.OS.M_Pro(iJe(); 

Vtsibte-GridpoinU += grid.Visibte-Profile-Pointe; 
Viewthad.Fractioft.V'ttjble := Vtsible_Gndpoints / Ray.Gridpoints; 


elevation value used. The method Cakulate-LOS.on-Profile takes the vector of elevation 
values which represents the one dimensional visibility profile and determines the 
visibility of each point in the profile. 


# Calculate_LOS_on_Profile method. 

Controlling-Slope := — oo; 

Points.Visible := 1; 

FOR i IN 1 ... Number .of_Profile_PoinU 

Current.Slope := (Profile.Elevation[i] + Target-Height - Observer-Elevation) / i; 
IF Current-Slope > Controlling-Slope THEN 

This .Slope := (Profile.Elevation[i] - Observer-Elevation) / i; 

IF This-Slope > Controlling-Slope THEN 
Controlling-Slope := This-Slope; 

Points.Visible += 1; 


Note that the counter of the number of points visible from the observer is 
initialized to 1 instead of 0 because the observer point is always visible from itself. 

5.2 Investigating Visibility Index Estimates 

An estimate of the visibility index for each point in an area of interest can 
be computed by constructing some number nyj of equally spaced LOS’s from each 
observer out to the range of interest and summing the number of points visible 






62 


from the observer along each LOS. This sum can be scaled to make best use of the 
representation being used to store the result; for a 16-bit signed integer, the result 
could be scaled to the range 0-32767. Evidence that using LOS samples from each 
observer point can produce a consistent visibility index estimate was gathered by 
performing the same analysis using 32, 64, 128, and 256 LOS’s from each observer 
point, using the VL algorithm. The results are shown graphically in Figure 5.1 by 
a sampling of 256 points from the terrain represented by cell N37E127, comparing 
results obtained using 32 rays and 128 rays. 


■p 

c 

•H 

o 

CL) 

d 

o 

id 

u 


M 

b 

co 

(0 

os 

00 

CM 


O' 

c 

•H 

co 

D 



2K 4K 6K 8K 

Using 32 Rays From Each Point 


Figure 5.1: 40-kilometer Visibility Index Values, 32 and 128 LOS 
Rays, N37E127. 


Each point in the figure represents the visibility index values of the same 
location on the ground as estimated using 32 and 128 rays. The evident near-linear 
relationship of results obtained from each ray density suggests that using 32 rays 
produces estimates very similar to those achieved using four times as many rays; 









63 


even fewer rays may be appropriate for this particular cell. Also evident is the 
relatively small number of points with visibility values at the high end of the total 
range. 

A gray-scale view of elevation in the northwest section of the DTED Level I 
cell N37E127 is shown in figure 5.2. This same area is shown in figure 5.3, with 
lighter levels of gray representing locations of high visibility. Initially, more than 
50 days of execution time on a Sun IPC workstation were required to produce the 
visibility image for a DTED Level I cell. Subsequent refinements (see Section 5.4) 
reduced the time required to between 4 and 5 days. Parallel processing of separate 
terrain areas on networked workstations reduced elapsed time by a factor essentially 
linear in the number of workstations used. 

5.3 Hypotheses and Characteristics 

Consider the possible distributions of visibility values. If a very small number 
of points occur that have visibility indexes in the high range of visibility values for the 
entire AOI, this would significantly reduce the number of locations to be examined by 
specific site selection algorithms or interactive on-line analyses. Analysis conducted 
on over a dozen terrain cells suggests that such distributions are common on real 
terrain. 

The density histogram of points having particular LOS values for the visibility 
indexes computed for N37E127 is shown in figure 5.4. The near linear decline in the 
log of the number of points as LOS value increases suggests a negative exponential 
relationship. For terrain where this or similar relationships occur, we can expect to 
find relatively few points with high LOS values. 

When spatial relationships among data points am ignored, an examination 
of corresponding elevation and visibility points in N37E127 reveals the lack of any 
obvious correlation between elevation and visibility values. Although the elevation 









64 



Figure 5.2: Gray-Scale Elevation Image of DTED Cell N37E127. 



Figure 5.3: Gray-Scale Visibility Image from DTED Cell N37E127. 













Figure 5.4: Log Distribution of LOS Index Values from DTED Cell 
N37E127. 


values of data points have significant spatial correlations, the elevation of a given 
location is by itself only slightly correlated to its visibility index estimate. A sample 
of 256 points in figure 5.5 shows this graphically. Chapter 6 provides a more detailed 
examination of possible relationships between elevation and visibility. 

Because it can be important in siting problems to identify a limited number of 
candidate sites with the best visibility (largest numbers of potential viewshed points 
visible), a legitimate question is whether some correlation exists between elevation 
and visibility for the best candidate sites, even if correlation is low overall. Evidence 
to date suggests that this is not the case. The pattern of elevation and visibility 
for the 110 points in N37E127 with the best VL visibility is shown in Figure 5.6. A 
more detailed presentation of the low correlation between the elevation and visibility 
index value at given points is provided in Chapter 6. 





66 



Figure 5.5: 


0 200 400 

Elevation 


Point-Matched Elevation and 
DTED Cell N37E127. 


600 


Visibility Values from 



0 200 400 600 800 1000 1200 1400 

Elevation 


Figure 5.6: Elevation and Visibility, 110 Best VL Visibility Points, 
N37E127. 





67 


5.4 Pseudo-Fixed Point Optimizations 

Although many programming languages lack built-in support for fixed point 
operations, careful partitioning of relative long (32-bit and greater) native integer 
representations can achieve the same effect, as long as attention is paid to the pre¬ 
cision required by the problem and the number of times values must be converted 
to and from the pseudo-fixed point format. For example, since the maximum dif¬ 
ference in elevation between any observer and target elevations on earth are known 
to be less than 16384 meters, only 13 bits in a 32-bit integer are needed to repre¬ 
sent any such difference value. During visibility calculations where the divisor is an 
integer grid count, this allows a relatively accurate (18-bit fractional component of 
the quotient) pseudo-fixed point division to take place by shifting such an elevation 
difference value left (a computationally inexpensive operation on typical hardware) 
by 18 bits and performing a standard integer division. Use of such techniques to 
eliminate most floating point operations during the execution of the VL algorithm 
improved execution time by approximately a factor of 4 on Sun IPC hardware. 

5.5 WeightF Algorithm 

Real terrain manifests a significant degree of spatial correlation. We do not 
observe a mountain peak within one grid point of a valley bottom in normal elevation 
datasets. If we hypothesize that this correlation extends significantly to visibility 
subregions, then we could add to the accuracy of the visibility index estimates 
produced by the VL algorithm by using the visibility value for each point on a LOS 
ray to infer the visibility characteristics about the unsampled points in its vicinity. 
A straightforward way of doing this is to weight each point on a LOS ray by the 
percentage of the area within the total potential viewshed that is closer to it than 
to any other point on any LOS ray. This percentage is proportional to the distance 
of the point from the observer, which serves as the basis for modifying the VL 




68 


algorithm to produce the WeightF (Weighting-Full) algorithm. 


# Weight-F algorithm. 

Viaw«liidJTactio«_VisiMe := 0; 

FOR i IN 1... Hiwnb er.ofJ.OS.Rays 

grid.Deine.EIevationJHofilef Observer,lOS-Ray.Eiidpoiat[i]); 
grid.Cai calaU.Wo i gl i to d_LOS-oa_ProWe(); 
Viewshed-Fraction. Visible += 

grid.Weighted-Points. Visible / grid.LOS.Ray.Arej; 
Viewshed-Fractioa-Visible /= Number.of_LOS_Rays; 


The individual LOS profile visibility calculation is modified to weight the count 
of visible points by the relative distance from the observer for each visible point. It 
also tracks a weighted count of all visible points which corresponds to the total area 
of the potential viewshed. 


# Modified LOS profile visibility calculation. 

ControlliiigJSIope := — oo; 

Weighted-Points.'Visible := 0; 

LOS.Ray.Area := 0; 

FOR i IN 1 ... Number-of-Profile.Points 
LOS.Ray.Area += i; 

CurrenLSIope := (Profile.Elevation[i] + Target.Height - Observer-Elevation) / i; 
IF Current.Slope > ControllingJSIope THEN 

THis^lope := (Profile.Elevation[i] • Observer-Elevation) / i; 

IF This_Sk>pe > Controlling-Slope THEN 
Controlling^lope := This-Slope; 

Weighted.Points. Visible += i; 


5.6 Weight Algorithm 

The standard implementation of the WeightF algorithm requires the accumula¬ 
tion of fractional contributions or other floating point representations to determine 



69 


the sum of contributions weighted by their true distance from the observer. An 
integer-only approximate weighting is employed in the Weight algorithm that uses 
the grid segment distance along the most rapidly varying axis as a proxy for distance 
from the observer. 

# Weight algorithm. 

Sum.Weightad.PoMU-Visible := 0; 

Sum_Weighted_Total_Area := 0; 

FOR i IN 1 ... Number_ofJ.OS.Rays 

grid.D«fio«_Elev»t ion _Profik(Observer,LOS_Ray_Endpoint[i]); 
grid.Calculate.Weighted_LOS_oa_Profile(); 

Sum.Weighted.PoinU.Visibte += grid.Weighted.PoinU.Visible; 
Sum_Weighted_ToUl_Arej += grid. LOS.Ray .Area; 

Vi e wslied.Fractkxi-Visible ;= Sum.Weighted.PoinU.Visibie / Sum.Weighted_ToUl.Area; 


This saves one floating point divide operation for each LOS ray computation 
and replaces a floating point addition with two integer additions. The impact of the 
resultant distortion in the weighting of points in LOS rays (off of the horizontal and 
vertical axes) is shown in Chapter 6. 

5.7 Visibility Images 

The VL, WeightF, and Weight algorithms have computational requirements 
that are low enough to allow the calculation of visibility index estimates over sub¬ 
stantial viewsheds for large numbers of observer points. This offers the potential to 
create visibility images providing full AOI coverage that can be used for interactive 
exploration and analysis. 

5.7.1 Magnitude-Only Visibility Images 

By mapping visibility index values to intensity values for gray-scale display 
screens or even black-and-white hardcopy media with adequately fine dithering, a 



70 


visibility image of an area analogous to an elevation gray-scale image can be ren¬ 
dered (for example, see Figure 5.3). The ability to produce such images in quantity 
(i.e., over many areas and using many combinations of LOS parameters) offers a new 
way of displaying large amounts of visibility information in an intuitive fashion. The 
military applications of magnitude-only visibility images are obvious ([Ray94]), but 
many users of GIS capabilities may also find such visual, non-numerical representa¬ 
tions appealing and useful. 

5.7.2 Directional Visibility Images 

Color displays have been used as an alternative to gray-scale representations for 
the purpose of depicting raster intensity values, such as elevation or visibility indexes. 
An alternative use of color capabilities is to map hues to directions, analogous to 
a color wheel. While this method does suffer from some limitations (such as the 
many-to-one mapping of visibility distributions to specific hues), it adds another 
way of visualizing visibility relationships in a region [FR94b]. 

5.8 Parallelism Considerations 

Characteristics of many visibility algorithms are well suited to parallel execu¬ 
tion at several degrees of granularity. During the conduct of this research, practical 
experience with parallelism on several levels was gathered. Even finer grained uses 
can be employed without significant changes to the basic algorithms. Operation of 
any of the VL-based LOS ray algorithms can be executed for the same observer by 
dividing the n total rays over m available processors (assuming m < n), although 
the efficiency of such a scheme would be heavily dependent on memory organization 
and capacity. Even an interactive use of the R2 algorithm could easily parallelize 
over 8 processors by assigning one octant per processor. 


J 





71 


• WAN-Connected Stations. Workstations geographically dispersed (at Troy, 
New York and at West Point, New York) executed visibility analysis proce¬ 
dures on locally stored copies of elevation and output datasets. Summary 
results and visibility datasets were collected via the Internet. One disad¬ 
vantage was the need for separate copies of input datasets. The advantages 
included reliability (both sites were down intermittently but never at the same 
time), ease of parallelization, and lack of mutual interference. Monitoring was 
straightforward via multiple windowed telnet sessions and Network File Sys¬ 
tem (NFS) file status monitoring and distribution. To fully exploit the overall 
reliability of geographically distributed computation, robust and automatic 
checkpoint and restart facilities were built into the software implementing the 
visibility algorithms. 

• LAN-Connected Stations. At each site, multiple workstations shared common 
input datasets via LAN-connected shared file systems. A disadvantage was 
that outage of key file servers could affect ail running analyses since the file 
system was not redundant. The ability to maintain single copies of input 
and intermediate files was an advantage. Problem sets as small as 30,000 
observer points were divided across physical workstations, with control and 
results consolidated through shared directories. 

• Multi-Processor Stations. Individual workstations, such as the Sun 670-MP 
with 2 to 4 physical processors each, were used to run visibility analysis on 
processors while leaving adequate processors free to service interactive loads. 
Although not exploited, physical memory and virtual memory demands could 
have been reduced through the use of shared memory . egments. 






72 


5.9 Summary 

In this chapter we have proposed three 0(n R ) visibility index estimation algo¬ 
rithms: VL, WeightF, and Weight. Investigation on real terrain showed consistent 
visibility estimates can be produced by the common underlying approach of sam¬ 
pling visibility by calculating a set of equally spaced LOS rays from an observer. 
The results of the visibility index analysis on a sample elevation dataset suggested 
that observer points with relatively high visibility index values may be a very small 
part of the total set of observer locations. Programming optimizations and use of 
parallel execution to speed up visibility analyses was also described. 








CHAPTER 6 

ACCURACY OF VISIBILITY ESTIMATES 


This chapter investigates the critical issue of how accurately various 0(R) visibility 
algorithms based on determining the visibility along a fixed number of LOS rays can 
estimate the true viewshed fraction from an observer. The R2 algorithm is used as 
an accurate reference value of true viewshed fraction because it requires significantly 
less computation than R3. 

6.1 Overview of Methodology 

Unless otherwise stated, all visibility calculations in the following examples 
were calculated usi"" a 40 km radius, a target height of 25 meters, an observer 
height of 5 meters, and were taken from the DTED Level I one-degree cell with a 
southwest corner located at 37 degrees north latitude, 127 degrees east longitude 
(N37E127). This cell contains elevation values ranging from 0 to 1462 meters above 
mean sea level. Mean elevation is 197 meters, median elevation is 152 meters, and the 
standard deviation of elevation values is 161.0 meters. Visibility measures are stated 
as the fraction of points in the potential viewshed at which a particular algorithm 
determined or estimated a target would be visible from the observer location. 

The algorithms employed include: 

• R2 The circumference sweep point-by-point visibility estimation algorithm 
described in Chapter 4. 

• VL The initial visibility estimation algorithm employing a specified number 
of LOS rays without weighting individual points described in Chapter 5. 

• WeightF A visibility algorithm employing a specified number of rays that 

uses an accurate area weighted technique, described in Chapter 5. 


73 





74 


• Weight A visibility algorithm employing a specified number of rays that uses 
an approximate point weighting technique, described in Chapter 5. 

A commercial curve-fitting and analysis software package was used in selecting 
and evaluating possible equations and parameters that could best describe the ob¬ 
servations. The equations selected for display in the following figures were selected 
based on best correlated variation, strongest relationships suggested by F-statistics, 
elimination of spurious fits (step functions, sinusoidal functions, peak functions, 
etc.) on observations that lacked any strong fit, preference for simpler functions to 
more complex ones when the degree of fit provided was essentially the same, and 
elimination of equations that showed nonsensical behavior immediately outside the 
region for which observations had been collected. The equations relating visibility 
measures, or elevation and visibility, that were most often selected include: 

• y = a + bx 

• y = a + bx 2 

• y = a + bx 3 

• y = a + 6 log x 

It is of interest to note that using the criteria mentioned above, equations with 
multiple x terms such as y = a + bx + cx 2 were generally not preferred to equations 
with only a single x term, such as y = a+bx 2 . This may have occurred in many cases 
because equations with quadratic and higher x terms were selected to fit observations 
where all curve fits were weak and no underlying lower-order trend justified inclusion 
of other terms. 

For many visibility and elevation samples used to analyze the various algo¬ 
rithms, a standard set of 1024 pseudorandomly distributed point coordinates was 
selected. The resulting pattern is shown in Figure 6.1. The x and y coordinate for 






75 



Figure 6.1: Spatial Distribution of 1024 Pseudorandom Points. 


each point were drawn from separate pseudorandom number generators provided in 
the class libraries furnished with the Free Software Foundation’s G++ compiler. Dif¬ 
ferent seeds were used to start each generator. The generators permute the output 
of a Linear Congruential method [Knu81] with a Fibonacci Additive Congruential 
generator to obtain improved independence between sequential samples. 

The R2 algorithm will be used as a reliable and computationally efficient 
reference to evaluate the accuracy of the visibility fraction estimates produced by 
other algorithms. Evidence of the agreement between the results from the R2 and 
R3 algorithms was presented in Chapter 4. 

6.2 Performance of the VL Algorithm 

Figure 6.2 suggests that the VL algorithm has substantial capability to predict 
visibility (as measured by R2 visibility values on the Y axis). The correlated varia¬ 
tion of 0.776 is evidence of a strong relationship, and the fitted equation y = a + bx 2 
is free from fitting artifacts and bizarre behavior at the extremes of the range of 










76 


the independent variable (the estimated visibility fraction calculated by the VL 
algorithm). 



Figure 6.2: N37E127 - VL and R2 Visibility for 1024 Random Points. 


Note that the estimates produced by the VL algorithm for most points sub¬ 
stantially overestimate the visibility fraction calculated by R2. For example, points 
with a VL value of around 0.15 have R2 values clustered close to 0.05. It would 
be expected that points with very high actual visibility (e.g., centered on an open 
plain) would be accurately estimated by VL. We can observe that this hypothesis 
is supported in Figure 6.2 because points of very high VL estimated visibility (the 
right side of the graph) are as a percentage less overestimated than points near the 
center of the graph. 

We might infer that relatively few points would be found beyond the VL values 
at which the fitted equation intersects y = x. This is because at and beyond that 
point the high degree of agreement between VL and R2 might indicate a boundary 
condition for the relation, just as a high degree of agreement occurs at the boundary 
of lowest visibility near (0,0). For the fitted equation in Figure 6.2 the intersection 





77 


between it and the y = x line would occur for a visibility fraction of about 0.4455 
for VL and R2, and in fact in this figure and in Figure 6.4 (which focuses on high 
visibility points) very few points are found near or beyond that value. 

The relative uncertainty in visibility associated with the VL estimated value 
decreases as the estimated visibility increases. This suggests that the VL algorithm, 
in spite of its poorer performance at lower values, might still be suitable for identi¬ 
fying the locations of highest visibility in an area. Sets of the 256 and 100 highest 
VL estimated visibility points will be used below to more closely examine visibility 
relationships present in high visibility points. 

By examining the residual error after fitting y = a -f bx 2 to the points in Fig¬ 
ure 6.2, we can gain a better understanding of the performance of the VL algorithm 
as a predictor of visibility over a range of visibility fraction values. 


N37E127 -1024 Random Points 


500 


pm 







4 


y 

IVVv 

9 * * * 

• 



* 500 
£ 

T 








a ' 10O °' 
** 

U- 

• 

• 








-2000. 










) 

0.1 

6.2 

VL Fraction 

0.3 0 


Figure 6.3: N37E127 - Fit Residuals, VL and R2 Visibility, for 1024 
Random Points. 


Figure 6.3 shows the percentage residual error after the curve fit to the VL and 
R2 values from Figure 6.2. Ignoring the large error percentages encountered for VL 
values close to 0 as manifestations of the mall number problem, it can be observed 









78 


that using the estimates employed by VL in conjunction with a simple adjustment 
calculation (i.e., the fitted equation from Figure 6.3) produces estimates of visibility 
fractions whose relative error decreases as the visibility fraction increases. This is 
further evidence of the potential utility of the VL algorithm in finding points with 
the highest visibility. 

Figure 6.4 suggests that for points of genuine high visibility, the VL algorithm 
provides better estimates (based on percent residual error) than for points of mod¬ 
erate to low visibility. Only points with the highest 256 VL estimated visibility 
values out of the total 1.44 million (1201 2 ) points in the N37E127 cell are shown. 
Note that only the low computational cost of the VL algorithm (compared to the 
R2 or R3 algorithm) makes is practical to compute visibility estimates for every 
point in the cell, consuming approximately four days of computing time on a Sun 
IPC workstation. 

N37E127 - Top 256 VL Pointt 
y-a+ta 

•~0.M2t0N2 

b-oomcooi 



VL Fraction 


Figure 6.4: N37E127 - VL and R2 Visibility for Top 256 VL Points. 


Comparing the range of actual R2 visibility values in Figure 6.4 with those 
in Figure 6.2 demonstrates that points of high visibility have in fact been selected 





















by choosing points with the highest VL visibility estimates. For points with VL 
estimated visibility values above 0.4 there are no R2 values less than 0.2, and in all 
of the 1024 pseudorandomly selected points in Figure 6.2 only one point had an R2 
visibility in excess of 0.2. 

The layout of points in Figure 6.4 again demonstrates that the VL algorithm 
for many points underestimates the visibility fraction, although with decreasing 
severity as the visibility values increase. For VL values above 0.48 in this set of 
observations, underestimation seems to have disappeared or been subsumed in more 
random error from the straight linear regression of R2 on VL. 

The ability to accurately predict which points will have high visibility values 
shows the suitability of the VL algorithm for many purposes that focus on points 
with the ‘best’ visibility. 


6.3 Relationships to Elevation 



Figure 6.5: N37E127 - Elevation and R2 Visibility for 1024 Random 
Points. 









80 


Appendix C provides a detailed investigation of elevation and visibility rela¬ 
tionships in different quadrants of the cell N37E127. To show an overview here of 
the relationships between elevation and visibility, a set of observations of elevation 
and R2 visibility fractions is shown in Figure 6.5. It plots, for the set of 1024 pseu- 
dorandomly selected points shown in Figure 6.1, the elevation in meters against 
the fraction of their total potential viewshed which was found visible by the R2 
algorithm. The fitted equation, y = a + bx 2 , might suggest a trend to a moder¬ 
ate increase in average visibility as the elevation of points increases. However, the 
scattering of points, low correlated variation (0.048), and presence of the highest 
visibility points in the middle of the elevation range argue that there is no basis for 
inferring a genuine, direct relationship between elevation and visibility. 

Figure 6.6 makes an initial use of the top 256 VL valued points from the 
N37E127 cell to examine possible relationships between elevation and visibility 
among points with high visibility values. 

N37E127 - Top 256 VL Point* 

y-*+t>(lnx) 

r*-0 45SS7S11« 



Elevation (meters) 


Figure 6.6: N37E127 - Elevation and R2 Visibility for Top 256 VL 
Points. 


A moderate relationship (r 2 = 0.4556) exists between elevation and visibility 














81 


using the equation y = a + 6 log z for this set of observations. However, recall that 
these points were not selected using their raw elevation values; they were selected 
using their estimated VL visibility fraction. What is shown in Figure 6.6 is a cor¬ 
relation between visibility fraction and elevation in a set of points known to have 
relatively high visibility. Even this correlation is not necessarily very useful. It only 
suggests a limited increase of visibility with elevation and provides no direct clues 
on how to use elevation values by themselves to select the highest visibility points, 
which in fact occur near the middle of the elevation range (around 550 meters). 



Figure 6.7: N37E127 - Elevation and VL Visibility for Top VL 256 
Points. 

Figure 6.7 shows the virtually nonexistent relationship between elevation and 
the higher visibility estimates produced by the VL algorithm, with an extremely 
low correlated variation of 0.0096 using a fitting equation of the form y = a + 
b\ogx. The virtual lack of correlation confirms the impression gathered by visual 
inspection, which is that raw elevation is not a suitable predictor of the visibility 
values estimated by the VL algorithm. As previously shown in Figure 6.4, in the high 
visibility ranges VL is a good predictor of the accurate visibility values produced by 








82 


R2. 


N37E127 - Top 100 VL Patna 




Figure 6.8: N37E127 - Elevation and VL Visibility for Top VL 100 
Points. 


Figure 6.8 demonstrates that when restricted to an even more select set of 
the highest 100 estimated visibility points, there continues to be no substantial 
relationship between elevation and visibility. The actual correlation value, 0.0235, 
has increased slightly but remains quite small, and the form of the fitted equation 
has changed to y = a + bx 3 . Note that the second derivative has changed from 
negative in Figure 6.7 to positive in Figure 6.8, suggesting further that even the 
best available curves represent spurious fits to data on measures (elevation and 
visibility) possessing essentially no direct relationship. 

Whether over the general range of elevations and visibility values, or when 
focusing specifically on the best prospective points for high visibility, individual 
elevation values provide no consistent predictive ability for visibility. This observa¬ 
tion holds true even when the points being sampled have been restricted to those 
predicted by a valid estimation algorithm to have high visibility. 










83 


6.4 Improvements to VL Estimation Algorithm 

As described in Chapter 5, with a relatively modest constant-time increase in 
the computational requirements of the VL algorithm we can attempt to make use 
of the spatial correlation of real terrain by weighting each point on a LOS ray in 
proportion to the area it ‘represents’ - the area to which it is the closest point fro,a 
all points in all LOS estimating rays. This weighting will be proportional to the 
distance of the point from the observer. The Weight algorithm uses an integer-based 
approximate weighting of sample points by their grid distance from the observer 
along a LOS ray. A full weighting using floating point arithmetic is employed in the 
WeightF algorithm. 

Figure 6.9 shows the impact on estimated visibility fractions of the previous 
set of 256 points with high estimated visibility of applying full weighting to the VL 
algorithm. 

N37E127 - Top ass VL Points 

I Hi ibK 

Ammhrn 

«■ 

b>100017S3 



0.3 OSS 0.4 0.45 0.5 0.55 
VL Fraction 

Figure 6.9: N37E127 - VL and WeightF Visibility for Top VL 256 
Points. 


Referring back to Figure 6.4 which depicted visibility results from the VL and 
R2 algorithms, we note a striking similarity. This suggests that when a definitive 












84 


comparison of the results of the WeightF algorithm is made against the standard 
benchmark results from the R2 calculations, there should be a strong correlation. 
This key result is confirmed in Figure 6.10. 


N37E127 - Top 256 W. Potato 




I* • I i— t -1 "T- 4 

0.1S 0.20 0.35 0.45 

WeightF Fraction 


Figure 6.10: N37E127 - WeightF and R2 Visibility for Top VL 256 
Points. 


There is an impressive agreement over the entire range of visibility values. In 
addition to the high correlated variation of 0.9227 and the appealing linear form of 
the fitted equation, there is evidence that the overestimation problem from the VL 
algorithm has been significantly mitigated. WeightF is shown to have the potential 
to be an outstanding visibility fraction estimator. 

Figure 6.11 shows the relationship between WeightF and R2 values as we 
broaden the focus from high visibility points to a range of pseudorandomly selected 
points. The correlated variation of 0.9821 is even higher than in Figure 6.10. Note 
that in this and some subsequent figures different (although still linear) x and y 
distance scales Me used to better show detail in the data and curve fits. 

The linear form of the fitted equation remains. The proportional term of 0.995 
is very close to one, and the constar.t term in the equation (0.00008965) is very small 














85 



Figure 0.11: N37E127 - WeightF and R2 Visibility for 1024 Random 
Points. 

in relation to the quantities being related. Together they suggest not only a good fit, 
but that there is no substantial over- or underestimation of visibility taken over the 
entire sample set. For the very high visibility points with WeightF and R2 visibility 
fractions above 0.15, the linear fit remains strong and all points are o rdered correctly. 
In other words, if we rank all locations in order using the WeightF algorithm and 
then using the R2 algorithm, the ordering would be the same. In addition to its 
general robustness, this is further evidence of the utility of WeightF in identifying 
points with the best visibility. 

Although a strong relationship to R2 is shown in Figure 6.11 when using 
WeightF with 32 LOS rays, this suggests the question of how much the correlation 
would improve with more rays. Continuing with the practice of using ray counts 
which are powers of 2 and mindful that many measures that are essentially descrip¬ 
tive statistical values improve not linearly but with the square or higher power of 
the sample size, an appropriate value for an increased ray count higher th:tn 32 will 
be 128. Figure 6.12 shows the results obtained by running WeightF on the same 







86 



Figure 6.12: N37E127 - WeightF (128 rays) and R2 Visibility for 1024 
Random Points. 

points used in Figure 6.11, but using 128 rays. 

The increase in correlation from an already high 0.9821 to 0.9984 is noticeable, 
but the utility of such an increase when compared to the increased computation cost 
(a factor of 4) will depend on the specific application and the inherent uncertainty 
in visibility values produced by errors presumed present in the data (see Section 6.8 
below). The RMSE decreases from 0.0030486 for 32 rays to 0.00090769 using 128 
rays, a decrease of approximately a factor of 3. Both in absolute and relative terms, 
the bulk of the error is concentrated in regions of low visibility. 

A visual inspection of Figure 6.11 and Figure 6.12 suggests that the linear 
model fits particularly well at higher visibility values, a conclusion supported by 
an examination of the values of residuals between the observed data and the fitted 
equations. As shown in Figure 6.13, increasing the number of estimating rays from 
32 to 128 for the set of high visibility points increased the correlation from 0.9227 
to 0.9940, an increase which might be justified in trying to sort out the points with 
the very highest visibility. A valid strategy for finding points with the best visibility 












87 



Figure 6.13: N37E127 - WeightF (128 rays) and R2 Visibility for Top 
256 VL Points. 

might be to run WeightF with a relatively modest ray count such as 32, selecting a 
set of candidate points based on this initial run, and then running WeightF with a 
higher ray count. This would only result in a constant time improvement, however, 
and so the utility of this approach will depend on the specific application. 

The Weight algorithm runs faster than WeightF by a constant factor because 
it weights points by their grid distance from the observer along the most rapidly 
varying axis instead of tracking its fractional distance out along the LOS using 
floating point operations. We would expect that any systematic error introduced 
by this approximate weighting method will result in an overestimation of visibility. 
This is because for rays furthest off of either axis, the contributions of their furthest 
points will be undervalued. This will result in an increase in the significance of 
points closest to the observer, which tend to have a greater fraction visible than 
points further away. Figure 6.14 shows the impact on accuracy resulting from using 
Weight instead of WeightF. 

Although a linear fit of estimated (Weight) visibility fractions to benchmark 







88 


N37E127 - 1024 Rmdom Ports 

fmmtb K 
AA7441MC7I 
*4.00177191tt 
MM7IM1 

0.3T-I-1-!-!- 



0 0.1 02 0.3 

Woight t.'action 


Figure 6.14: N37E127 - Weight and R2 Visibility for 1024 Random 
Points. 

(R2) visibility fractions is maintained, the correlated variation has decreased from 
0.98210 in Figure 6.11 to 0.74418; the y-axis intercept has not drifted greatly but 
the proportional term has decreased by about 10%, suggesting the Weight is over¬ 
estimating average visibility for this case much more than WeightF did. This is in 
accordance with our expectations. The overall poorer performance of Weight when 
compared to WeightF suggests that its modestly lower computational requirements 
would not generally justify its use. 

6.5 WeightF to R2 Correlation as a Function of Ray Density 

By varying the number of rays used in running the WeightF algorithm and 
comparing the resulting correlation to the corresponding R2 results, we can observe 
how the accuracy of the estimated visibility provided by WeightF increases with 
computational effort. 

In Figure 6.15 we cam observe a strong fitted relationship between the num¬ 
ber of rays employed in producing WeightF visibility estimates and the correlated 


I 






89 



Figure 6.15: N37E127 - Correlation Variation of R2 to WeightF. 

variation observed between the corresponding WeightF and R2 values (based on the 
y = a + bx model). The lowest number of rays employed in this sample was four, 
and the ascending ‘tail’ of the curve as the number of rays approaches 1 should be 
discounted as an artifact. The correlated variation of 0.99883 observed using the 
equation 

logy = a + (6 log x)/x 2 (6.1) 

is suggestive of both the logarithmic relationships often encountered in terrain-based 
characteristics and the inverse square relationship between many statistical sample 
sizes and the accuracy of the results derived from those samples. 

As a specific instance of the manner in which the WeightF estimates degrade 
as ray density decreases, we can examine the plot of comparative WeightF and R2 
results in Figure 6.16 using a ray density of 8. The overall correlation has decreased 
from 0.98210 in the 32 ray example from Figure 6.11 to 0.82943, with increasing 
magnitudes of error in the higher visibility regions. An examination of residuals 
shows the error in the WeightF estimates remains approximately proportional to 









90 



Figure 6.16: N37E127 - Correlation Variation of R2 to WeightF (8 
rays). 


visibility magnitude over the entire set of observed values. Referring back to Fig¬ 
ure 6.15, we see that we could increase the correlated variation between WeightF 
and R2 values from 0.82943 to 0.91795 by increasing the ray count by 50% to 12 
rays from each observer. 

6.6 Analysis of Other Locales 
6.6.1 Dijon Area 

A natural question regarding the results presented so far is to what degree 
they are specific to the particular terrain (cell N37E127) that has been used for ex¬ 
perimentation. It is a relatively complex region on the Korean Peninsula, proximate 
to the Yellow Sea, which has been shaped by geological forces much different from 
those in many other areas of real terrain. For comparison, an analysis is presented in 
Figure 6.18 that uses terrain from the DTED cell N47E005 in the vicinity of Dijon, 
France. Elevation ranges from 176 meters to 635 meters, with a median elevation 









91 


of 257 meters above mean sea level. The mean elevation is 280.4 meters, with a 
standard deviation of 76.8 meters. 

The gray-scale elevation image of cell N47E005 is shown in Figure 6.17. This 
image was generated by median sampling 6 point (east-west) by 4 point (north- 
south) subcells in the elevation data for each image pixel. This compensates to 
within 5% for the unequal average east-west (92.61 meters) and north-south spacing 
(62.57 meters) between grid points. 

This terrain encompasses plains regions from the Saone and Doubs rivers, as 
well as higher plateau and mountain areas. The same (x,y) pseudorandom coordi¬ 
nates used for the examples from N37E127 were used here to minimize the impact 
of differences in the spatial distribution of sample points as a source of variation. 

The general appearance of the sample data, the y = a + bx form of the fitted 
equation, and the correlated variation of 0.9922 all compare favorably and consis¬ 
tently with the values from the data in Figure 6.11 for the N37E127 cell. The 
constant and proportion terms also suggest a very consistent relationship between 
WeightF and R2, without substantial over- or underestimation. The improvement in 
the 32 ray WeightF to R2 correlated variation from 0.9821 for the N37E127 terrain 
to 0.9922 conforms to our subjective expectations, given the less complex nature of 
the terrain in the Dijon area. 

6.6.2 Pripyat’ Area 

As a final local to investigate the performance of the WeightF algorithm and 
the properties of the resulting visibility output, an analysis was conducted for the 
one degree DTED cell N52E028 (also drawing on its eight surrounding cells as 
required), which is near the center of the Pripyat’ Marshes in Beloruss, northwest 
of the Ukrainian city of Kiev. Figure 6.19 shows the elevations of cell N52E028 as 
a gray-scale image. This cell was median-sampled on 4 point by 5 point subcells. 



Figure 6.17: N47E005 - Gray-Scale Elevation Image. 





93 



Figure 6.18: N47E005 - WeightF and R2 Visibility for 1024 Random 
Points. 

The N52E028 cell differs significantly from previous examples in two ways. 
First, the longitudinal spacing changes from 3 arc seconds to 6 arc seconds, result¬ 
ing in half as many longitudinal divisions in the cell with an average longitudinal 
distance between points of 112.76 meters instead of distances in the 60-75 meter 
range as in previous examples. Due to the decreased longitudinal density, each cell 
will have only 721,801 observer locations instead of the 1,442,401 locations for cells 
in previous examples. In addition, the N52E028 area consists of terrain with ele¬ 
vations that are relatively low and uniform. Elevations only range from 96 to 198 
meters above mean sea level, with a mean elevation of 123 meters and a standard 
deviation of only 11.5 meters. Median elevation is 121 meters. In spite of the re¬ 
duced standard deviation and range of elevations when compared to N37E127, the 
elevation histogram of N52E028 in Figure 6.20 shows the same log-linear form over 
most of the range of elevations as was shown in Figure 3.2. 

N52E028 was selected to see how the accuracy of the WeightF visibility esti¬ 
mates and the distribution of visibility values would be affected by relatively low, 







94 



Figure 6.19: N52E028 - Gray-Scale Elevation Image. 















95 



Figure 6.20: N52E028 - Elevation Histogram for 1024 Random Points. 

uniform terrain. Intuitively, we might suspect that the accuracy of the estimates 
and overall visibility would increase due to the less complex nature of the terrain. 
Figure 6.21 shows the results of running the WeightF algorithm on the same pseu¬ 
dorandom spread of 1024 points used in previous examples. 

The same strong linear trend that has been observed in previous examples is 
still present. As expected, the range of visibility fractions extends into significantly 
higher values. For the complex terrain in cell N37E127, in Figure 6.11 the highest 
observed R2 visibility fraction is 0.25646 and for N52E028 (as shown in Figure 6.21) 
this has increased to 0.56028, more than doubling. In spite of the general increase 
in visibility, however, very high visibility points (for example, those above an R2 
visibility fraction of 0.4) still occur in very small numbers (also shown in Figure 6.23). 
The correlated variation of 0.987477 is somewhat higher that the corresponding value 
of 0.982097 obtained in cell N37E127, which is in accord with the hypothesis of the 
impacts of the greater spatial cohesion present presumed present in N52E028. 










Figure 6.21: N JE028 - WeightF and R2 Visibility for 1024 Random 
Points. 

To investig^ s how visibility relationships change with differing LOS parame¬ 
ters, Figure 6.22 displays the results obtained by performing the same calculations 
on the sarnie points as in Figure 6.21 but with an alternative set of LOS parameters. 
This parameter set uses a radius of visibility of 4 km instead of 40 km, an observer 
height above ground of 1 meter instead of 5 meters, and a target height above ground 
of 3 meters instead of 25 meters. This set of parameters is more representative of 
visibility problems involving human vision instead of antennas. 

Using this new set of parameters, a strong linear relationship between the 
WeightF estimates and the R2 benchmark values remains. The correlated variation 
has decreased slightly from 0.987477 using the originad parameters to 0.970265 using 
the adternative parameter set. A greater change is observed in the constant of 
proportionality. This decreaises from 0.99329 to 0.92305 and suggests that under 
these conditions of a smaller potentiad viewshed, WeightF is still very successful 
in assigning relative visibility estimates but overestimates visibility ats meaisured by 
R2. Still apparent is the pattern of very small numbers of points with high visibility 















N52E028 -1024 Random Points 

y »i b « 

Aaiwmcm 



_ WrtghtF Fraction _ 

Figure 6.22: N52E028 - WeightF and R2 with Alternate LOS Parame¬ 
ters for 1024 Random Points. 

values (for example, R2 values above 0.6). We also note a modest increase in the 
maximum visibility value from 0.56028 using the original parameters to 0.71719 
under the alternative parameter set. 

Figure 6.23 shows a histogram of R2 visibility values from N52E028 using 
the original LOS parameter set. All visibility observations were categorized into 32 
equally spaced slots. Use of a log scale on the y-axis brings out a strong linear trend 
over much of the range of plotted points, corresponding to a decreasing exponential 
relationship between visibility values and the number of points. Log-linear regions 
are common in the distributions of many real world characteristics ([FR94a]) and 
reflect a product-form instead of an additive relationship between underlying con¬ 
tributing factors. For example, two factors contributing to the total visibility from 
an observer location are the configuration of the terrain in its immediate vicinity 
and the extent of visibility barriers at intermediate ranges. Neither factor by itself 
is adequate to ensure high visibility at longer ranges where the majority of the total 
viewshed lies; only when both of these factors are favorable can a high total visibility 









98 



Figure 6.23: N52E028 - Histogram of R2 Visibility for 1024 Random 
Points 


value occur. The impact of both factors being favorable has a much greater than 
additive effect in producing a high count of total points visible. 

The data displayed in Figure 6.23 support the subjective observations from 
previous figures that a relatively small set of points will have visibility values distin- 
guishably higher than the bulk of points in a given area. This relationship reinforces 
the value of searching through an entire area for the points with highest visibility 
when siting small numbers of observers. 

We can see the trend between visibility and frequency of points again in Fig¬ 
ure 6.24, which is a histogram similar to that in Figure 6.23 but for the alternative 
set of LOS parameters. This figure also displays substantial linearity in the visibility 
range between 0.2 and 0.6 when plotted using a y-axis log scale. The onset of this 
trend at a value of 0.2 occurs at a higher visibility than in Figure 6.23, where it oc¬ 
curred at approximately 0.05. This is consistent with the generally higher visibility 
values present in the alternative LOS parameter setting. 















99 



Figure 6.24: N52E028 - Histogram of R2 Visibility with Alternate LOS 
Parameters for 1024 Random Points 


6.6.3 Summary 

The WeightF algorithm is a highly effective and efficient modification to the 
basic VL algorithm, extending the predictive power of VL for high visibility points 
to the entire range of points. 

6.7 Confidence in Finding Highest Points 

To further verify the accuracy of the WeightF algorithm and to investigate 
how many of the points with the highest WeightF visibility index estimates should 
be selected in order to have high confidence that the n highest ‘true’ (R2 valued) 
visibility points had been included, an exhaustive study of the central 400 by 400 
grid region of the DTED cell N37E127 was undertaken. This required approximately 
45 days of CPU time to calculate R2 values for each point; actual execution was 
shared across several workstations. Execution time for calculating WeightF values 
was approximately 6.1 hours. 

One practical measure of the utility of the WeightF algorithm in finding the 










100 



Figure 6.25: Center of N37E127 - WeightF Points Required to Find the 
Highest R2 Points 


n highest visibility points in a region is given by determining how many of the top 
WeightF points in addition to n must be evaluated by R2 to have high confidence 
that all of the true highest n valued points have been found. Figure 6.25 shows how 
many WeightF points had to be evaluated to find a given number of top R2 visibility 
points in the central 400 by 400 grid of N37E127. From the inflection of the curve 
it is evident that the ratio of WeightF points required to R2 highest points desired 
is highest in the region located approximately between 10000 and 100000 highest 
points. This can be seen more clearly by plotting the ratios of required to desired 
points, which is done in Figure 6.27. It is useful to bear in mind when examining the 
relationship between desired and required points that in practical siting problems, 
the number of locations to be selected for analysis will be very much less than the 
total number of points in the elevation grid. 

Figure 6.26 suggests that the number of highest visibility points that have 
values in the top 50% of the total range of visibility values encountered may be only 
a few dozen out of over a 160,000 total points. In fact, in the top third of the total 








101 



Figure 6.26: Center of N37E127 - Histogram of R2 Visibility Values 


range of visibility values, only 50 points occur. 

While the general shape of the observations in Figure 6.26 suggests a log¬ 
normal distribution, if such a distribution is fitted to the peak in the data it is too 
high for values to the left of the peak and too low for values above it. Nevertheless, 
the fitted curve of log y = a + by/z + c/x% does exhibit many of the characteristics 
of the log-normal distribution. 

Figure 6.27 shows the variation between the required number of highest WeightF 
points to find a given number of highest R2 points by plotting the ratio of the two 
versus the number of highest R2 points desired. Note that the ratio of WeightF 
points required to find a given number of highest R2 points never exceeds 4, and 
for the range of R2 highest points from 1 to 10000 it never exceeds 3. Using more 
than 32 rays in the WeightF estimated visibility values would be expected to reduce 
the ratios of required WeightF to desired R2 highest points, and a tradeoff in exe¬ 
cution time between number of WeightF rays and subsequent R2 evaluations could 
be performed on specific hardware and problem settings. 







102 



Figure 6.27: Center of N37E127 - Ratio of WeightF Points Required to 
Number of Top R2 Points 

6.8 Impact of Errors in Elevation Data 

One reason why evaluating the impact of errors in elevation data is important 
is because it can help evaluate the relative significance of other uncertainties in the 
results produced by particular visibility estimation techniques. In [Fis92] one con¬ 
clusion was that introducing simulated error reduces viewsheds. This is consistent 
with the concept that the existence of substantial visibility is related to the pres¬ 
ence of significant spatial correlation in real terrain. As an experiment, elevations 
for the cell N37E127 and its surrounding cells were perturbed pseudorandomly from 
a Gaussian distribution with mean of zero, standard deviation of 7 meters, and with 
no introduction of spatial correlation in the errors. 

Figure 6.28 shows clearly that the points least impacted by the introduction of 
errors in the elevation data are those points with the highest visibility values. This 
lends credence to the use of consistent and unbiased visibility estimation techniques 
for the purpose of identifying the points in an area with the highest visibility values 












103 



Figure 6.28: N37E127 - R2 Visibility for Original and Perturbed Ele¬ 
vations 

in spite of the presence of errors in the elevation data. 

The existence of efficient algorithms to calculate visibility fractions (R2) and 
to identify points in with the highest visibility values (WeightF) makes it possible 
to make inferences such as mentioned above regarding Figure 6.28, because large 
numbers of points and their visibility results can be examined, calculated, and ag¬ 
gregated. 

6.9 Summary 

In this chapter, we have evaluated the accuracy of several algorithms that use 
a fixed number of LOS rays to estimate the visible fraction of a potential viewshed 
from a given observer. The R2 algorithm was used to provide the reference value 
for viewshed fraction. It was determined that the most direct LOS ray algorithm, 
VL, was useful in identifying points with high or low relative visibility, but had 
a correlated variation of only 0.776 using the standard data cell and visibility pa¬ 
rameters. It suffers from a non-linear systematic overestimation of visibility that is 






104 


most sever for points of moderate visibility. An algorithm which weights each point 
along an LOS ray by its distance from the observer produces excellent viewshed 
fraction estimates, with a correlated variation using the standard data cell and visi¬ 
bility parameters in excess of 0.98 and relatively uniform performance for the entire 
range of visibility values. This performance was evaluated using a variety of LOS 
ray densities, three different geographic settings, and an alternative set of visibility 
parameters. 


CHAPTER 7 

EXECUTION TIMES 


7.1 Overview 

In this chapter we investigate the execution times of the R3, R2, and LOS 
ray estimation algorithms and compare empirical measurements to predicted per¬ 
formance 

A sample set of 256 points was drawn from the standard set of 1024 pseu- 
dorandomly generated coordinates described earlier. These points were sorted into 
ascending order according to data layout to minimize the impact of paging on the 
timing results. Timing values were based on the sum of ‘user’ time and ‘system’ 
time reported by the UNIX ‘time’ command. The output of this command is in¬ 
tended to measure time spent by processors in running or supporting (e.g., paging) 
the execution of the problem program, regardless of ‘real’ (wall clock) time. Each 
run was executed as one process on a 4-processor Sun 670-MP. Total available real 
memory was 128MB. 

To reduce variations in timing measurements unrelated to running the visibil¬ 
ity calculations, several precautions were taken. Runs took place on identically con¬ 
figured Sun 670-MP computers with negligible workloads (network daemons, etc.). 
Output volume was minimal — a few hundred characters of textual summary in¬ 
formation. A delay of 180 seconds was imposed between successive runs to reduce 
the impacts of coincidental cached resources from one run to the next. To minimize 
the effects of possible caching of disk-based input data by the operating system, 
the 25.9MB input file was kept on the disk system of a separate Sun 670-MP and 
accessed by means of remote mounting using the Network File System (NFS) pro¬ 
tocol. 


105 


106 


7.2 R3 Execution Time Performance Evaluation 

We should expect that using the R3 algorithm we will observe startup time 
(loading the elevation file, adjusting the effective heights of the elevation data for 
curvature of the earth, establishing a working set in real memory, etc.) to be rela¬ 
tively constant between all runs, and the time of 225.2 seconds for the run with the 
shortest radius (1250 meters) provides an upper limit for the estimated value of the 
startup time. The execution time for the R3 algorithm should vary with the cube 


Radius 

(km) 

Run Time 
(seconds) 

Fitted Model 
Time(seconds) 

1.25 

225.2 

230.3 

2.50 

229.4 

233.7 

5.00 

259.0 

260.8 

7.50 

335.4 

334.6 

10.00 

487.3 

478.3 

15.00 

1088.2 

1068.4 

20.00 

2243.3 

2217.6 

30.00 

6944.1 

6938.8 

40.00 

16084.4 

16132.6 


Table 7.1: R3 Execution Times as a function of the Visibility Radius 

of the radius. Figure 7.1 shows the run times for the R3 algorithm using conditions 
similar to those used to compile execution time data on R2, with the exception that 
8 points were used instead of 256 on each run and longer ranges were not measured 
due to the excessive run times required. 

To account for knowledge that the run times of the shorter radii form a ceiling 
on the startup time, the y = a + bx 3 form of the fitted equation was selected by 
using raw timing data. Then, in order to more properly reflect the significance of 
the lower radii run time values in establishing a ceiling on startup time (the value 
of the a parameter), the fitting procedure was run again with each point weighted 
by the reciprocal of its run time. 

The fit to weighted data is what is displayed in Figure 7.1. Weighting the 



107 



Figure 7.1: N37E127 - 8 Random Points; Weighted Curve Fit 

fit changes b from 0.24776 to 0.24848 and decreases the correlated variation from 
0.99999296 to 0.9999503, but brings the value of a (which should reflect average 
startup time) from 238.3 to 229.8 seconds, with 95% confidence interval boundaries 
of 237.0 and 222.6 seconds. This is a noticeable improvement in agreement with 
the two lowest run time values of 225.2 and 229.4 seconds. The high correlated 
variation using either weighted or unweighted data for a fitted curve of the form 
y = a + bx 3 confirms the anticipated cubic relationship between the run time of the 
R3 algorithm and the radius of visibility. 

7.3 R2 Execution Time Performance Evaluation 

The number of points within the visibility radius for which the R2 algorithm 
must make a visibility determination is shown in Figure 7.2 as it varies with radius 
length. Use of log scale on both axes reveals a relatively straight line relationship 
that brings out the simple power relationship (in this case quadratic) between radius 
and points within the boundary followed by R2. 











108 



Figure 7.2: N37E127 - Points within R2 Radius 


Note that the radius units are meters, not grid points. For the cell N37E127 
the average spacing in meters between points is 73.475 meters per 3 arc seconds of 
longitude and 92.614 meters per 3 arc seconds of latitude. The relation between the 
radius and the points within it takes the anticipated form, in that 

y/y = a + bx (7.1) 

is tantamount over our range of interest to 

y = a' + b'x + cx 2 (7.2) 

where 

a' = a 2 = 1.5942 
6' = 2 ab = 0.054259 
c = ft 2 = 0.00046167 


The non-zero y-intercept of 1.5942 corresponds to an intuitive limiting case 
of one point for a zero radius, that point being the observer location itself. The 










109 


number of LOS rays computed by R2 is determined by the number of boundary 
points (see Chapter 4), which is in turn proportional to the circumference around 
the area of the points within the given radius of visibility. Since the number of 
operations involved in computing a single LOS is also proportional to the visibility 
radius, the work required in the R2 algorithm in running all LOS’s to the boundary 
points is proportional to the square of the radius. As shown above, the number of 
operations to set visibility values for interior points (based on results from running 
LOS’s to the boundary) is also quadratic in the visibility radius. Therefore, in terms 
of performance we expect that the execution time of R2 would vary with the square 
of the visibility radius, after accounting for program startup time. In Figure 7.3 and 
Table 7.2 we can examine this hypothesis in the context of actual run data. 



Figure 7.3: N37E127 - R2 Execution Times for 256 Random Points 

Superficially, Figure 7.3 suggests that a good fit between the observed data 
and the predicted quadratic behavior of the R2 algorithm has been achieved, fitting 
the expected form of 

y = a + bx 2 


(7.3) 







110 


The correlated variation of 0.99908 seems high, but there are warning signs that 
an improved fit can be found. For radius values in the range between 17.5 km and 
60A:m the fitted curve consistently overestimates the observed data. For radius values 
less than 17.5A:m, the fitted curve consistently underestimates the observed data. 
In particular, the y-intercept value of 103.8 seconds substantially underestimates 
the clear evidence of the points with the smallest radius values that the minimum 
execution time (due to startup operations) is very likely to lie between 200 and 250 
seconds. 


Radius 

Run Time 

Fitted Model 

(km) 

(seconds) 

Time(seconds) 

1.25 

227.3 

223.5 

2.50 

242.3 

240.1 

3.75 

268.4 

267.8 

5.00 

302.8 

306.6 

6.25 

356.1 

356.6 

7.50 

411.6 

417.6 

8.75 

485.6 

489.7 

10.00 

563.4 

572.9 

12.50 

775.4 

772.5 

15.00 

1012.1 

1016.5 

17.50 

1305.2 

1304.9 

20.00 

1643.0 

1637.7 

20.00 

1643.0 

1688.7 

25.00 

2491.2 

2479.0 

30.00 

3487.6 

3477.6 

40.00 

6187.4 

6124.6 

50.00 

9682.1 

9672.6 

60.00 

14087.8 

14155.8 

80.00 

26058.9 

26039.8 


Tfeble 7.2: R2 Execution Times as a function of the Visibility Radius; 
Separate Fitted Curves for each Group 


By experimenting with breaking the observations into two groups, varying the 
break point, and evaluating the curves fitted to each group, substantially better 
results were obtained. The results of the separate fits are presented in Table 7.2 


Ill 


and in Figures 7.4 and Figure 7.6. A breakpoint of 20km was selected by examining 
how the F-statistic for each of the fitted curves varied as the breakpoint varied. 

For points with radius values in the range between 0 and 20 km, the form of the 
fitted equation -emains y = a+bx 2 . The y-intercept value has changed from 103.8 to 
217.9, which is a significant improvement in the estimate of the minimum execution 
time over the previous value obtained by fitting a curve to the entire range of points. 
The correlated variation has improved from 0.99908 to 0.99984, representing a nearly 
five-fold decrease in the amount of uncorrelated variation remaining. 

An examination of the residuals between the observed and modeled execution 
times also shows improvement when specifically fitting the 0 to 20fcm range. When 
fitting the entire range, the trend of the residual percentages forms a smooth, con¬ 
tinuous pattern of decreasing values from 0 to 30 km, with the first 10 of the 12 total 
observations between 0 and 20 km being positive. After fitting y = a + bx 2 to the 
range of 0 to 20 km, the residuals pattern becomes much more erratic, a sign of an 
improved fit. The longest run of residuals with the name sign is now 5 instead of 
10, with an even total split of 6 positive and 6 negative residuals. 

The constant of proportionality on the x 2 term on the equation fitted in Fig¬ 
ure 7.4 has decreased to 3.55 from the value of 3.98 obtained when fitting the entire 
range of radius values. This is consistent with the increase in the y-intercept value 
while improving the fit with all of the data points in the 0 to 20 km range. 

For convenience, Figure 7.5 shows the execution times for R3 and R2 from 0 
to 20 km on the same graph, using a logarithmic scale for time. The dotted line 
represents the time performance for R3 for 256 points, derived from the curve fitted 
to R3 observations for 8 points in Figure 7.1 by multiplying the constant on the 
x 3 term by 32. The solid line with boxes indicating observed values represents the 
same R2 information depicted in Figure 7.4. The pronounced time advantage of R2 
over R3, especially for longer radius values, is readily apparent. 






112 



Figure 7.4: N37E127 - R2 Execution Times for 256 Random Points 
(lower range) 



Figure 7.5: Comparison of R3 and R2 Execution Time for 256 Points. 





















113 


Figure 7.6 and the second portion of Table 7.2 show a significant improvement 
resulting from separately fitting the range of points between 20 and 80 km. When 
fitting the entire range of points, the 7 residuals in the 20 to 80&m range were 
negative for the first 6 observations. By specifically fitting the 20 to 80/cm range the 
residuals split to 5 positive and 2 negative values, with four positive values being 
the longest consecutive constant sign run of values. The correlated variation has 
increased from 0.99908 to 0.99997, representing a 30-fold decrease in uncorrelated 
variation. 



Figure 7.6: N37E127 - R2 Execution Times for 256 Random Points 
(upper range) 


The most significant item to note about the curve best fitted to the data values 
in the 20 to 80 km range is that the form of the fitted equation has changed from 

y = a + bx 2 (7.4) 


to 


y = a + bx c 


(7.5) 














114 


Although the power coefficient value c = 2.1665 is not far from the value of c = 2 
for a simple quadratic relation, it represents a distinguishable difference from the 
original fit to the entire 0 to 80fcm range and to the improved quadratic fit on the 
0 to 20&m range. 

Since the R2 algorithm performs the same pattern of steps regardless of the 
visibility radius, one other place to look for an explanation of the small upward 
deviation in run time is to the architecture of the underlying hardware. As with 
many current system architectures, in addition to system main memory the processor 
modules on a Sun 670-MP contain small on-chip data and instruction caches. There 
is also an external cache between each processor and system main memory. Although 
the on-chip processor caches are too small to account for any performance change 
observed for radii above about 2-5 km (depending on assumptions about cache 
operation), interactions between the problem size and the external cache may have 
some impact on execution time for larger radius values. Given a finite cache size 
and the known characteristics of the R2 algorithm, it is likely that Equation 7.5 
represents a transition zone between shorter ranges where cache hits dominate data 
accesses and longer ranges where cache misses dominate data accesses. In each of 
these zones a quadratic relationship in the form of Equation 7.4 would describe the 
relationship between visibility radius and execution time, but with a higher b value 
at longer ranges. 

An approximation of the possible relationship between external cache size and 
the radius value at which the problem size and access patterns may start to generate 
a substantially increased cache-miss rate (and therefore longer execution times) can 
be formulated as follows. We assume that most but not all of the external cache 
can be used to hold elevation data to speed processing of the visibility calculations, 
because some of the cache will be used to hold program code, operating system 
functions, or will be malutilized in the visibility calculations due to address mapping 







115 


or related conflicts. Since we assume that more than half but less than all of the 
cache will be used for the visibility calculations, we arbitrarily chose an estimate 
of 75% cache use for visibility calculation elevation data. Each elevation value 
requires two bytes to represent. The minimum number of points per kilometer in 
cell N37E127 is 

10.8 points per km = (1000 meters)/(92.6 meters per North-South grid) 

and the maximum number of points per kilometer is 

13.6 points per km = (1000 meters)/(73.5 meters per East-West grid) 

so we will use an average of 12 points per kilometer in this approximate analysis. 
Since we have an empirical indication of a change in run time performance charac¬ 
teristics at a radius of around 20 km, this would correspond to an external processor 
cache size of approximately: 

Cache Size (bytes) = irr 2 points x 2 bytes per point/Cache Utilization 

= ir x [20 kilometers x 12 points per kilometer] 2 x 2/0.75 
= 482549 bytes 

or approximately 471KB. This agrees with the 1024KB actual size of the external 
cache on the Sun 670-MP processors to within about a factor of two. This should not 
be taken as confirmation of the hypothesis regarding the size and manner in which 
cache size will affect the execution time of the R2 algorithm, but only as support for 
its plausibility. Further research using larger problem sizes and processors with easily 
controllable cache operation (such as 80486 or Pentium class personal computers) 
would be able to better determine the interactions between problem and cache size. 

7.4 WeightF Execution Time Performance Evaluation 

The WeightF algorithm exhibits a strong linearity in execution time for visi¬ 
bility ranges out to 60 km. The same measurement methodology used for measuring 








116 


the performance of R2 and R3 algorithms was employed, with the exception that 
the 256 point sample set was executed 16 times during each run to obtain longer 
times that stood out better from ‘noise’ variations for shorter ranges. Figure 7.7 
shows the accurate linear fit. 










































P5i 


















Figure 7.7: N37E127 - WeightF Execution Times for 16 runs of 256 
Pseudo-Random Points (out to 60 kilometers) 


The same break from the linear trend at longer ranges occurred for WeightF 
as was observed for R2, although at a longer specific range (60 km instead of 20 
km). This break is clear when comparing the 70 km and 80 km values (which were 
excluded from the linear fit) to the fitted line. 

A different phenomenon here is responsible for the higher than expected exe¬ 
cution times at higher ranges than was observed for execution time for R2 at longer 
ranges. The processor time devoted to system functions (approximately 140-170 
seconds) is much higher for the 70 km and 80 km values in Figure 7.7 than for 
runs at smaller ranges (approximately 25-35 seconds), which was not the case for 
R2 runs. This is more likely to be attributable to paging operations than memory 
cache behaviors. 














117 



Figure 7.8: N37E127 - WeightF Execution Times for 16 runs of 256 
Pseudo-Random Points (from 70 to 90 kilometers) 


Figure 7.8 shows observations of WeightF execution times conducted for vis¬ 
ibility ranges between 70 km and 90 km, with a 1 km change in range between 
adjacent runs. These runs were conducted in parallel on separate processors on the 
same Sun 670-MP in groups of 10 or 11 consecutive ranges per processor, with each 
run being preceded by a ‘dummy’ run at the lowest range in the group in an attempt 
to establish a common base of buffered data files, etc. 

Starting out each sequence of runs with a ‘dummy’ run and using a fine range 
granularity (1 km) produces a sequence of execution times for the 70 km to 90 km 
range much more in accord with the observations for lesser ranges from Figure 7.7, 
with the exception of the observation for the 89 km range (this point was manually 
excluded from the curve fit). The system time for this point was approximately 140 
seconds, more than 100 seconds above the system times observed for the remainder 
of the observations. The user time for this point is also somewhat higher than what 
would be predicted by the fitted curve. If the jump in system time is caused by 
increased paging requirements to bring the elevation data required by the longer 










118 


LOS lengths into memory, this modest jump in user time may be due to increased 
context switching charged to the user process, increased cache misses, or other 
inefficiencies. 

7.5 Summary 

Strong empirical evidence exists that the predicted execution times for R3 
[©(/i 3 )], R2 [9(f? 2 )], and WeightF [0(f2)] match the behavior observed running the 
algorithms using real data on physical hardware. Anomalies have been observed for 
some points or transition zones which may be due to caching or paging behavior. 










CHAPTER 8 

SUMMARY AND CONCLUSIONS. 


The potentially immense demands involved in computing visibility and the utility 
of visibility analyses makes finding ways of reducing the computational complexity 
of suitable visibility algorithms an important problem. We have focused here on 
developing and validating algorithms that are accurate, significantly faster than 
existing methods, practical to implement, and robust on real world data. This 
chapter summarizes methods and results, and then lists some areas for extensions 
and future research. 

8.1 Summary 

To facilitate development and evaluation of algorithms dealing with terrain 
datasets of significant size, a variety of access routines coded in C++, an object- 
oriented third generation computer language, were written, tested, and refined. 
These routines provide transparent access to five types of USGS data, DMA (Defense 
Mapping Agency) DTED Level I data, TEC (U.S. Army Topographic Engineering 
Center) TerraBase data, and to any other raster based layout that can be described 
by parameters such as grid orientation, geographic location, ground spacing, etc. A 
uniform format for representing datasets of gridded elevation and visibility values 
was devised to provide for efficient storage and access. 

To be able to investigate and validate very fast [©(/?)] LOS ray methods of 
computing visibility indexes, it was necessary to develop an accurate viewshed com¬ 
putation algorithm that was substantially more efficient than [©(R 3 )] naive methods 
such as those used in the common R3 algorithm. The R2 algorithm [0(R 2 )] devel¬ 
oped here meets these requirements. For the standard case of a 40 km viewshed 


119 


120 


in the DTGD Level I cell N37E127 with common startup time discounted, view- 
shed calculation time on a Sun 670-MP for 256 observer locations using R3 takes 
approximately 5.89 days but only 1.66 hours using R2. 

In order to produce accurate visibility index values in 0 (R) time, a method to 
exploit the spatial correlation of real terrain was designed. This used a fan of equally 
spaced LOS rays from observer points to sample the visibility of the surrounding 
terrain and produce an estimated visibility index. 

The initial LOS ray algorithm, VL, was evaluated on a large number of terrain 
datasets and found to be useful in identifying points of high visibility but was less 
accurate for other points and tended to overestimate true visibility index values (as 
measured by R2). Investigation and experimentation demonstrates that a weighted 
version of the VL algorithm, WeightF, provides very accurate (correlated variation 
to R2 above 0.9) visibility index estimates over the entire range of values for only 
a minimal constant time increase in computation. Accurate estimates for visibility 
index values requiring approximately 1.66 hours to compute using R2 (see above) 
can be calculated in less than 10 minutes using WeightF. 

To gain confidence in the visibility algorithms developed, experiments were 
run on terrain from several parts of the world ranging from mountainous to river 
valley to marshland. Over two dozen DTED Level I cells were analyzed, each with 
over 1.4 million observer locations and viewsheds of over 740,000 points from each 
observer. Parallelism in various degrees was employed to spread the work over 
several workstations and processors. 

Verification of predicted execution time behavior as a function of problem 
parameters was performed by controlled runs of the algorithms on several platforms. 
Some variation from expected timing results for R2 was observed, which may be 
attributable to memory cache characteristics. 



121 


8.2 Significant Contributions and Accomplishments 

There are several contributions realized by the results of the research described 
in this thesis. 

• The existence of significantly faster algorithms for viewshed and visibility index 
calculation broadens the range of applications that can be developed and the 
scope and nature of further visibility research that can be undertaken. 

• The existence of efficient and implementable viewshed and visibility index 
algorithms provides benchmarks for developing even faster methods based on 
genetic algorithms, hierarchical data structures, or other means. 

• Experimental evidence that terrain datasets routinely contain a very few well 
defined highest visibility points is important to the manner in which solutions 
to certain types of siting problems are undertaken. 

• A specific elevation model and visibility characteristics taxonomy is presented 
to enable the results of this research to be understood and extended in a 
manner that facilitates further development and meaningful comparisons with 
other work. 

Several issues and goals relating to this work were described in Chapter 1. 
They have been addressed as follows: 

1. We use all relevant data from the elevation raster. No points are dropped 
by simplifying the dataset, and the criterion for adequacy and criterion for 
appropriateness are satisfied. 

2. Significant improvements in performance over existing implemented and accu¬ 
rate algorithms have been achieved for total viewshed determination by R2 in 
9 (R 1 2 ) time and for visibility index estimation by WeightF in 0 (R) time. 




122 


3. The accuracy and utility of the algorithms developed have been confirmed on 
examples of different types of terrain (mountain, river valley, marshland) and 
for different LOS parameters (antenna, human vision). 

4. Visibility indexes on real terrain have been observed to follow distributions 
resembling negative exponential or log-normal forms; a very limited number 
of points with high visibility index values for their region can be identified and 
used as input to various site selection algorithms. 

5. Parallelism at various degrees of granularity and architectural dispersion have 
been successfully employed to tackle visibility analyses with large, real world 
problem sizes. This exploitation of parallel execution included robust recovery 
and input data sharing. 

6. The algorithms developed have been shown to be practical and implementable; 
demonstration viewers for inexpensive platforms have been made available 
to user communities to confirm the utility of visibility index images and to 
identify areas for further research. The utility of these algorithms have been 
demonstrated on 1 meter spacing elevation datasets. 

Any existing algorithms or applications that make use of visibility rankings or 
viewsheds can potentially benefit from using faster visibility determination methods. 
GIS-related applications are obvious instances. Use of R2 instead of R3 in an on-line 
viewshed display application reduces the cost of visibility computation to the same 
order as for screen pixel update operations. Outside of the traditional GIS area, the 
visibility image applications described in [DFJN92] and [Ray94] benefit from total 
scene analysis times reduced from 0(R®) using naive methods to Q(R*) by using R2 
and (if accurate enough in the specific problem domain) to ©(.R 3 ) using WeightF. 

Given the existence of a set of portable, reusable, and extensible implementa¬ 
tions of algorithms to conduct fast and accurate visibility analysis of large elevation 




123 


datasets, a variety of new approaches to existing problems can be undertaken. Com¬ 
putationally cheap and robust visibility calculations can open new possibilities for 
analyzing the occurrences and nature of errors in elevation datasets. In addition to 
the analytical possibilities (such as the impact of elevation data errors on visibility 
in Figure 6.28), new visual aids in error identification become possible. 

Figures 8.1 and 8.2 show an instance of such an application. They show the 
elevation and visibility gray-scale images of a portion of the terrain in the southeast 
comer of DTED cell N37E126. Note the dark vertical line in the northeast comer of 
the visibility image. This line is barely distinguishable in the elevation image, even 
knowing where to look for it, and is virtually undetectable to the unaided eye on a 
standard gray-scale workstation display of elevation data. The line corresponds to 
an erroneous drop of 10-20 meters in the elevation values recorded on the boundary 
of a DTED cell. This appears to be another instance of the nature and magnitude of 
DEM strip error mentioned in Section 2.3. It is difficult to identify in the elevation 
image, but the visibility values for the corresponding points in the visibility image 
decrease in intensity by 60% to 80% and show the problem area clearly. 

8.3 Future Work 

• Faster Real Terrain Visibility Methods Tools already created can facili¬ 
tate the investigation of even faster visibility determination algorithms. Such 
methods could capitalize on correlated viewshed subregions between adjacent 
points, terrain configuration local to an observer point, genetic algorithms, 
neural networks, quadtree representations of minimum and maximum eleva¬ 
tion values within subregions, and various other approaches. The ultimate 
objective of such investigations would be to produce algorithms that run sig¬ 
nificantly faster than WeightF and R2 but yield the same relative degrees of 
accuracy (or better). 



Figure 8.1: Elevation Vicinity n37el26y 



Figure 8.2: Visibility Vicinity n37el26y 








125 


• Variations in Elevation Point Density Given the availability of reliable ele¬ 
vation datasets of varying densities for the same area, the relative impact of 
random error and grid density on visibility results can be investigated. Sim¬ 
ple elimination of points from an existing grid may not prove suitable, since 
an agent producing a reduced density DEM for real applications would be 
likely to take into account issues of ‘representativeness’ and apply some type 
of adjustment to the chosen values. 

• Terrain Complexity Categorization Computationally efficient means of cat¬ 
egorizing terrain complexity can be evaluated by observing the relationship 
between their measures and the number of WeightF LOS rays needed to ob¬ 
tain a specified degree of visibility accuracy for given terrain data sets. The 
general concept of using the emergence of patterns between GIS operations 
as a means of identifying valid and properly scaled measurement methods is 
mentioned in [Ope89]. Existing methods such as the 2D-Fourier Transform 
and Fractal Dimension Estimation are computationally intensive, and sim¬ 
ple statistics (e.g., elevation standard deviation) are insufficient to distinguish 
between terrain types such as gradually sloping plains and rolling hills. Pos¬ 
sible measures to investigate include the number of rays required for accurate 
WeightF estimates, slope sign changes along rows or columns of elevation data, 
and spatial correlation of elevation data. 

• Relationship of Aspect Error to Visibility As mentioned in Section 2.3, ran¬ 
dom DEM errors may have significant impacts on the apparent directional 
aspect of a terrain feature represented in a DEM. Aspect errors may result 
in significant visibility errors for some terrain features by narrowing or ex¬ 
panding their potential field of view around obstructions. The algorithms 
developed during this research can be used to help investigate variations in 







126 


visibility strongly related to error-induced aspect variations. The visibility 
images described in Section 5.7.2 can aid in visualizing aspect and visibility 
relationships. 

Other areas for extensions and future research include investigation of other 
terrain areas (including those on Venus or Mars by using data from NASA), incre¬ 
mental application of W'nghtF calculations for early identification of best visibil¬ 
ity points, modeling memory cache behavior during visibility calculations on large 
viewsheds, and weighting point contributions to viewsheds using the calculated LOS 
height at the point. 









LIST OP SYMBOLS 


S c The controlling slope at any specific state in the process of a visibility 

determination. 

B The set of target points on the boundary of an an area viewed from 
a given observer. 

C A class of observed point associated with a specific weight vector for 
the value of covering it by various numbers of observer facilities. 

Do Distance from the observer in earth curvature calculations. 

A Ec Change in elevation due to earth curvature. 

GD\p a ,Pb] The ground distance between points P a and Pb in L. 

G a One specific arrangement of facilities within Sj. 

L The surface of the landscape of an AOI. 

M A matrix of points corresponding to T. 

Oh The height of an observer above ground level. 

P a A point in 3D space on or above L. 

R The maximum range for which visibility is significant. 

Rave The average range for which visibility is significant. 

Re Effective radius of the earth. 

S A 2]D surface where elevation is defined only for locations corre¬ 
sponding to grid points or grid lines in M. 

Sj Set of points in 5 where it is possible to place an observer facility. 

Sxy The set of (x, y) coordinates in S for which elevation is defined. 

T A tessellation of integer valued elevations which approximates L. 

V a b Visibility function of observer at P a and target at Pb- 

Ci The class of the point at location * in S. 

Cia The class of the point at location i using facility arrangement a. 

/l A function mapping a 2D geographic coordinate to the elevation of 

L at that point. 

dj The error in the approximation of L by T at a given point. 

h t The lowest height above ground for which it is necessary to establish 

a LOS to a target. 

m tx Constant terrain distance between all longitudinally adjacent points 
in M. 


127 








128 


rrigy Constant terrain distance between all latitudinally adjacent points 
in M. 

nc Number of classes of covered points in S. 

rij Number of observer facilities available. 

n <3 The number of possible distinct arrangements of facilities. 

n\t The number of points in M. 

njc The number of points improperly contributing to a LOS. 

npc The number of points properly contributing to a LOS. 

n R The average number of grid point or grid segment crossings along 
an LOS of length R. 

ns The number of elevation data points in S. 

ns, The number of potential observer site points in Sj. 

nvi The number of LOS rays used in calculating a visibility index esti¬ 

mate. 

p a A point in M corresponding to a point P a in L. 

wid Weight (value) of covering a point in class Ck by t facilities. 

zl The elevation of a point in L. 

zs The elevation of a point in S where elevation is defined. 

0() notation bounds a function to within constant factors. 

0() notation gives an upper bound for a function to within a constant 

factor. 

f2() notation gives a lower bound for a function to within a constant 
factor. 







GLOSSARY 


4-Neighbor Adjacent Relationship between two points that are immediately 
north-south or east-west adjacent on a grid. 

8-Neighbor Adjacent Relationship between two points that are immediately 
north-south, east-west, northwest-southeast, or northeast-southwest adjacent 
on a grid. 

ALBE Air-Land Battle Environment. An evolving set of terrain analysis applica¬ 
tions sponsored by TEC which serves as a testbed for new concepts. 

AM Altitude Matrix. A regular rectangular grid of point measures that forms the 
most common type of Digital Elevation Model [Wal89]. 

AOI Area of Interest. The total area which must be represented to evaluate a 
potential siting solution for a given situation. 

Controlling Slope With respect to a given observer, the slope of a line segment 
beneath which no point is visible on or over the terrain for which that slope 
is the controlling slope. 

Covering Triangle A triangle defined by two grid line segments with one common 
endpoint which is assumed to be no lower than any corresponding terrain 
points. 

DEM Digital Elevation Matrix. 

DMA Defense Mapping Agency. 

DTED Digital Terrain Elevation Data produced by DMA, typically in cells repre¬ 
senting an area of one degree of latitude by one degree of longitude. DTED 


129 











130 


Level I data has a typical grid spacing of three-arc-seconds, while typical 
DTED Level II data has grid spacing of one-arc-second. 

FSO Field Support Office, TEC. 

GIS Geographic Information System. 

GPS Global Positioning System. A system of orbiting satellites and associated 
standards for receiving their signals which allows determination of relatively 
precise 3D locations anywhere on the surface of the earth. 

HAWK A mobile medium-capability air defense missile and radar system capable 
of tracking and eng a g in g targets within a 360° circle of its emplacement site 
subject to minimum and ma ximum target range and altitude constraints. 

KB Kilobytes — approximately 1 thousand bytes. 

LAN Local Area Network. A network connecting stations residing in essentially 
the same geographic location. Throughput is typically on the order of 1 MB 
per second (max) delivered to each station. 

LOS Line-of-Sight. 

LOS Height The lowest height at which a target at a given location is visible from 
the observer. 

MAUP Modifiable Areal Unit Problem. Affects on the values, relationships, and 
inferences derived from spatial data due to the number and size of the zones 
used to report the data. 

MB Megabytes — approximately 1 million bytes. 


NFS Network File System. 







131 


Radius of Visibility The maximum geographic distance (meters or kilometers) 
for which visibility from the observer is significant. This might be the maxi¬ 
mum range of a radar, or of optical visibility under prevailing weather condi¬ 
tions. 

RMSE Root Mean Squared Error. 

SPECmark A relatively standard measure of CPU performance between different 
processors. 

TEC U.S. Army Topographic Engineering Center. 

TerraBase A terrain database system for MS-DOS based personal computers orig¬ 
inally developed at the U.S. Military Academy and now supported at the FSO. 

TIN Triangulated Irregular Network. 

USGS United States Geological Survey. 

Viewshed The points within the radius of visibility that are visible from the ob¬ 
server. 

Visibility fraction The ratio of the number of points in the viewshed to the total 
number of points within the radius of visibility. 

Visibility Index A single numeric measure of the amount of terrain which is visible 
from a particular point compared with other possible observer points in the 
same dataset. 

WAN Wide Area Network. A network connecting stations that are geographically 
distributed, generally with throughput an an order of magnitude less than 
found on LANs. 











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Symposium Proceedings, pages 112-121, July 1992. 




APPENDIX A 

Computation and Storage Costs of Visibility Determinations 


As discussed elsewhere, the general case of establishing a LOS from an observer to 
all surrounding grid elevation points requires running a unique evaluation to each 
point for which an existing collinear LOS (to a previous point) cannot be extended. 
The actual execution time on a Sun IPC workstation with a performance level of 
approximately 11 SPECmarks provided a mean throughput of 256 point-to-point 
LOS evaluations per second with a maximum radius of visibility of 40fcm in DTED 
cell N37E127. This distance was selected from [Jan89] and [JanSO] as representative 
of existing aH projected medium capability air defense radar ranges. 

A standard DTED Level I cell represents a rectangular area approximately 
lOOfcm on a side, which is a realistic magnitude for communications or radar plan¬ 
ning. The grid for such a cell is typically 1201 x 1201 for a total of 1,442,401 
elevation points. Given a three-arc-second grid spacing in each cell, the ground 
distance (North-South and East-West) between points varies between 60 and 100 
meters, depending on the specific latitude. The rest of this analysis uses an average 
figure of approximately 80 meters (this matches the 1200 grid spaces per side of a 
DTED Level I cell covering approximately lOOfcm). Since each elevation point is 
measured in meters, a two-byte integer (which can represent values between 32,767 
to -32,768 meters of altitude from mean sea level) is a common storage unit for ele¬ 
vations. The total storage requirements to represent a 10, OOOArm 2 area is therefore 
approximately 3MB. However, the need to include surrounding areas out to the de¬ 
sired radius of visibility requires additional storage. Extending data coverage 40 km 
in each direction requires a grid of approximately 2200 x 2200 points for a storage 
requirement of approximately 10MB. 

A program to calculate visibility values on the Sun IPC was coded in the 


138 




139 


G++ implementation of the C++ programming language, using all-integer operations 
in critical program segments. Actual throughput for 40 km LOS evaluations was 
approximately 128 evaluations per CPU second. When evaluating LOS’s to each 
point on a fixed number of LOS rays within a AOkm radius of visibility, the average 
distance will be 20 km, implying a throughput of approximately 256 LOS evaluations 
per CPU second. Allowing for am improvement by a factor of four in some other 
common environment (more powerful hardware, better optimizing compiler, etc.), a 
throughput of approximately 1000 LOS evaluations per second under the conditions 
stated above seems reasonably obtainable, implying 1 millisecond of CPU time per 
LOS evaluation. 

The computational cost to evaluate a given LOS will vary linearly with the 
density of grid points to ground distance. This is because determining the LOS 
between two points requires values (or estimates) of elevation based on all elevation 
data points in between. If the cost to calculate a LOS between two points is 0.001 
seconds when the average grid spacing is 80 meters, it would take approximately 
0.002 seconds if the average grid spacing were 40 meters. 

The estimated CPU time to evaluate the LOS from each point in a lOOOOfcm 2 
area to each point on the elevation data grid within a 4 0km radius of visibility in 
terms of the ground spacing between grid points can be calculated from the following 
relations: 

Akm = (lOOfcm) 2 
Nkm = lOOOm/S* 

No = A k jN 2 km 
N c = ( nxR 2 v )/S 2 g 


Tio, = (0.08sec/m)/S s 


(A.l) 





140 


T iot = No x N e x (A.2) 

where 

Atm Area containing potential observer points, in km 2 . 

Nkm Number of observer points per linear km. 

N c Number (approximate) of target points in a circle of radius Ry meters from a 
given observer point. 

N 0 Number of observer points. 

Ry Radius of visibility, 40,000 meters in this case. 

S g Average ground spacing between points, in meters. 

Tio, Time (mean) to evaluate a single 20km LOS, estimated as 0.001 seconds when 
S a — 80 meters. 

T to t Total time to evaluate a LOS to each target point from each observer point, in 
seconds. 

If D g is defined as the average linear density of grid points per kilometer 
(lOOOm/5*), then from the above relations an expression for total time is found to 
be: 


T^t = (lOOfcm) 2 x D] x 7T x (40fcm) 2 xD|x 0.00008 x D g 
This reduces to: 

T tot = (1280tt)D® (A.3) 

so clearly 

t m = e(D‘) = 0(DD = SJ(OJ) 
for this particular visibility algorithm, where [CLR91]: 

• 6 notation bounds a function to within constant factors. 

• O notation gives an upper bound for a function to within a constant factor. 


141 


• ft notation gives a lower bound for a function to within a constant factor. 

Therefore, the total time to evaluate the LOS from each potential observer point to 
all corresponding target points will vary with the linear data point density raised to 
the fifth power. 





APPENDIX B 

Formal Definition of the HAWK Siting Problem 

The variety of approaches to the problems of visibility determination and facility 
site selection have naturally led to different forms of notation in describing specific 
problem domains. The following description attempts to make use of aspects com¬ 
mon to several existing definitions ([DFN 88 ], [Lee91a], [SI71], [CR74]). The model 
of the elevation data and its relationship to the physical terrain setting are described 
in Chapter 3. 

Consider a surface S which contains a total of ns elevation data points and 
corresponds to the landscape L that contains the AOl for a given siting problem. 
Let h a be the effective height of the observer (in this case a HAWK radar antenna) 
above the ground and let h t be the minimum height above ground level at which it 
is deemed necessary to track a target. To be able to accurately track (as opposed 
to just detect) an aerial target, the target must be visible from the observer. In this 
model points P a and Pb in three-dimensional space are each visible from the other 
if a straight line between them is not beneath the surface S at any point where 
the elevation zs is defined. Stated differently, if for any point (x/, yi, zi) on the line 
connecting P a and Pj where (x/, yi) € Sxy it is true that zj > zs then P a is visible 
from Pb and P 4 is visible from P a . 

The symmetry in visibility between P a and Pb in general three-dimensional 
space no longer holds when we consider points which are the projections onto S 
of potential observer and target locations. If an observer O a is positioned at point 
(*a,y«, 2 «), where (x a ,y a ) € S xy and z a = /s(x s , y a ), then for visibility determina¬ 
tion we consider lines-of-sight to originate at (x a , y a , (z a + h 0 )) and extend out to 
(x&>y»> (*fc + /»<)) where (x^yj) € Sxy. Since in general h 0 ^ h t , if an observer at P„ 
can see a target at Pb this does not imply that an observer at Pb could see a target 


142 








143 


at P a . Define the visibility function Vab to have the value 1 if a target no lower than 
height h t above point Pb on surface S is visible from an observer at height h Q above 
point P a on surface S and the distance between P a and Pb is not greater than some 
constant R. Let n R be the average number of crossings corresponding to points in 
M (see Chapter 3) where a LOS of length R crosses a grid line segment. 

V a b has a value of 0 otherwise. Note that the value of V a b implies nothing about 
the value of V^,. If V a b = 1 we say that point Pb can be covered, from an observer at 
point P a . 

Let nj be the number of facilities (HAWK units) available. The number of 
units (0...n/) covering the various points in S determines the effectiveness of a 
particular arrangement of HAWK units in an AOI. For each location i € S, where 
i = 1,2,... ,ns let /, = 1 if there is a facility at location i and /, = 0 otherwise. 
The number of facilities at any given location is always either 0 or 1. 

Not all points in S will have the same importance for coverage. Each point in 
S is considered to belong to exactly one of nc classes for the purposes of evaluating 
the value (weighting) of covering it. Let c,- be the class of the point at location i. 
Within each class Cfc, k = 1... n c , a different weight exists which corresponds to the 
number of facilities (0.. .n/) covering a point in that class. Let tvy be the weight 
assigned to the value of covering a single point in class Ck by exactly j facilities. We 
impose no particular restrictions on the values chosen for these weights other than 
—oo < w < oo, although a typical scenario would have 0 < tv < co with the value 
of Wkj increasing as j increases. 

Let 5/ be the set of points in S where it is possible to place a facility, and 
let ns, be the number of such points, ns, < ns, and generally ns, = 0 (« 5 ) over 
various AOI’s. If k € 5/ then /* = 1. In typical real world situations nj <C ns r Let 
G a be an arrangement of facilities, a € 1 ,..., no, such that for each arrangement 
there are exactly n/ locations * € 5/ for which /< = 1. There will be no possible 







144 


distinct arrangements, where 


no = 



n/!(n s , - n/)! 


(B.1) 


For a specific problem with a fixed number of available facilities n/ = k and where 
nj C ns ,, then n/\ = k\ and we can approximate the variation in no with ns, as 


nc « ( T *s / ) n/ 


(B.2) 


Observe that if the linear density of points on the ground (see Table 1.1) doubles, 
then the number of possible arrangements can be expected to increase by a factor 
of approximately 4*. 

We can now state the initial, or static weight HAWK siting problem as finding 
an arrangement G a which maximizes 


where 


•es 


(B.3) 


I>,= Z Ki 

p€G m 

few locations p € G a . Note that this formulation implicitly assumes that there is no 


change in weights between different arrangements, and therefore no value is imputed 
to coverage of one facility by another. 

Since in the real world this is often not the case (air defense units are sited to 
cover each other, when possible), an alternative formulation to represent the value 
of overlapping coverage becomes finding an arrangement G a which maximizes 

£ w «.>i ( B ’ 4 ) 

«€5 

where c, 0 is the class to which location i belongs under arrangement a. This problem 
is referred to as the dynamic weight HAWK siting problem. The class of a given 
location will vary depending on whether coverage of that location is considered 
significant in defending one or more HAWK sites in any given arrangement. 







145 


It is useful at this point to emphasize that it is necessary to perform the sum¬ 
mations in either problem formulation for all locations t € S, not just locations 
where specific targets (or HAWK sites, in the dynamic weight formulation) are lo¬ 
cated. This is due to need to intercept or deter potential attackers before they have 
approached close enough to their targets to release stand-off munitions, which have 
a range of several kilometers or more. The need to evaluate the contribution of all 
covered points in S also conforms well to the problems related to siting communi¬ 
cations facilities and other applications influenced by visibility considerations. 







r 


APPENDIX C 

Investigation of Elevation-Visibility Relationships 

A variety of factors influence the degree of correlation observed between elevation 
and visibility at given points. These include but are not limited to the roughness 
and extent of the sample area, presence of significant coastline and sea level areas, 
considering all or only selected sets of points in the sample area, the model used 
to relate elevation and visibility, and the method used to calculate the visibility 
index for observer points. The results of some of these correlation calculations are 
presented here in tabular form for two models. The linear model assumes that for 
the area sampled 

Vi = 0o + 0\Zi + £j (C.l) 

while the quadratic model assumes 

Vi = 0o + 0i zf + e,- (C.2) 

where V is the visibility index at location *, Z{ is the elevation at location t, 0q and 0i 
are parameters, and Ci is the error between the predicted and actual visibility index 
at location *. The empirical basis for not including a linear term in Equation C.2 
is presented in Chapter 6. The most significant results of the regression analysis 
are the sign and magnitude of the correlation coefficient r. The value of r 2 (the 
correlated variation) reflects the portion of the variation in the value of Vi that is 
explained by the assumed relationship to z,. Many other models relating elevation 
and visibility can be constructed; here we attempt only to examine the results of 
these two simple models over a variety of terrain areas for which elevation data is 
available and visibility index values have been computed. 

Results obtained by applying the two models described above to several DTED 
cells are shown in Table C.l. Points with elevation values of zero (i.e., sea level) were 


146 









Correlation of Visibility to Elevation: Best Points 


Cell 

Name 

Non-0 

Elevs 

Vis Index 
Range 

Best 

Points 

r 

Linear 

r 

Quadratic 

Cell 

Type 

N37E127 

1441304 

144-16976 

2071 

■9 

0.161028 

Mountain 

Lowland 

N36E127 

1439567 

160-19800 

9980 

KH 

0.201289 


N36E128 

1442399 

160-16648 

690 

KfiH 

0.176861 

Mountain 

Upland 

N36E126 

759014 

184-31704 

1529 

-0.024374 

-0.052767 

Coastal 

N37E127-W 

1441304 

8-14624 

582 

0.068689 

0.018259 

Mountain 

Lowland 


Table C.l: Best Points Visibility—Elevation Correlations. 


excluded from the correlation calculations, but not in the calculation of the visibility 
index values for points that are considered. As mentioned elsewhere, the potential 
values for a visibility index for a given point may range from 0 (perhaps from the 
bottom of a well!) to 32767 (all points within radius R are visible). The number 
of observations used in calculating correlations varies with the number of sea level 
points excluded. N36E128 has almost no points excluded (from a maximum possible 
of 1,442,401), while N36E126 is almost one-half ocean and has almost 50% of its 
points excluded. The ‘-W’ in the cell name N37el27-W indicates that in computing 
the visibility index values, the contribution of each point along each of the 32 LOS’s 
calculated was weighted approximately according to its distance from the observer 
point. This variant is included to evaluate the impact of treating each point along 
a LOS as representing a sample of all unevaluated points closer to it than to any 
other evaluated (i.e., on a LOS) point. 

Several significant trends can be noted in Table C.l. First, the number of 
points with visibility values in the top half of the total range of values for a given 
cell is much smaller than the total points in the cell. For example, for N37E127 out 
of 1,441,304 non-sea level points in the cell, only 2071 had visibility index values 
















































148 


in the top half (i.e., V< > 8560). This provides evidence for the hypothesis that 
visibility index calculations can identify a relatively small group of points out of 
the total set of potential observer locations for further investigation. N37E127-W 
produces an even smaller number (582) of points with visibility values in the top 
half of the total range (i.e., Vi > 7316). However, more detailed investigation reveals 
that points selected from weighted processing tend to cluster together more than 
in unweighted processing when ranked by visibility index, and can actually require 
consideration of a larger number of points to achieve a desired minimum number of 
distinct geographic locations as candidate observer areas. 

Another observation to be made from Table C.l is that magnitude of the 
correlation between elevation and visibility index is never large, and for the cells 
shown the value of r 2 for either model never exceeds 0.073684 — hardly a vote of 
confidence for using either of these elevation-based models to find the best observer 
points. In some cases the correlation (such as it is) is in fact negative. The results 
for N36E126 may be heavily influenced by the presence of substantial coastal areas 
(with relatively low elevations) which have extensive visibility out over the ocean. 
Whether or not visibility over the ocean is of the same import as visibility over land 
is a function of the specific application; here they are treated equally. 

Achieving marginal results in this initial attempt to use elevation as a guide to 
determine points with the best visibility, Table C.2 shows the results of applying the 
same models to the more general problem of relating elevation and visibility for all 
(non-sea level) points throughout an area. Also shown are some results of the impact 
of changing the extent of the total area correlated by presenting values for subareas 
of N37E127-W. Cell names with lowercase letters and suffixed with a lowercase ‘w’, 
‘x’, ‘y’, or V are quadrants of a complete DTED cell. A ‘w’ suffix indicates the 
southwest quadrant, ‘x’ the northwest quadrant, ‘y’ the southeast quadrant, and V 
the northeast quadrant. 








149 


Correlation of Visibility to Elevation: All Points 

Cell 

r 

r 

Remarks 

Name 

Linear Model 

Quadratic Model 


N37E127 

-0.119575 

-0.016950 


N37E127-W 

0.196629 

0.243252 

Weighted 

N36E127 

-0.236553 

-0.098787 


N36E128 

0.041409 

0.11115 


n37el27w-W 

0.291576 

0.378529 

SW Quadrant 

n37el27x-W 

0.214607 

0.263673 

NW Quadrant 

n37el27y-W 

0.257636 

0.281329 

SE Quadrant 

n37el27z-W 

0.500397 

0.557214 

NE Quadrant 

n37el27z-W 

0.205598 

0.170632 

164 Points in Top 
50% of Vis Range 


Table C.2: All Points Visibility—Elevation Correlations. 


For several complete cells the results are no better — or even significantly 
worse — than correlations for best visibility points from Table C.l. In some cases 
(N37E127, N36E127) the sign of the correlation has switched from positive to neg¬ 
ative. However, the correlation for N37E127-W for both models has manifested a 
marked improvement. Selecting this cell for further investigation, subsequent en¬ 
tries in the table show the impact of narrowing the scope of the areas examined to 
cell quadrants. There is significant variation in the degree of correlation between 
elevation and visibility among the four quadrants, with n37el27z-W manifesting the 
highest correlations. For this area, in both the linear and quadratic models nearly 
25% of the variation in visibility can be matched to elevation. 

It could be hypothesized that by starting with the cell and LOS estimation 
method yielding the highest initial correlations (N37E127W), examining subareas 
within it (its quadrants), and selecting the subarea with the highest internal corre¬ 
lation between elevation and visibility (n37el27z-W) that for this restricted set of 
data it might be possible to obtain useful results in using elevation values to predict 
the best visibility points. However, as shown in the last entry for Table C.l, the 










150 


correlated variation for the 164 points occupying the top 50% of the range of visi¬ 
bility values is still quite low (r 3 of approximately 0.04 or less) and are in fact below 
the results of Table C.l using unweighted visibility index values for all of N37E127. 

In summary, over the visibility measures, models, and sample areas and sizes 
presented here, there is no evidence of any straightforward relationships between 
elevation and visibility that would be useful in quickly selecting most of the best 
observer sites in a given region. Other models relating elevation to visibility are 
of course possible, such as those making use of several adjacent elevation values. 
Given the presence of visibility information for a sufficiently encompassing region, it 
may be possible to produce a variety of methods more effective than the two models 
used here through genetic algorithm development or by employing various heuristics. 
More sophisticated models will of course involve higher computation costs, and in 
any case would still require a base of visibility estimates or rankings (such as those 
used here) to evaluate their success in selecting promising observer points. 








APPENDIX D 

Demonstration to User Community 


Although the generation of a visibility grid corresponding to the elevation data 
for an AOI provides the basis for extracting concise information for a variety of 
siting algorithms, it also offers the opportunity to provide a display to augment 
existing ways of visually representing terrain considerations for planners and decision 
makers. Many users of digital terrain data who could provide useful insights into 
various methods of rendering a visibility image may not have access to workstation 
environments common in research and academic settings. 

In an attempt to make some form of visibility display available to users with 
relatively restricted computing resources, a standalone demonstration system was 
developed whose primary screen display is shown in Figure D.l. It can execute on 
any Intel 80x86 architecture running an MS-DOS or compatible operating system 
with a standard VGA (640 x 480 x 16 color) display. The entire demonstration 
package is distributed on a single 1.2MB 5.25 inch floppy disk, which can hold the 
demonstration program, background and operating information text, and elevation 
and visibility data for 12 areas corresponding to one-degree DTED Level I cells. 
Also provided for each area are lists of the 16 best and 64 best visible points, as 
selected from the entire cell and as selected from within 16 equal sub regions with 
a cell to reduce clustering. 

To accommodate the size and color limitations consistent with reaching a 
large audience, the available elevation (16-bit) and visibility (effectively 12-bit) data 
are sampled from a 1201 x 1201 grid down to a 300 x 300 grid. Elevation and 
visibility values are each scaled into thirteen discrete values in a 4-bit nibble, leaving 
three reserved colors for ocean coloring and two separate highlighting colors for 
selections of best visibility points. As a result of this reduction, a combined total 


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152 





Figure D.l: Screen from PC Demo Application. 


















153 


of approximately 5.8 MB of elevation and visibility data in one DTED Level I 
cell is transformed into approximately 90 KB of data for use in the demonstration 
program. While certainly lacking the detail present in the original data, initial 
response from the ALBE group at TEC indicates that the reduced data remains 
useful and representative. 

Comments on the initial implementation of the demonstration set led to an 
enhanced version which provides for legend displays. User configurable palettes 
were suggested and implemented to allow unanticipated uses of the data, such as 
hi ghli ghting areas of very limited visibility (for concealment) by reversing the normal 
palette. An application based on this demonstration set is being implemented at 
TEC in a UNIX X-Window environment to be integrated into the existing ALBE 
software suite for fielding in the first part of 1994.