the windows are absolutely irreplaceable, phillipe villeneuve told us. these treasures were spared. there are artisans from about 20 different specialties at work here, some in this medieval place using the most modern of implements. including a drone fitted with special imaging technology. >> it's a high resolution photograph. so i already have the cathedral on my computer. >> in cathedral in the computer. >> reporter: phillipe dillmann is at france's national center for scientific research. he showed us what he calls notre dame's digital twin. >> we have 3-d maps to understand the way they were built, but also to restore them. >> reporter: and they can compare these images with high resolution ones taken before the fire. they've examined how the monument moved, where it was stressed by the fire, and the temperatures at which it burned. they're trying to understand where specific pieces were placed. some materials like stone can be reused. but this piece of wood, for instance, has been burned. that's not going to be able to put back in place. >> exactly. >> reporter: why does it ma