A nice article on the BBC Website about the use of Stereoscopy in the Second World War. While the technique was generally used to help draw contours onto vertical images, the war-time effort employed stereoscopy to identify vertical structures that could be used to launch rockets. Then by using parallax calculations the height of the structures could be calculated.
OK, so what? Well the scale of the operation was immense. Around 36 million images were collected to profile mainland Europe. These images had to be developed and then analysed manually. Factor in the fact that images were collected behind enemy lines by unarmed fighter planes which had to fly low to ensure the resulting images were of suitable resolution to identify key structures and the scale of the mission becomes clear.
Stereoscopy is now performed on computers, the maths is all done for you and editing the resulting DEM can be generated in a handful of hours. The old method of mirrors and moving images around on your desk seems archaic but it wasn’t that long ago that it was standard practice. It is nice to see some recognition for the “back-room” staff who contribute by providing reliable data which can make all the difference. These “back-room” staff still exist today in the armed forces, but i imagine the have access to some serious hardwear.