The Russians used satellite images, high altitude aerial reconnaissance photographs, and spies on the ground to gather the information and send it back to 50,000 cartographers in Moscow. www.sovietmaps.com has a nice write-up on the history of the most extensive mapping project ever undertaken, complete with KGB-era maps of most of the U.K. According to the website, these maps were discovered in abandoned train carriages in Latvia and Estonia following the 1991 collapse of the Soviet Union and the Soviet Military's subsequent speedy retreat from the Baltic states. Fascinating.