Before satellite images and drones could pinpoint the exact location of enemy targets, warfare was often more like a game of Battleship: a complex series of guesses based on spotty information.

During World War II the British began to change that. They sent their fastest planes not to drop bombs, but to take pictures. By flying over German territory in planes outfitted with several cameras, pilots built a set of images that overlapped one another and could be viewed stereoscopically—that is, in 3-D. These three-dimensional images enabled the British to see for instance, camouflaged ships that would have not been easily spotted in 2-D images.

In this clip from "3D Spies of WWII" by PBS's NOVA, historians and pilots relive the early days of surveillance imaging to explain how those stereoscopic images worked and how they were used. The entire program, airing on PBS Wednesday, January 18 at 9 P.M. Eastern time, reveals more examples of how these images bolstered British war efforts.

Watch Sneak Peak: 3D Spies of WWII on PBS. See more from NOVA.