Toronto. During the war, aircraft would routinely pass over enemy territory using cameras such as this beast (used by the late George Hunter long after the war) to take long runs of photographs.
Stereo uses two images at slightly different angles. If each image is restricted to one eye, the two images are fused in the brain to create a stereo image. If the angle is exaggerated, the images still merge but as hyper stereo images.
War time, selected photographs from the long run became stereo pairs. This pair of photos were viewed with a tall dual lens viewer and translated as topographical outlines on a map.
In hyper-stereo, what looked like flat lands suddenly became hilly or even mountainous. Things hidden under a camouflage cover suddenly became outlined and dimensional, no longer part of a forest or field.