What is Stereoscopic Overlap?

In air or satellite imagery, scenes are often captured with an overlap of 30-40% so that the images can be viewed in stereo (3D). This means that part of an image is present in many images, but from a different angle. An example of stereo overlap is the human eyes, which see the same objects from different points, with overlap, which allows the determination of distance and depth. This technique allows the measurement of the height of objects and of the topography, and is how many of the older topographic maps were made.