The technology had originally been developed for military purposes such as missile targetting. Here's a video of how the mapping data is obtained:
Technology Review previously described the process:
C3's models are generated with little human intervention. First, a plane equipped with a custom-designed package of professional-grade digital single-lens reflex cameras takes aerial photos. Four cameras look out along the main compass points, at oblique angles to the ground, to image buildings from the side as well as above. Additional cameras (the exact number is secret) capture overlapping images from their own carefully determined angles, producing a final set that contains all the information needed for a full 3-D rendering of a city's buildings. Machine-vision software developed by C3 compares pairs of overlapping images to gauge depth, just as our brains use stereo vision, to produce a richly detailed 3-D model.