Ryan has a couple of posts on the difference between orthophotography — geometrically corrected aerial photography — and “true” orthophotography; true orthophotos “add the dimension of correcting for the distortion of buildings. Or, simply stated, true orthophotos do not show building lean.” Significant in urban areas rich in skyscrapers, where the building (or its shadow) can obstruct the view of at least part of the surrounding area. Producing true orthophotos requires tighter overlap in the stereo imagery taken or special processing techniques. See Eastern Topographics’ page on orthophotography for an example of the former, and Morten’s 2004 master’s thesis on orthophoto generation for an example of the latter. (Both linked from Ryan’s posts above.) True orthophotos are, in fact, mosaics of several images.