Comentario oficial
A Flat Map is taken by just taking the images and plopping them down on the ground where the pointing tags say they intersect with the earth. There is no attempt to match features. The only merging is done by blending the edges. The idea is that it gives you something "close enough" when feature matching fails. This is useful when working in dense trees, snow, sand or large swaths of water when feature matching is likely to fail.