1. A method for orthorectifying oblique photographic imagery, comprising: (a) projecting an oblique photographic image onto three-dimensional model of terrain; (b) sampling points of the projected photographic image from a viewport having a nadir perspective, the points being at the intersection of the three-dimensional model of terrain and parallel rays extended from the viewport; (c) assembling the sampled points into an orthorectified image; and (d) adjusting the orthorectified image or the sampled points approximately according to a difference in tilt angle between a camera that captured the oblique photographic image and the viewport having the nadir perspective to generate a foreshortened orthorectified image, wherein each location in the foreshortened orthorectified image corresponds linearly to a corresponding location in a two-dimensional map.