1. A computer-implemented method for orthorectifying oblique photographic imagery, comprising: (a) for respective points in a nadir viewport, determining a point on a three-dimensional model of terrain located at the intersection of a normal ray extended from the respective point on the nadir viewport and the three-dimensional model of terrain; (b) projecting each point determined in (a) to a position of a camera that took an oblique photographic image to sample a point on the oblique photographic image that corresponds to the respective point in the nadir viewport; (c) assembling the sampled points into an orthorectified image such that each location in the orthorectified image corresponds linearly to a corresponding location in a two-dimensional map according to an angle between the nadir viewport and the camera that took the oblique photographic image; and (d) adjusting a vertical direction of the orthorectified image to correct for distortion caused by a difference in perspective between the nadir viewport and the camera that took the oblique photographic image such that the sampled points are sampled more frequently in a horizontal direction than the vertical direction.