An original method that rigorously generates digital orthophotos from SPOT images is presented. In place of the ray tracing method, we have developed an algorithm that determines the corresponding image coordinates for a ground element, provided that the time dependent orientation parameters and a DTM are available. In this paper, a rigorous bundle adjustment program for pushbroom scanning images that models the dynamic orientation for SPOT data is first developed. Incorporating the DTM, a non-linear equation is formulated that determines the image coordinates in the flight direction in terms of the sampling time for a ground element. The Newton-Raphson method is then applied in order to solve the non-linear equation. The along-track and across-track image coordinates for the corresponding ground element can thus be calculated. After a bilinear interpolation, the pixel grey value for the ground element in the orthophoto is determined. Experimental results indicate that the generated orthoimages attain an accuracy better than two-thirds of a pixel.
