Digital true orthophotos generation
Ettarid, Mohamed ... [et al.]
This paper aims at developing software for the establishment of digital true orthophotos. First, a review of different approaches developed for the generation of true orthophotos is given. The Z-buffer approach that has proved to be efficient in detecting and dealing with hidden and occulted areas on the aerial images is adopted in this paper. The technique is based on the fact that forward features, near to the perspective centre, will hide backward features on the object surface. Therefore, a Z-buffer matrix is built, which memorizes, for each pixel, the minimum distance between the camera and the surface along the perspective ray. Hence only the first point of the surface met by the perspective ray will appear on the orthophoto image; the other points on the surface along this same ray are occulted and will be omitted and their corresponding pixel on the ortho are considered void. The different modules of the software were developed in C language, and experiments were conducted on aerial photos at scale 1/7500 flown on a sub urban area of Meknns region in Morocco. To generate the digital orthophoto, a digital terrain model (DTM) derived by conventional photogrammetric stereo plotting was combined with 3D building stereo plotting to build a digital surface model (DSM) used in the ortho rectification process. The quality of the generated orthophoto was based on the root mean square error computed from differences on check pointts coordinates measured on the orthophoto against those determined by conventional surveying methods. Vector data acquired by conventional stereo plotting is also superimposed on the generated true orthophoto and has shown perfect correspondence of features.
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