ANALYSIS OF DEMs FROM ACROSS TRACK STEREO PAIRS FOR A PART OF KACHCHH REGION

Orbital stereo data is acquired by covering same area from two exposure stations in space with oblique viewing capability. It is used for generating digital elevation models (DEM) and three dimensional viewing of the terrain in a softcopy photogrammetric workstation. A good base to height ratio (B/H), usually greater than 0.5, is necessary for generation of good quality absolute DEM. Processing of the data requires a few GCPs, image matching and interpolation. Therefore DEM accuracy also depends on these factors. Good image matching is one of the most important factors in governing the altitude accuracy. Two IRS 1C/1D PAN stereo pairs covering the part of Kachchh sedimentary basin were processed for extraction of DEM and its analysis for geological study. The DEM image in the first pair clearly shows the drainage, the topographic high and variation of elevation in the hilly part of the terrain. The topographic high (water divide) with brighter tone on gray scale image indicates a higher elevation than adjoining terrain. The large difference in solar elevation angles of the images of second pair results in difference in radiometry and finally resulting in difference in image matching hence a poor DEM is obtained. A height accuracy of 5-15m is obtained in first case whereas in second case due to large plannimetric errors a height evaluation has not been attempted. Though Cartosat-1 satellite, a dedicated Indian mission for along track stereo acquisition and DEM generation, is already in orbit, however experience gained from using across track PAN data from Indian Remote Sensing satellites IRS 1C and 1D, is highlighted here.