Hi, I have some problem with result of orthorectification my SPOT image. I work with SPOT Level 1b. I have some shapefiles of rivers. Both have the same projection, but the rivers doesnt fit on SPOT, there are shifted about 200 meters. So I decide to provide orthorectification. I created DEM in ArcGIS from contour lines (interval: 5m) by the help of Topo to Raster. But I dont know how orthorectify my image correctly. I tried Orthorectification with RCP, but the result is wrong. I dont know why. I get RCP from the tool Map - Orthorectification-Generic RPC and RSM-Orthoretify RPC or RSM. I choosed one point in middle of DEM and found his latitude and longtitude, than I used calculater on web to compute geoid height (it was 43 meters). But the result is wrong. Than I tried orthorectify with GCP. At first I get RCP (the tool Map - Orthorectification-Generic RPC and RSM-Orthoretify RPC or RSM) and than I choosed 10 GCP - longtitude and latitude and also elevation. But in Help is written that elevation must be relative to the value of geoid offset. I dont understand it. Should I fill the elevation minus 43 or not? I am confused, I try both - just elevation from my DEM as well as elevation minus 43. Results are bad. Can anybody help me? Please, I need help. Could be problem that my DEM cover just small par of whole image? I choosed the points which are also in DEM. I am searching some method, read tutorials, but nothing works.