SPOT data should come with metadata that allows the georeferencing and/or orthorectificaiton to be automated via Map ->Georeference SPOT or Map ->Orthorectification ->SPOT.  Does your SPOT data come as DIMAP (with a .dim file)? If not and you must perform a manual registration, there is no one answer to your question.  More complex surfaces such as those in mountainous areas typically require polynomial equations to model them.  But even better would be to perform an orthorectificaiton with an associated DEM to account for the complex terrain.

Thank you very much for your suggestion.   Unfortunately I lack the necessary RCP file. I'll try to see if I manage to build them myself.

Hello again. I tried to georeference my image with Georeference SPOT but since my leader file is XML, ENVI shows an error. The orthorectification doesn't work either given the lack of the RCP file. So I tried the option for retaining the RCP information suggested on the ENVI Help, but no luck, I think again is because of my file XML extension. Is there any workaround for this? The problem that I have with buidling the RCPs myself, is that I don't have the elevation information for my GCPs.   Thank you

For your landsat image, you can find an MRLC landsat image of same path/ row then do an automatic image to image registration (warp) in ENVI. The MRLCs have already been terrain corrected.

Thank you!