Dear all  I am orthorectifying some Spot and Landsat images for my study site with ENVI 4.5, and I have a few questions regarding the correct workflow and some important details: 1) A total of 5 Spot images and 4 Landsat make the picture for my study site. So I was wondering if I should carried out the radiometric correction and the fusion of the images prior to the orthorectification, or I can do the orthorectification for each one, and the carry out the other corrections and the mosaic to make the final out put image. 2) I will be using a SRTM (DEM) for the orthorectification together with GCPsgathered from a hydrography vector layer digitased from a hard-copy map. I am not sure about the correspondance that needs to exist between the GCP elevation and the DEM. Should the GCP's elevation data be the one from the DEM or that of the hard-copy map? The DEM fits well the hard-copy map but it is not perfect, and I don't see away to register the DEM to the map. 3) I was wondering if there is a way in which on top of GCPs I could also add tie points (like it is done on ENVI 4.6) given the overlap between images.   I hope you can help me with this   Shena