Hi I am using ENVI 4.1 to orthorectify some aerial photos I have. The process seems to be working fine except that I have areas on black on the finished product. I think that this is a problem with the DEM I am using where there are areas of No-Data possibly, causing this omission of the photo. The DEM I am using is a geotiff which has been converted from an asc file. But I don’t think it is in the best format. (Or data was not converted properly) I have the original ASC file, can anyone suggest the best format and process to convert the ASC file into a reliable DEM which will hopefully give me better results in the Orthorectification process. Any help will be greatly appreciated. Neil