The RegridRaster task actually does something a little different than what you are trying to do. RegridRaster reprojects a raster from one standard coordinate space to another, while allowing the user to define the pixel size and coordinate system to be projected to. ENVI has a number of tasks related to this and the underlying idea is that when performing any sort of change detection on multitemporal data, the data must be in a common grid and have the same pixel size in order to achieve accurate results. To convert from RPC coordinates to a standard projection I think that the task that you would want to use would be the RPCOrthorectification task. Please take a look at the following link: http://www.exelisvis.com/...ctificationtask.html The input raster must be an image in RPC space with valid RPCs associated with it. The example in the above link includes reference to a Ground Control Point file and an Output Pixel Size, but these are optional parameters. If you do not have an associated DEM, you can use the GMTED2010.jp2 World DEM that ships with ENVI. That is located here: C:\Program Files\Exelis\ENVI5x\data Please let me know if this doesn't do the trick.