You handle this using the DIMS keyword. You need to figure out the DIMS you need for the map extents you want. If you know the coordinates of the extents, then you can use ENVI CONVERT_FILE_COORINATES to find the image coordinates to use based on those map extents.

That doesn't quite produce the desired result. The output extents of course form a rectangle, but in the input coordinate system, those extents from a parallelogram. In only being able to specify the min,max X and Y dimensions, I must set a rectangle that encloses the output extents' parallelogram but which will include some data that will not really be in the output scene. So this approach might reduce the amount of extra data, but it's not really going to produce an exact match to an existing file. In one of my searches on this issue I saw a suggestion of perhaps using theenvi_layer_stacking_doit with the /exclusive flag. It will cause the output to include the 'sample' band in the output data which could subsequently be removed. Seems to me there must be a better, albeit undocumented, way to do this since the interactive GUI can do it and the layer stacking process does something very similar.

Unfortunately, ENVI_CONVERT_FILE_MAP_PROJECTION does not currently provide a mechanism to reproduce the behavior that you can get in interactive ENVI by choosing the "Match Existing File" option. I agree that it would be great if it did. I think you might be right that the fastest way to get what you want would be to use ENVI_LAYER_STACKING with the /exclusive flag. Your inputs would be your image of interested and another image in the desired projection to which you want to match the extent. Then after the layer stacking remove the one extra band that you used to specify the extent. - Peg