How Can I export a Portion of a Terrain Element from a Raster DEM

I am trying to get a DTM in InRoads from an ERDAS IMG file. I need only about 10% of the coverage from the file. I cannot get the Scaleable Terrain Model to work in Descartes but I can load the ERDAS IMG file. But the file is too big to export a DTM from it - the software throws an error after churning for a good 20 minutes.

Is there a way to either trim a terrain element to a fence outline or element so the resulting element is a fraction of its original size?