Thanks EJ, but I don't think we're talking about the same thing. In vw11.5, after making a DTM, you could select a 2D polygon and go to AEC>DTM Processor>Set Border. That would set the class of the polygon to "DTM-border" and would cut the DTM like a cookie cutter the next time it was calculated. I'm trying to do the same thing now in VW12.