I have a large DXF format three dimensional DTM triangulation file from a LiDAR survey (about 3.5 GB file). I wish to extract part of it from inside a polygon. I was impressed that GRASS successfully imported the DTM using the GRASS import menu (v.in.dxf) as several other tools I have tried failed to do this. I have also imported the polygon which I intend to use to clip the DTM using the same menu.
Could someone provide guidance by means a simple example, as to how I achieve the clipping to the boundary, prepare contours of the clipped region to a desired interval, and then export clipped DTM and contours as 3D .dxf file so I can use these results in other software packages such as QGIS?