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?


The clipping can be done with:

v.overlay ainput=<dtm_tin> binput=<polygon> operator=and

See: the manpage for v.overlay

Regarding the DTM and contours, can you check the attribute table of your imported triangulation and report back if the elevation appears?

My guess is that it will be easier to just import the original lidar points (not the triangulation) and create your DEM and contours from the points directly.

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