Point clouds from dense-matching photogrammetry are popular. Stored in the LAS format – or its compressed LAZ twin – these “near-LiDAR” points are easily ingested into LiDAR processing software to, for example, generate a Digital Surface Model (DSM). “Can LAStools also create a Digital Terrain Model (DTM) from such data?” is a question we often get asked by private email or via the LAStools user forum. At INTERGEO Euroasia 2014 we gave a demo of this at the UltraCam booth. We repeated this demo with different data at the Pix4D booth during the Geospatial World Forum 2014. Now the folks from Visionmap have also become curious …
Above the “topography_cadastre_switzerland_densified.las” point cloud we got at the Pix4D booth. You may have already seen this colorized point cloud at some other Pix4D event. The original points were in a highly incoherent spatial order that negativly affected all subsequent processing. Therefore we first use lassort to reorder the points into a space-filling Hilbert curve. At the same time we rescale the coordinate resolution to centimeters and compress the file into the LAZ format.
lassort -i topography_cadastre_switzerland_densified.las ^ -rescale 0.01 0.01 0.01 -olaz
lasground -i topography_cadastre_switzerland_densified.laz ^ -city -ultra_fine ^ -odix _g -olaz
las2dem -i topography_cadastre_switzerland_densified_g.laz ^ -keep_class 2 -thin_with_grid 0.125 -extra_pass ^ -step 0.5 ^ -ocut 2 -odix _dtm -obil
las2dem -i topography_cadastre_switzerland_densified_g.laz ^ -thin_with_grid 0.125 -extra_pass ^ -step 0.5 ^ -ocut 2 -odix _dsm -obil
Did you know that lasview can visualize BIL files via on-the-fly conversion from grids to points? Above you see the generated DTM and below the corresponding DSM. So yes, LAStools can create DTMs from points that are result of dense-matching photogrammetry … under one assumption: there is not too much vegetation.