I've made a DEM from first pass LiDAR data which I want to invert and run watershed segmentation and basin delineation over in order to hopefully extract tree canopy outlines and heights - I'm newish to GIS.

  • What GIS software are you using? – PolyGeo Feb 19 '16 at 2:35
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    Multiply your raster by -1 or subtract it from 0. – user2856 Feb 19 '16 at 2:53
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    Possible duplicate of Canopy Height Model of LiDAR data in QGIS? – Andre Silva Feb 19 '16 at 9:09
  • Thanks - I think I need to both - subtract the DSM from the DTM but also run the watershed segmentation and basin delineation over the inverted DSM to try and extract canopy outlines and approximate trunk location - (Edson 2011) - though it's an urban condition so it may just fail due to the buildings. – N-particle Feb 20 '16 at 10:14

I would subtact the last pass and the first pass elevation if you want the height. You could also possibly add a constant (e.g. 100) in order to have positive values but this is more an esthetic consideration.

by the way, DEM is used for ground elevation, in your case it is a DSM.

in the raster calculator (assuming you are using QGIS)

dem@1 - dsm@1

replace dem@1 with any constant value if you don't have a DEM at the same resolution as your DSM.

  • Thanks - I do have both at the same resolution so will try this and then just use the inverted DSM to hopefully get canopy outlines and high points... – N-particle Feb 20 '16 at 10:23

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