[pdal] Calculation of the difference between DSM and DTM

[Raúl Casado Barbero]

Hi,

my aim is to calculate the fitovolume of a shrub vegetation area. To do 
that I calculate the DSM, classify a point cloud with ground (2) and low 
vegetation (3) and then create a DTM.

I use 2 different pipelines (see below) to do that modified from this 
post <https://github.com/manaakiwhenua/pycrown/issues/18>. But I have 2 
issues:

  * When I calculate the difference between DSM and DTM I get negative
    values
  * The pixels of the DTM are horizontally displaced with respect to the
    DSM by half a pixel.

Does anyone know why I am getting these negative values using this 
pipelines? Is anything wrong in the pipelines I`m using?


Raúl.

Pipeline for DSM:

[
     "nubeRecortadaPdal_1.laz",
     {
         "type": "filters.range",
         "limits":"returnnumber[1:1]"
     },
     {
         "type": "writers.gdal",
         "filename": "nubeRecortadaPdal_1_mds.tif",
         "output_type": "idw",
         "gdaldriver": "GTiff",
         "resolution": 0.05,
         "nodata": -9999,
         "radius": 0.07,
         "window_size": 50,
         "data_type": "float32"
     }
]

Pipeline for DTM:

[
     "nubeRecortadaPdal_1.laz",
     {
         "type":"filters.overlay",
         "dimension":"Classification",
         "datasource":"D:/Datos/clasificadasVector.gpkg",
         "layer":"ndviRandomForest",
         "column":"Id"
     },
     {
         "type":"filters.range",
         "limits":"Classification[2:2]"
     },
     {
         "type": "writers.gdal",
         "filename": "nubeRecortadaPdal_1_mdt_ndviRf.tif",
         "output_type": "idw",
         "gdaldriver": "GTiff",
         "resolution": 0.05,
         "nodata": -9999,
         "radius": 0.07,
         "window_size": 100,
         "data_type": "float32"
     }
]

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