pdal_notes_README.md

Notes on PDAL Pipelines

spestana@uw.edu

PDAL - Point Data Abstraction Library

Splitting LAS Files

• Specify input filepath
• Use filters.divider with the following options:
  • mode - "partition" (default) or "round_robin" processing
  • count - Provide number of output files to create from single input
  • capacity - Alternatively, provide the number of points to be contained in each output file
• Use writers.las to write out new las files
  • filename - include "#" character in output filename for incremental numbering of split files

{
  "pipeline":[
    "\input\filepath\filename.las",
    {
      "tag" : "splitLasFile",
      "type" : "filters.divider",
      "mode" : "partition",
      "count" : "10"
    },
    {
      "tag" : "writeOutLasSplits",
      "type":"writers.las",
      "filename":"\output\filepath\filename_split_#.las"
    }
  ]
}

Filter Discrete Returns

• filters.range simple filter for specified dimension(s):
  • limits - dimension name and range to include
    • e.g. filter to values of R where x <= R <= y --> "R[x:y]
    • e.g. filter to values of R where R is NOT a < R <= b --> "R!(a:b]"

 ...
    {
      "tag" : "filterReturns",
      "type" : "filters.range",
      "limits" : "ReturnNumber[1:1]"
    },
 ...

Write Out a Raster

• writers.gdal GDAL raster writer:
  • resolution - length of raster pixel edges (in same units as x,y from las file)
  • radius - radius from pixel center to calculate pixel value (default radius = resolution * sqrt(2))
  • dimension - value of pixel to output taken from points within the pixel (min, max, mean, idw, count, or stdev)
  • gdaldriver - "GTiff" for GeoTiff (default)
  • filename - output filename for raster
  • see online documentation for further optional parameters

 ...
   {
      "tag" : "pointDensityRaster",
      "type" : "writers.gdal",
      "resolution": 10,
      "output_type" : "count",
      "gdaldriver" : "GTiff",
      "filename" : "pointDensityRater_10m.tif"
    }
 ...

Specifying Pipeline Command Inputs

• Within a pipeline JSON file, outputs from one step can be used as inputs for following steps by use of the "inputs" option:
  • tag - provide a tag name for previous steps to reference in later input arrays
  • inputs specifies an array of tags from previous steps to use as inputs for the current step

 ...
    {
    ...
      "tag" : "firstStep",
    ...
    },
    {
    ...
      "tag" : "secondStep",
    ...
    },
    {
    ...
      "tag" : "lastStep"
      "inputs" : [
                    "firstStep",
                    "secondStep"
                 ],
    ...
    }
 ...

---

Executing Pipelines

• From a terminal, call up the pipeline json file that is to be executed
  • (Optionally include variables as arguments)

    pdal pipeline filename.json

DTMs

Extracting ground points

pdal translate "input_file.las" -o "output_file.las" smrf range --filters.range.limits="Classification[2:2]" -v 4

Extracting Non-ground points

pdal translate "input_file.las" -o "output_file.las" smrf range --filters.range.limits="Classification![2:2]" -v 4

Writing to raster GeoTiff

{
    "pipeline": [
        "input_filename.las",
        {   
		    "type" : "writers.gdal",
			"gdaldriver" : "Gtiff",
            "resolution": 5,
            "filename":"output_filename.tif"
        }
     ]
}

LAStools - lasdiff

lasdiff -i "input_ground_surface.las" -i "input_nonground_surface.las" -o "output_diff.las"