filters.hexbin.rst

filters.hexbin

A common questions for users of point clouds is what the spatial extent of a point cloud collection is. Files generally provide only rectangular bounds, but often the points inside the files only fill up a small percentage of the area within the bounds.

Hexbin output shows boundary of actual points in point buffer, not just rectangular extents.

The hexbin filter reads a point stream and writes out a metadata record that contains a much tighter data bound, expressed as a well-known text polygon. In order to write out the metadata record, the pdal pipeline command must be invoked using the --pipeline-serialization option:

Example 1

The following pipeline file and command produces an JSON output file containing the pipeline's metadata, which includes the result of running the hexbin filter:

{
    "pipeline": [
        "/Users/me/pdal/test/data/las/autzen_trim.las",
        {
            "type" : "filters.hexbin"
        }
    ]
}


$ pdal pipeline hexbin-pipeline.json --metadata hexbin-out.json

{
  "stages":
  {
    "filters.hexbin":
    {
      "area": 746772.7543,
      "avg_pt_per_sq_unit": 22.43269935,
      "avg_pt_spacing": 2.605540869,
      "boundary": "MULTIPOLYGON (((636274.38924399 848834.99817891, 637242.52219686 848834.99817891, 637274.79329529 849226.26445367, 637145.70890157 849338.05481789, 637242.52219686 849505.74036422, 636016.22045656 849505.74036422, 635983.94935813 849114.47408945, 636113.03375184 848890.89336102, 636274.38924399 848834.99817891)))",
      "boundary_json": { "type": "MultiPolygon", "coordinates": [ [ [ [ 636274.38924399, 848834.99817891 ], [ 637242.52219686, 848834.99817891 ], [ 637274.79329529, 849226.26445367 ], [ 637145.70890157, 849338.05481789 ], [ 637242.52219686, 849505.74036422 ], [ 636016.22045656, 849505.74036422 ], [ 635983.94935813, 849114.47408945 ], [ 636113.03375184, 848890.89336102 ], [ 636274.38924399, 848834.99817891 ] ] ] ] },
      "density": 0.1473004999,
      "edge_length": 0,
      "estimated_edge": 111.7903642,
      "hex_offsets": "MULTIPOINT (0 0, -32.2711 55.8952, 0 111.79, 64.5422 111.79, 96.8133 55.8952, 64.5422 0)",
      "sample_size": 5000,
      "threshold": 15
    }
},
...

Example 2

As a convenience, the pdal info command will produce similar output:

$ pdal info --boundary /Users/me/test/data/las/autzen_trim.las

{
  "boundary":
  {
    "area": 746772.7543,
    "avg_pt_per_sq_unit": 22.43269935,
    "avg_pt_spacing": 2.605540869,
    "boundary": "MULTIPOLYGON (((636274.38924399 848834.99817891, 637242.52219686 848834.99817891, 637274.79329529 849226.26445367, 637145.70890157 849338.05481789, 637242.52219686 849505.74036422, 636016.22045656 849505.74036422, 635983.94935813 849114.47408945, 636113.03375184 848890.89336102, 636274.38924399 848834.99817891)))",
    "boundary_json": { "type": "MultiPolygon", "coordinates": [ [ [ [ 636274.38924399, 848834.99817891 ], [ 637242.52219686, 848834.99817891 ], [ 637274.79329529, 849226.26445367 ], [ 637145.70890157, 849338.05481789 ], [ 637242.52219686, 849505.74036422 ], [ 636016.22045656, 849505.74036422 ], [ 635983.94935813, 849114.47408945 ], [ 636113.03375184, 848890.89336102 ], [ 636274.38924399, 848834.99817891 ] ] ] ] },
    "density": 0.1473004999,
    "edge_length": 0,
    "estimated_edge": 111.7903642,
    "hex_offsets": "MULTIPOINT (0 0, -32.2711 55.8952, 0 111.79, 64.5422 111.79, 96.8133 55.8952, 64.5422 0)",
    "sample_size": 5000,
    "threshold": 15
  },
  "filename": "\/Users\/acbell\/pdal\/test\/data\/las\/autzen_trim.las",
  "pdal_version": "1.6.0 (git-version: 675afe)"
}

Options

edge_size

If not set, the hexbin filter will estimate a hex size based on a sample of the data. If set, hexbin will use the provided size in constructing the hexbins to test.

sample_size

How many points to sample when automatically calculating the edge size? [Default: 5000]

threshold

Number of points that have to fall within a hexbin before it is considered "in" the data set. [Default: 15]

precision

Coordinate precision to use in writing out the well-known text of the boundary polygon. [Default: 8]