Create concave hull

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Q: I have a set of points and would like to compute the concave hull of it (this is not the convex hull as done by v.hull):

A: You can approximate a concave hull through the following steps:

GRASS 6

  • create map of lines connecting all points (I find it easier to do this with v.net.visibility, but v.distance works also)
  • add a table with a column 'cat' and a column 'length double precision' and categories to these lines (v.db.addtable, v.category)
  • upload the length of each line to the table (v.to.db)
  • visually identify a length threshold above which the lines should not be used
  • extract only those lines below that threshold (v.extract where="length<MAXLENGTH)
  • convert the lines to boundaries (v.type)
  • break overlapping lines (v.clean tool=break, there shouldn't be any duplicate lines, but just in case you can also use rmdupl)
  • add centroids with the same category value to the boundaries (v.centroids step=0)
  • dissolve the areas into one (v.dissolve)

GRASS 7

The addon v.concave.hull (src) creates a concave hull using Delaunay triangulation.

The image shows the results:

Concave hull.jpg