Create concave hull

Q: I have a set of points and would like to compute the concave hull of it.

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

• 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)

The image to shows the results:

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