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 GIS 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 GIS 7
The addon v.concave.hull (v.concave.hull (src)) creates a concave hull using Delaunay triangulation.
The image shows the results:
