View source for Create concave hull

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 {{cmd|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 {{cmd|v.net.visibility}}, but {{cmd|v.distance}} works also)
* add a table with a column 'cat' and a column 'length double precision' and categories to these lines ({{cmd|v.db.addtable}}, {{cmd|v.category}})
* upload the length of each line to the table ({{cmd|v.to.db}})
* visually identify a length threshold above which the lines should not be used
* extract only those lines below that threshold ({{cmd|v.extract}} where="length<MAXLENGTH)
* convert the lines to boundaries ({{cmd|v.type}})
* break overlapping lines ({{cmd|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 ({{cmd|v.centroids}} step=0)
* dissolve the areas into one ({{cmd|v.dissolve}})

=== GRASS GIS 7 ===

The addon {{AddonCmd|v.concave.hull}} ({{AddonSrc|vector|v.concave.hull|version=7}}) creates a concave hull using Delaunay triangulation.

The image shows the results:

[[Image:Concave_hull.jpg|500px]]


[[Category:FAQ]]