Difference between revisions of "Create concave hull"

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A: You can approximate a concave hull through the following steps:
 
A: You can approximate a concave hull through the following steps:
  
=== GRASS 6 ===
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=== 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)
 
* 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}})
 
* 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}})
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* dissolve the areas into one ({{cmd|v.dissolve}})
 
* dissolve the areas into one ({{cmd|v.dissolve}})
  
=== GRASS 7 ===
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=== GRASS GIS 7 ===
  
The addon {{AddonSrc|vector|v.concave.hull|version=7}} creates a concave hull using Delaunay triangulation.
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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:
 
The image shows the results:

Latest revision as of 06:45, 3 June 2015

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:

Concave hull.jpg