[postgis-users] ConcaveHull generation

[Paragon Corporation]

Brian,
 
I guess I should ask first what were you expecting to happen?
 
The concave hull always returns a single polygon.  It should never return a
multipolygon.  So given that constraint, I don't see how it could avoid
splitting the edge of your example.
 
Note the union of your polygon is a multipolygon
 
SELECT ST_Astext(ST_Union('POLYGON((13 0,13 20,23 20,23 0, 13
0))'::geometry, 'POLYGON((23 20, 23 28, 30 28,30 20, 23 20))'::geometry));
 
That said any collection of polygons when unioned that does not result in a
single polygon will have edges split when forming the concave hull.
 
Hope that helps,
Regina
http://www.postgis.us
 
 


  _____  

From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Brian
Sanjeewa Rupasinghe
Sent: Sunday, May 13, 2012 2:27 PM
To: postgis-users at postgis.refractions.net
Subject: [postgis-users] ConcaveHull generation


Hi, 

I have two polygons with following coordinates:

poly1: (13,0 13,20 23,20 23,0 13,0)

poly 2: (23,20 23,28 30,28 30,20 23,20)

Once generated concaveHull, i end up with the polygon having following
coordinates:


"POLYGON((13 3e-016,13 5.64256204109163,13 11.2851240821833,13
16.9276861232749,13 20,17.4061007032357 20,23 27.049761125177,23
28,28.6425620410916 28,30 28,30 22.3574379589084,30 20,23 14.674220666202,23
9.34844133240395,23 4.02266199860593,23 3e-016,17.3574379589084 3e-016,13
3e-016))"




When investigating the coordinate list, it is observed that some existing
edges are split. [Eg. edge connecting 13,0 and 13,20]. Why is taht?




Here is the SQL code i used






SELECT ST_AsText (ST_cleangeometry((ST_ConcaveHull(ST_Collect(geom),
0.99))))

FROM concavepoly

WHERE cluster_id = 'VC-2';




Cheers, Brian



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