[postgis-users] Concave hull problem & delaunay triangulation -help needed please

[pcreso at pcreso.com]

Thanks,

All things come to those who wait :-)

My data is 2D at present...

If I can't get thr PL/R solution working I think I'm better off waiting for GEOS 3.4/Postgis 2.1

Cheers,

  Brent


--- On Sat, 9/1/12, Paragon Corporation <lr at pcorp.us> wrote:

From: Paragon Corporation <lr at pcorp.us>
Subject: Re: [postgis-users] Concave hull problem & delaunay triangulation -help needed please
To: "'PostGIS Users Discussion'" <postgis-users at postgis.refractions.net>
Date: Saturday, September 1, 2012, 4:48 PM



 
 

Brent,
 
Is your data 2D or 3D?  You might want to try out the new 
ST_DelaunayTriangles in 2.1
 
http://www.postgis.org/documentation/manual-svn/ST_DelaunayTriangles.html
 
Right now I beleive it's just 2D, but work is going on in GEOS 
land to make it 3D.: http://trac.osgeo.org/geos/ticket/570
 
 It does require upgrading your system to 2.1 development 
and Geos  3.4.0 dev so might not be an option for you 
yet.
 
We also have windows build bot builds for those on 
windows who want to try which gets built whenever there is a change in the 2.1 
code base
 
http://www.postgis.org/download/windows/experimental.php  
(for 9.2 both 32-bit/64-bit and for 9.0,9.1 just 64-bit 
windows)
 
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 
pcreso at pcreso.com
Sent: Friday, August 31, 2012 11:42 
PM
To: PostGIS Users Discussion
Subject: [postgis-users] 
Concave hull problem & delaunay triangulation -help needed 
please




  
  
    Hi,

I'm trying to come up with an approach using 
      Postgis to come up with a polygon representing the outer bounds of a set 
      of points, ie: a polygon passing through all the outer points.

Much 
      useful info (as usual - thanks guys!!) at spatialdbadvisor & bostongis 
      but my limited understanding is stalling progress for now.

I came 
      across the ST_Concavehull() function, which fails. Indeed, I was surprised 
      to see this actually expands the polygon extent representing the point 
      set, as shown at:
http://www.bostongis.com/postgis_concavehull.snippet 
      as this buffers the points in some cases, and often gives quite unexpected 
      (to me anyway) results. 

Looking for something that fits better, 
      using existing capabilities, I figured I could pretty easily use Delaunay 
      triangulation to create the outer hull, then realised this bridges every 
      second outer point (pretty much by definition). That's easy, just remove 
      any triangles from the polygon where they share a line segment with the 
      polygon. I like Postgis!!

I have Simon's code working fine, at 
      least using his example invocation, from:
http://www.spatialdbadvisor.com/postgis_tips_tricks/290/r-based-delaunay-triangulation-function-for-postgis-using-the-deldir-package

But 
      I can't figure out how to make it work with my point data. I don't know R 
      or PL/R well enough to adapt it.

I have a set of sample points in a 
      table:

  Column 
      |      Type       
      |                                   
      
 --------+-----------------+-----------------
  
      id     | 
      integer         | not null 
      default
  geom   | geometry(Point) | 

Can 
      anyone explain, ideally in words of one syllable or less, how Simon's 
      function from the link above needs to be adapted to read my points dataset 
      as input?

Once I get the triangles as an output set I can use 
      Postgis for the rest...

Thanks,

   Brent 
      Wood




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