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<TITLE>RE: [postgis-users] Get concave hull from an array oflongitude inlatitude points</TITLE>
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<P><FONT SIZE=2>Its one of those frustrating things I guess where I visually plot, yet can't figure out how to program :(. Well maybe some day.<BR>
<BR>
-----Original Message-----<BR>
From: postgis-users-bounces@postgis.refractions.net on behalf of Martin Davis<BR>
Sent: Mon 11/24/2008 2:37 PM<BR>
To: PostGIS Users Discussion<BR>
Subject: Re: [postgis-users] Get concave hull from an array oflongitude inlatitude points<BR>
<BR>
Yeah, I saw that too. Unfortunately the algorithm is only available by<BR>
email, and is implemented in Mathematica. It also sounds somewhat<BR>
complex.<BR>
<BR>
But I guess this sort of answers my question - there are algorithms<BR>
which people have called "concave hull", and they produce concave<BR>
polygons from sets of points in some kind of way. I don't think there's<BR>
any consensus on what algorithm to use, though.<BR>
<BR>
One interesting approach is here:<BR>
<A HREF="http://grass.osgeo.org/wiki/Create_concave_hull">http://grass.osgeo.org/wiki/Create_concave_hull</A><BR>
<BR>
In any case, obviously this isn't in PostGIS right now.<BR>
<BR>
Obe, Regina wrote:<BR>
><BR>
> Evidentally its this thing<BR>
> <A HREF="http://ubicomp.algoritmi.uminho.pt/local/concavehull.html">http://ubicomp.algoritmi.uminho.pt/local/concavehull.html</A><BR>
><BR>
> So I guess basically you take the boundary and you get rid of points<BR>
> that have points sitting to the right/left/top/or bottom of it and then<BR>
> you form a polygon from the remaining points.<BR>
><BR>
> Well at least that is what it seems like to me visually. Just not<BR>
> quite sure how to program such a thing without writing yuck procedural<BR>
> code.<BR>
><BR>
> Hope that helps,<BR>
> Regina<BR>
><BR>
><BR>
> -----Original Message-----<BR>
> From: postgis-users-bounces@postgis.refractions.net on behalf of<BR>
> Martin Davis<BR>
> Sent: Mon 11/24/2008 1:58 PM<BR>
> To: PostGIS Users Discussion<BR>
> Subject: Re: [postgis-users] Get concave hull from an array of<BR>
> longitude inlatitude points<BR>
><BR>
> What's a concave hull?<BR>
><BR>
> Ales Kecman wrote:<BR>
> > Hi!<BR>
> ><BR>
> ><BR>
> > I'm new to Postgis and would really like to get some help on getting a<BR>
> > concave hull from an array of points.<BR>
> ><BR>
> > For example:<BR>
> ><BR>
> > I have an array of points - longitude and latitude of buildings in<BR>
> > particular area in the city. I already managed to get a convex hull<BR>
> > polygon from this array of points, but this is not really helpful.<BR>
> > What I'm trying to get is some kind of concave hull polygon from an<BR>
> > array of points, which I would further like to use in Google Earth kml<BR>
> > file.<BR>
> ><BR>
> > Thanks.<BR>
> > ------------------------------------------------------------------------<BR>
> ><BR>
> > _______________________________________________<BR>
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> > <BR>
><BR>
> --<BR>
> Martin Davis<BR>
> Senior Technical Architect<BR>
> Refractions Research, Inc.<BR>
> (250) 383-3022<BR>
><BR>
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<BR>
--<BR>
Martin Davis<BR>
Senior Technical Architect<BR>
Refractions Research, Inc.<BR>
(250) 383-3022<BR>
<BR>
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