# [postgis-users] Get concave hull from an array oflongitude inlatitude points

Obe, Regina robe.dnd at cityofboston.gov
Mon Nov 24 11:48:42 PST 2008

```Its one of those frustrating things I guess where I visually plot, yet can't figure out how to program :(.  Well maybe some day.

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net on behalf of Martin Davis
Sent: Mon 11/24/2008 2:37 PM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Get concave hull from an array oflongitude	inlatitude points

Yeah, I saw that too.  Unfortunately the algorithm is only available by
email, and is implemented in Mathematica.   It also sounds somewhat
complex.

But I guess this sort of answers my question - there are algorithms
which people have called "concave hull", and they produce concave
polygons from sets of points in some kind of way.  I don't think there's
any consensus on what algorithm to use, though.

One interesting approach is here:
http://grass.osgeo.org/wiki/Create_concave_hull

In any case, obviously this isn't in PostGIS right now.

Obe, Regina wrote:
>
> Evidentally its this thing
> http://ubicomp.algoritmi.uminho.pt/local/concavehull.html
>
> So I guess basically you take the boundary and you get rid of points
> that have points sitting to the right/left/top/or bottom of it and then
> you form a polygon from the remaining points.
>
> Well at least that is what it seems like to me visually.  Just not
> quite sure how to program such a thing without writing yuck procedural
> code.
>
> Hope that helps,
> Regina
>
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net on behalf of
> Martin Davis
> Sent: Mon 11/24/2008 1:58 PM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Get concave hull from an array of
> longitude inlatitude points
>
> What's a concave hull?
>
> Ales Kecman wrote:
> > Hi!
> >
> >
> > I'm new to Postgis and would really like to get some help on getting a
> > concave hull from an array of points.
> >
> > For example:
> >
> > I have an array of points - longitude and latitude of buildings in
> > particular area in the city. I already managed to get a convex hull
> > polygon from this array of points, but this is not really helpful.
> > What I'm trying to get is some kind of concave hull polygon from an
> > array of points, which I would further like to use in Google Earth kml
> > file.
> >
> > Thanks.
> > ------------------------------------------------------------------------
> >
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>
> --
> Martin Davis
> Senior Technical Architect
> Refractions Research, Inc.
> (250) 383-3022
>
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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022

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