[Qgis-user] concave hull/alpha shapes in QGIS

[G. Allegri]

AFAICS ST_ConcaveHull is written in PL/PgSQL, and doesn't use GEOS methods
[1].
I suppose it is based on the common "alpha shapes" [1] approach.
Martin has written something for JTS, but I think he hasn't release it
yet...

giovanni

[1]
http://trac.osgeo.org/postgis/browser/trunk/postgis/postgis.sql.in.c#L4419
[2]  http://www.mpi-inf.mpg.de/~jgiesen/tch/sem06/Celikik.pdf



2012/1/3 Andreas Neumann <a.neumann at carto.net>

> Hi,
>
> Postgis SVN (2.0) has a function called ST_ConcaveHull (
> http://www.postgis.org/**documentation/manual-svn/ST_**ConcaveHull.html<http://www.postgis.org/documentation/manual-svn/ST_ConcaveHull.html>)
> - it says it builds upon GEOS - (but probably only in geos-svn) - so there
> should be some way to expose it in a future QGIS version.
>
> See also this thread:
> http://www.mail-archive.com/**postgis-users@postgis.**
> refractions.net/msg10785.html<http://www.mail-archive.com/postgis-users@postgis.refractions.net/msg10785.html>
>
> and Regina's examples:
> http://www.bostongis.com/**postgis_concavehull.snippet<http://www.bostongis.com/postgis_concavehull.snippet>
>
> I currently can't find the GEOS class/method that calculates it, but since
> Postgis makes a reference to GEOS it has to be somewhere in GEOS.
>
> If you want to speed up the process, you can perhaps pay a developer to
> include it.
>
> Andreas
>
> On Tue, 3 Jan 2012 11:36:28 +0800, maning sambale wrote:
>
>> We have a nifty convex hull in ftools/vector plugin. I'm looking for
>> ways to create concave hulls.  I have a set of csv points as a source
>> to generate the polygon.  This is updated regularly by non-geo field
>> workers.  Any workaround to generate concave hull in QGIS?
>>
>> Mappy new year to all!
>>
>> PS. I've seen old discussions in the qgis-dev list.
>>
>
> --
> --
> Andreas Neumann
> Böschacherstrasse 10A
> 8624 Grüt (Gossau ZH)
> Switzerland
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