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

[maning sambale]

Thanks for the tips Alex, Andreas and Giovanni.

Unfortunately, I need to "turnover" the maintenance of the data to a
non-geo user, hence I need a workflow that can avoid R or postgis for
now.

On Tue, Jan 3, 2012 at 6:13 PM, G. Allegri <giohappy at gmail.com> wrote:
> 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) - 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
>>
>> and Regina's examples:
>> 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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>
>
>
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-- 
cheers,
maning
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