Re: [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  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 
>>
>> 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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>> http://lists.osgeo.org/mailman/listinfo/qgis-user
>
>
>
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maning
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Re: [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 

> 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
> __**_
> Qgis-user mailing list
> Qgis-user@lists.osgeo.org
> http://lists.osgeo.org/**mailman/listinfo/qgis-user
>
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Re: [Qgis-user] concave hull/alpha shapes in QGIS

[Andreas Neumann]

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.


--
--
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Böschacherstrasse 10A
8624 Grüt (Gossau ZH)
Switzerland
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Re: [Qgis-user] concave hull/alpha shapes in QGIS

[Alex Mandel]

On 01/02/2012 07:36 PM, 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.

They can be done in R, and therefore done using ManageR QGIS plugin.
The R package is alphahull

Though you might want to look at other methods depending on what your
actual goal. I messed around with them in regards to territories from
GPS telemetry and I believe Anne Ghisla has a whole thesis on the topic.

In the end a professor of mine came up with using a density function as
the fastest method to get good results.

I can find the R code if it will help you.

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

[maning sambale]

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.
-- 
cheers,
maning
--
"Freedom is still the most radical idea of all" -N.Branden
wiki: http://esambale.wikispaces.com/
blog: http://epsg4253.wordpress.com/
--
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