[postgis-users] simplifying (homogenize) a polygon

[Denis Rouzaud]

wow, thanks a lot!

indeed, much much nicer!

thanks again,

Denis


On 21. 11. 13 15:47, Nicolas Ribot wrote:
> Thanks for the explanation.
>
>
> You could correct the multipolygons in one step, by using the
> st_buildArea function, that "merges" a linework of linestrings,
> preserving holes.
> To extract polygons boundaries as linestring, st_boundary() is very handy:
>
> The "bad_multipg" table contains invalid multipolygons with adjacent
> parts (http://imgur.com/MP6Cy4f)
>
> select gid, st_buildArea(st_boundary(geom))
> from bad_multipg
> group by gid;
>
> Gives: http://imgur.com/RqlowFI
>
> Nicolas
>
> On 21 November 2013 13:50, Denis Rouzaud <denis.rouzaud at gmail.com> wrote:
>> Hi Nicolas,
>>
>> I edit my multi polygons in QGIS, and you're right the geometry becomes
>> invalid if some parts have adjacent segments.
>>
>> In a simple case, I would have a multi polygons with 3 parts with 2 parts
>> adjacent.
>> http://imgur.com/zvxPFeR
>>
>> Although the geometry is invalid, my intention is to make it valid, and
>> therefore merge the two parts, which is impossible in QGIS - AFAIK.
>>
>> So my intention was to save the geometry as this, and make it valid by a
>> command.
>>
>> I could achieve this with a function:
>> CREATE OR REPLACE FUNCTION distribution.simplemultipoly(geom geometry)
>> RETURNS geometry AS
>> $BODY$
>>      DECLARE
>>          num_geom integer;
>>      BEGIN
>>          SELECT ST_NumGeometries(geom) into num_geom;
>>          SELECT
>> ST_Multi(ST_CollectionHomogenize(ST_Union(ST_GeometryN(geom,n))))
>>              FROM generate_series(1,num_geom) n
>>              INTO geom;
>>          RETURN geom;
>>      END;
>> $BODY$
>> LANGUAGE plpgsql;
>>
>> But I wondered if there was a more direct way.
>>
>> Cheers,
>>
>> Denis
>>
>>
>> On 21. 11. 13 12:42, Nicolas Ribot wrote:
>>
>> Hi Denis,
>>
>> In your example, I don't understand how the parts of your
>> multipolygons can be adjacent. Are they valid ?
>> Do you have a visual example ?
>>
>> Thank you
>>
>> Nicolas
>>
>>
>> On 21 November 2013 07:32, Denis Rouzaud <denis.rouzaud at gmail.com> wrote:
>>
>> Hello Rémi,
>>
>> I was hoping a simplest request without enabling topology but thanks anyway!
>>
>> Cheers,
>>
>> Denis
>>
>>
>> On 20. 11. 13 18:26, Rémi Cura wrote:
>>
>>
>>  From what I understand of your needs, Postigs topology was designed for
>> this.
>> Cheers,
>>
>> Rémi-C
>>
>>
>> 2013/11/20 Denis Rouzaud <denis.rouzaud at gmail.com>
>>
>> Hi all,
>>
>> I am drawing some multipolygons in QGIS and sometimes, I have parts of
>> them which are adjacent and I'd like to homogenize them to have less parts
>> and no adjacent parts.
>>
>> I could do this with a quite complex method:
>>
>> 1. get the number of parts Z:
>> SELECT ST_NumGeometries(geometry) FROMmytable WHERE id=123;
>>
>> 2. do the simplification using ST_CollectionHomogenize:
>> SELECT
>> ST_Multi(ST_CollectionHomogenize(ST_Union(ST_GeometryN(geometry,n)))) FROM
>> mytable CROSS JOIN generate_series(1,Z) n WHERE id = 123;
>>
>> 3. update manually the result in the table (copy paste).
>>
>>
>> Does someone has something more straightforward???
>>
>> Thanks a lot!
>>
>> Best regards,
>>
>> Denis
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>>
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>>
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>>
>>
>>
>> _______________________________________________
>> postgis-users mailing list
>> postgis-users at lists.osgeo.org
>> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users