[postgis-users] simplifying (homogenize) a polygon

Nicolas Ribot nicolas.ribot at gmail.com
Thu Nov 21 06:47:07 PST 2013

```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:

parts (http://imgur.com/MP6Cy4f)

select gid, st_buildArea(st_boundary(geom))
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
> 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
>
> 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
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```