[postgis-users] Reduce precision using ST_union with gridSize

Palacios García Cuevas, Elena epgcuevas at indra.es
Tue Apr 6 07:28:25 PDT 2021


I am trying to simplify a polygon by reducing the precision using ST_Union with the gridSize parameter ( I am using  postgis 3.1  and GEOS 3.9.1). I have read that it is possible to do that by a union of a geometry with an empty geometry (http://blog.cleverelephant.ca/2020/12/waiting-postgis-31-3.html).   However, does not works for me ( maybe is my fault because I am new with postgis). I have tried as follows:

CREATE TABLE public.error_union_empty
    geom geometry(MultiPolygon),
    gid integer NOT NULL DEFAULT nextval('fallo_gid_seq'::regclass),
    CONSTRAINT error_union_empty_pkey PRIMARY KEY (gid)
INSERT INTO error_union_empty
(SELECT ST_union(geom,ST_GeomFromText('POLYGON EMPTY'), 0.001152) geom
FROM f_polygon);

But the result is an empty table.
Then I tried to do the same but by doing the union with the polygon himself, and this approach seems to work ok.

CREATE TABLE public.error_union_empty2
    geom geometry(MultiPolygon),
    gid integer NOT NULL DEFAULT nextval('fallo_gid_seq'::regclass),
    CONSTRAINT error_union_empty2_pkey PRIMARY KEY (gid)
insert into error_union_empty2
select  ST_CollectionExtract(ST_union(f_polygon.geom,f_polygon.geom, 0.001152)) geom
from f_polygon;

Why is not working the first approach? Have I missed something in the query?
Any help or comments are welcome.
Does anyone knows if  is expected to have a simplify with gridsize in the future?



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