[Qgis-user] aggregate polygon overlay

[Karl Magnus Jönsson]

Hi Simon!
Seems like a common problem that GIS should handle easy, but it is perhaps not that easy. I had a similar problem that ended up in below. I have my data in Postgres + PostGIS so it is handled by the database in in a view. The code disassembles the polygons in the rings, union the rings and then build polygons again from the lines. Then I do a join with the original data (in this case not exactly) based on location on the centerpoint. In your case I would do a suitable group by and concatenate the attributes. Hope this helps even it is not QGIS.

WITH orings AS (
         SELECT st_exteriorring((st_dumprings((st_dump(omr.geom)).geom)).geom) AS geom
           FROM qdp2.omr
        ), oboundaries AS (
         SELECT st_union(orings.geom) AS geom
           FROM orings
        ), opoly AS (
         SELECT (st_dump(st_polygonize(oboundaries.geom))).geom AS geom
           FROM oboundaries
        )
SELECT row_number() OVER () AS id,
    p.namn,
    'delområde'::text AS delomr,
    p.plan_uuid,
    p.status,
    p.publicerad,
    op.geom
   FROM opoly op,
    qdp2.plan_omr po,
    qdp2.plan p
  WHERE st_contains(po.geom, st_pointonsurface(op.geom)) AND po.plan_uuid = p.plan_uuid

Med vänliga hälsningar

Karl-Magnus Jönsson

Från: QGIS-User <qgis-user-bounces at lists.osgeo.org> För Simon Dietmann via QGIS-User
Skickat: den 17 februari 2026 15:01
Till: qgis-user <qgis-user at lists.osgeo.org>
Ämne: [Qgis-user] aggregate polygon overlay

Dear list,

I am trying to solve the following problem:

I have one polygon layer with many polygons  that are in many cases overlapping and with different attribute valus, e.g. a, b, c:
[cid:image001.png at 01DCA028.5EA7E630]

My goal is to generate a polygon layer with polygons for every overlap constellation containing a concat string of those attributes, in this example seven polygons with the attributes a, b, c, ab, bc, ac, abc.

I started with a union of my original layer, the result is a layer with 12 polygons where overlapping areas of the original polygons result in overlapping intersections containing one of the original attributes each, for example in the middle, there are 3 polygons, one with a, one with b and one with c.

How is it possible to summarize those overlapping polygons into one polygon each with all the attributes, given that there is a large number of polygons and different attributes?

Thank you for any hints,

regards

Simon


i.A. Simon Dietmann
Projektleiter | Geograph


[cid:image002.png at 01DCA028.5EA7E630]

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