I am curious how can I find all polygons that are touching with each other but are completely separate from the rest of the polygons is the same dataset, using PostGIS.
For better understanding I uploaded a picture.
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Are they MultiPolygon data type?– TarasAug 30, 2021 at 8:44
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Yes, but usually they consists of one polygon. Every polygon you see on the picture is separate from others, but every one is defined as MultiPolygon in a database.– DrJacobyAug 30, 2021 at 8:49
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4I don't really understand even with the picture ... Is the question from your picture is : how to find an entire square ? If so, merge / dump and intersect with itself, if no intersections and contains more than 1 origin polygon, it is what you search.– J. MonticoloAug 30, 2021 at 8:50
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PgRouting should work but it requires some pre-processing. Nodes that you cannot access from a start node belong to other islands.– user30184Aug 30, 2021 at 13:15
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1I think you may be looking for clustering based on intersection? If so, try this.– geozelotAug 30, 2021 at 15:17
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2 Answers
As suggested in this answer, use ST_ClusterDBSCAN to assign an id to each touching group of polygons:
SELECT *,
ST_ClusterDBSCAN(geom, 0, 1) OVER() AS clst_id
FROM poly_table;
You can merge touching polygons, see this answer:
https://stackoverflow.com/a/49970447/2816941
This will produce a layer for these "islands".
Then you can simply join back onto the islands from the original data using st_intersects(islands.geom, st_pointonsurface(original.geom))
answered Aug 30, 2021 at 8:53
