I have a multipart shape file created from a GeoJSON. The original geoJSON is from the Vermont Open Geodata Portal, so a lot of the polygons have errors and overlaps.

To fix the overlaps, after I convert the original GeoJSON to single parts, I use SAGA's self-intersection tool to identify overlap using the self-intersection tool

I then eliminate selected polygons (all the NULLs and join by the largest common boundary), but one of the polygons becomes a GeometryCollection.enter image description here

The GeometryCollection is, as far as I know, unsupported in QGIS and returns errors when I try to dissolve with the other polygons to combine the "district" types into multipart. Is there a way to change the GeometryCollection type or allow it to dissolve with polygons?

  • Welcome to GIS SE. As a new user, please take the Tour. Something is only a GeometryCollection if it can't be dissolved with polygons. Instead of asking for the impossible, you need to focus on how to extract the polygon from within the collection, or how to prevent collections from being created in the first place.
    – Vince
    Aug 14, 2023 at 20:04
  • Did you run a Check Validity or Fix Geometries tool prior to either the intersection or after, or both, before trying to dissolve? If you run a GRASS v.clean (I would use break, snap and bpol) and then the QGIS delete duplicate geometries tool, does that work?
    – John
    Aug 14, 2023 at 22:53
  • There is also a ProcessX-PlugIn which has a tool to remove self overlapping areas in a polygon layer based on a condition I haven't tried it myself.
    – John
    Aug 14, 2023 at 22:58

1 Answer 1


I don't have enough points to comment or upvote sadly, but am running into the same issue with the 'Eliminate Selected Polygons' tool -- did you end up finding a solution?

  • I will put is as a comment for you later
    – GforGIS
    Aug 30, 2023 at 0:14

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