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I am trying to use union processing on a number of overlapping polygons (approx 50-100) in the same layer. This raises the error "GEOS geoprocessing error: intersection failed", without any further details.

I have tried "check validity", "fix geometries", "check geometries" and "topology checker" on my data, and none of them show me any errors. I have tried this both in 3.22.3 and 3.16.16 (LTR).

I think this thread talks about the same problem (albeit without providing any helpful solution): https://github.com/qgis/QGIS/issues/37032

Is QGIS just not able to handle Union-processing of this magnitude, or is there anything I can do?

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  • Try buffering with a very small buffer, like 0.0001 m. Sometimes it can fix problems like yours
    – BERA
    Jan 6 at 19:04
  • I got the same error while clipping a large file (global file with large size) to a country level in QGIS. I used ArcMap to first clip the original file to a slightly bigger area (rectangle area) than the designated country area and then clipped the resulting file with desired country area. It worked in ArcMap. However, in QGIS same procedure could not resolve the issue. You may try, may be it works.
    – rgt
    Apr 16 at 9:07

1 Answer 1

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it does seem to be a limitation of the GEOS engine, as described in your github link. But that was 2020 so I'm not sure what's happened with it. I'm also getting this issue with Intersection tool on linework vs polygon.

I ended up using the SAGA Line-polygon intersection tool instead which worked.

For you, there is a SAGA Polygon union tool, this may help you with your work. As a tip as well, I always try and specify a non-shapefile output with all of the GUI based tools, I just find the 'temporary' dataset too problematic.

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  • Could you please describe what you mean by "... too problematic"?
    – Stu Smith
    Jun 23, 2022 at 13:59
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    @Stu Smith - yer for sure. It varies, but as an example, if I have a source dataset which has attribute values which are longer than the default allows, the actual tool itself will fail. Also the GDAL/SAGA/GRASS tools also tend to really want a file path specified, even though it appears they will accept a temporary dataset as output. ......
    – nr_aus
    Jun 24, 2022 at 7:07

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