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I am using QGIS version 3.14.0. I have about 20 polygon .shp-files with overlaps between files. All are in the same projection. I've been using the difference tool to remove the overlaps and then merge the files to create one shapefile. This process works well for about 6 files, then I get an error that says:

GEOS geoprocessing error: difference failed

I've tried to restart the process at the dataset that causes the error, and am able to continue for a few years until the error comes up again. Online research suggests two things:

  1. Geometry or topology is causing the error.
  2. The difference tool may be maxing out computer memory causing a failure.

Has anyone else worked through this error before and has a fix?

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Not a very useful error message is it!

Firstly, lets see if we can do something about that. You can check the QGIS log panel. (Right click on tool bar > Log Messages Panel) This may give you more context to the error.

We probably need more information about how you are doing this task to diagnose it, but to your specific question:

It is possible that there are some geometry errors in your dataset. Run the 'Check Validity' tool first on each dataset. Also run the Fix Geometries.

Also just do a sanity check on the geometry - is there a polygon record that has a large number of vertices? Its possible a really complex geometry may cause the process to chew on memory. However my personal experiences with QGIS and memory overuse, is that it doesn't throw an error, it just freezes or becomes non-responsive, so I don't think it is this.

So its going to be geometry. Try Differencing and merging single datasets at a time, such that you can pinpoint the problem to a specific dataset. Then you can run fixes/checks across a single dataset. Also check those logs I mentioned above!

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This sounds like an error message from QGIS indicating that the GEOS overlay algorithm has encountered a robustness problem. This is usually due to nearly-coincident linework, which can happen when working with polygon differences.

GEOS 3.9 contains a completely new overlay algorithm which essentially eliminates this kind of problem. Hopefully this will be incorporated in QGIS sometime soon.

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I used clip tool to find a common part and then the result in Difference tool and it worked in QGIS 3.4 But not in all cases.

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    Please don't post double answers. Either edit or delete your other, comment-like answer. Thank you. – Erik Jul 27 at 10:44

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