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I tried to dissolve a couple of polygons and found out that it creates little holes in the Polygon (see: QGIS Dissolving and holes). I knew how to 'fill' the holes (with v.clean) but I am wondering why this happens.

I read on the internet there are some 'topological errors' in my files but I can't figure out what this exactly means. Could anyone explain me in understandable language (because I am not a QGIS specialist) why this happens?

EDIT: see pictures below (before - after). So I think it only happens on the edges of the polygons because the data is not optimal. Anyone know why my data is faulty?

before the operation

after the operation

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  • Have you made sure you havent got small gaps between the original polygons? Commented Feb 25, 2022 at 16:36
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    topological errors not topographical (topographic refers to terrain) Commented Feb 25, 2022 at 22:58
  • @BERA Yes, I am sure. The original polygons didn't have any gaps between. I could fix the problem but Im wondering why this is happening so I can fix the problem in advance. Commented Feb 26, 2022 at 14:34
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    How are you sure there were no topological gaps? A visual check will usually miss those that are very small. If you install the Topology Checker plugin and run it with the topology rules of must not have gaps and must not overlap on your original layer, I expect you might find many errors. If you don't want to run it, or after running it and finding many errors, you might run the v.clean tool with different tolerances andthen check topology. If the log reports duplicates run the Delete Duplicate Geometries tool. You could try the Snap Geometries to Layer tool instead of v.clean. Commented Feb 26, 2022 at 23:20
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    Maybe you can share your data for inspection? Othewise, it's difficult to say Commented Feb 27, 2022 at 10:21

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