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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

user2856
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eagleadmiral
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  • Have you made sure you havent got small gaps between the original polygons? – BERA Feb 25 '22 at 16:36
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    topological errors not topographical (topographic refers to terrain) – user2856 Feb 25 '22 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. – eagleadmiral Feb 26 '22 at 14:34
  • @user2856 so you think its because the Polygons were overlapping a bit?.. but the holes did't werde between the polygon but in the polygons itself. I attached the before-after result in the question itself. – eagleadmiral Feb 26 '22 at 14:36
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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. – John Feb 26 '22 at 23:20
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    Maybe you can share your data for inspection? Othewise, it's difficult to say – Babel Feb 27 '22 at 10:21
  • Thanks @johns for your comment. I checked my data with your suggested Plugin 'Topology Checker' and there were indeed little topological gaps in the data. I downloaded them directely from a goverment website but I think they messed it a bit up. I will report them the issue so they can look over it again and update their data if needed. Thank you for your time :D – eagleadmiral Feb 27 '22 at 22:38
  • @Babel Thanks for offering your help. As I mentioned to johns i found little topology errors in the data. – eagleadmiral Feb 27 '22 at 22:40
  • Can't share your data? – Babel Mar 20 '22 at 20:47

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