Its about QGIS. In the attached screenshot, the topology checker does show error for the overlap for the two large polygons (1 and 2); but it does not show an error for the smallest (3) and the left polygon (1). I have tried both 'must not overlap' and 'must not duplicate' and still 1 and 3 don't give an error. enter image description here

  • Your symbolization doesn't allow for visual error detection. Change the angle of the line fill so it's different or each feature, and you'll probably discover that 3 is a hole in 1, and therefore does not conflict.
    – Vince
    Mar 10 '18 at 18:46
  • Vince. I have updated the screenshot. Its after I have run the topology checker i.e. no errors are displayed for polygon 1 and 3. Looking forward to a solution or a reason.
    – Roger Snow
    Mar 10 '18 at 22:11
  • Forgot to mention in original post that its QGIS I am working with.
    – Roger Snow
    Mar 10 '18 at 22:22

I have replicated the example of the features and the topology rules and indeed there is not a rule to define the topological relationship between polygons 1 and 3.

POLYGON((242.559661 -504.936524, 44.741727 -312.553149, 147.998341 -170.167713, 440.377595 -169.080801, 445.812154 -359.290353, 242.559661 -504.936524));

POLYGON((315.382747 -307.11859, 309.948188 -185.384477, 418.639361 -57.128893, 874.055374 -59.302717, 908.836549 -158.211684, 861.012433 -283.206532, 503.418475 -434.287262, 315.382747 -307.11859));

POLYGON((216.47378 -376.680941, 186.040251 -316.900796, 234.951279 -269.07668, 281.688483 -322.335354, 269.732454 -377.767852, 216.47378 -376.680941));



The explanation is that there is no overlap relation between polygons 1 and 3 (Adjacent polygons should not share common area) or duplicate geometries (must not have identical geometries), the rule being that polygon 3 is within polygon 1, or vice versa is that polygon 1 contains polygon 3.


  • Thanks a lot for the explanation. But what can I do if, within the same layer, there are these small polygons fully contained in a larger polygon. I do not want any overlaps of this kind (1 and 3) in the cleaned layer. Selection by location does work in this case either (as they are in same layer). I am ok even if the smaller polygons can be selected and deleted somehow, but don't know how.
    – Roger Snow
    Mar 11 '18 at 9:34
  • In this case I would recommend the Geometry Checker Plugin better, as you can see in the image you can identify and correct the errors you mention. i.imgur.com/PoEL8yW.png For more information you can check the official documentation: docs.qgis.org/2.18/en/docs/user_manual/plugins/…. html
    – SamTux
    Mar 11 '18 at 14:26

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