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I have two polygon layers, A and B, both in the same projection. I am trying to create a new layer, C, which is all polygons in A less the areas where polygons in A and polygons in B intersect.

I am using v2.14.1 - Essen.

I have checked the geometries are valid in both layers, removed duplicate notes and dealt with crossovers. Running the validity query in QGIS shows that there are no errors.

I ran the 'difference' geoprocessing tool, and, for the most part, this works. C is created, which includes all A polygons, less those areas that overlapped with B polygons.

However, there is one polygon in A which, for some reason, does not seem to work as part of the process. When I create C, the entirety of this polygon has disappeared, despite there only being a small overlap with several polygons in B.

Can anyone point me towards a reason why this might be?

(Apologies for A, B and C, but thought I would generalise the question)

  • It would be great if you could edit your question to include screenshots of your problem ;) – Joseph Oct 7 '16 at 11:07
  • Try selecting those several polygons in B, toggle the editing button and use the Edit > Merge Selected Features tool, save the edits and run the Difference tool again and see if the polygon from A is there? – Joseph Oct 7 '16 at 11:34
  • @Joseph Thanks for that. Data is a bit sensitive (commercial etc), so wary about sharing. Will try your suggestion. – Chris Pope Oct 7 '16 at 12:51

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