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Identifying overlapping polygons in single layer using QGIS?

As described this question there are 3 conditions for two polygon on a same layer.

  1. These 2 polygons can have intersection area. (How to detect-Solved)
  2. These 2 polygons can have dublicated. (How to detect-Solved)
  3. One polygon can cover other polygon. How to detect? (My question)

Layer: QGIS - SHP layer

  • If you want to detect only fully covered polygons, you can try this process : 1/ compute the intersection of all polygons 2/ merge the resulting (intersection) layer and the original layer 3/ detect duplicated polygons. A fully covered polygon will stay identical after step 1 and thus will appear as duplicated after step 2. I don't know if there's a shorter way. – ArMoraer Jan 15 '16 at 9:59
  • I don't really get the difference between #1 and #3. – wiltomap Jan 15 '16 at 10:46
  • ArMoraer : Your way is suitable for 2 polygons or for few polygons. I want to analyze too much polygons at same tame. wiltomap : In first condition there is a intersection area and polygon doesn't covered fully. Third condition one polygon fully covered by other polygon. – Tuna Jan 15 '16 at 11:35
  • @Tuna: you're right. I posted another solution that should work, if I understand correctly what you want. – ArMoraer Jan 15 '16 at 12:23
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Otherwise you can use (or install it if needed) the plugin topology checker and add a rule "not overlap" on your layer and it will show you polygons that overlapping. Here more informations

  • "not overlap" works just for first condition. It doesn't work on third condition. It works for adjacent polygons. – Tuna Jan 15 '16 at 11:41
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Well, I think I found a way to detect polygons that are fully covered by other polygons. This is a bit complicated and I am surprised there seems to be no other way (I might have missed a plugin), but here it is (home tested) :

  1. Dissolve your input polygon layer (Vector > Geoprocessing Tools > Dissolve). In the field "Dissolve field", choose "-- Dissolve all --". Let's call diss the output layer.

  2. Apply an inner buffer to diss : Vector > Geoprocessing Tools > Buffers. Here's the thing : you have to choose a small negative value for "Buffer distance", e.g. -0.0001 (depending on your projection). It is necessary before the next step (it will set the distance tolerance between covered and covering polygons). Let's call diss_buff the output layer.

  3. Use the "Select by location" tool (Vector > Research Tools). First input layer : choose your original layer. Second input layer : choose diss_buff. In the options, select only "Include input features completely within the selection features". Click OK: all (and only) your covered polygons should be selected.

  4. (Optional) You can save the selection with "Save As" and select "Save only selected features.

Tested on GQIS 2.8.

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