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I have a single layer of polygons. Some of the polygons overlap.

polygons overlap

I wish to make a new layer of just the parts of the polygons that overlap.

enter image description here

What is the best way to do this in QGIS seeing it's all in one layer?

I tried the clip and intersect in the geo-processing tools, but they expect two layers.

  • So you want Intersecting polygons? Vector > Geoprocessing Tools > Intersect. – Mapperz Feb 3 '15 at 18:17
  • Ideally you would be able to export topology errors. Go to Vector - Topology Checker. Then configure: yourPolygon must not overlap, then click Validate All. That should highlight the parts you are talking about, right? Now the question is how to export these highlighted parts. There does not seem to be a way using the topology tools. Are you into programming at all? There surely must be a way using PyQGIS. Considering the parts are highlighted there also must be a way to save them to a new layer. PS: if you have access to ArcGIS, I am pretty sure I have exported errors before. – BritishSteel Feb 3 '15 at 19:57
  • Thanks @BritishSteel. The Topology Checker did highlight the parts I wanted. I too couldn't find a way to export. I might explore ArcGis and Grass to see if I can do it in there. – R3b3cca Feb 4 '15 at 10:22
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There's is a rough and dirty way to do it. I say rough and dirty because I've been getting strange intersect results which I'll open a bug for.

  1. Use Vector > Geoprocessing Tools > Intersect... with the layer as both the inputs.
  2. Use the Spatial Query (spatial query buytton) tool and select features from the intersect tools output that "Equal" your source layer.
  3. Start and editing session to remove those selected features in your intersect results layer.

Depending on the complexity or count of the features, running these can take a couple mins.

Some optional additional steps could be if you have an ID type of field with common or unique attributes do a dissolve then a Singleparts to Multipart.

  • Thanks for your answer @SaultDon. I got it to work! :) I did the spatial query then Toggled Editing and clicked on 'Cut Features'. – R3b3cca Feb 4 '15 at 10:31

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