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Using QGIS, what is the quickest way to check whether a shapefile has polygons that overlap each other?

I guess I could import it into PostGIS, then see whether:

SUM(ST_Area(geom)) 

is the same as:

SUM(ST_Area(ST_Union(geom)))

But that doesn't feel like it's likely to be the most efficient way to do it, especially since my shapefile has about 40k features.

Any ideas?

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    I reduced the scope of this question to prevent it being closed as too broad, and to match the only answer. To ask the same question for another product just ask a new question.
    – PolyGeo
    Apr 25, 2017 at 20:35
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    Thanks - I've opened the same question for PostGIS; gis.stackexchange.com/questions/243565/…
    – Richard
    Jun 11, 2017 at 20:52

1 Answer 1

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Without knowing what software you are using I can only answer if you use QGIS. Simply running the 'Topology Checker' tool will identify where all overlaps occur in a single layer

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  • Thanks - how? I've opened the plugin, do I need to add a rule of some kind? I've tried adding one saying that my layer is layer #1 and it "must not overlap", with no layer #2 - is that what you mean? It's running, but is very slow because my layer is large, so I'm not sure what it will find.
    – Richard
    Apr 25, 2017 at 15:38
  • Ah just found this: gis.stackexchange.com/questions/119625/…
    – Richard
    Apr 25, 2017 at 15:40
  • Unfortunately this doesn't work for me because my shapefile is too large... QGIS just stops responding.
    – Richard
    Apr 25, 2017 at 16:08
  • A faster method might be to throw your layer into PostGIS and search for overlaps by their bounding boxes but I'm not entirely sure how to accomplish that Apr 26, 2017 at 6:37

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