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How do I find the differing polygons between two layers in QGIS?

After running the difference operation, however, I get results like this:

diff

I am only interested in the solid (blue) polygons, not the weird lines - which I suspect are the result of rounding error, or previous conversion of the polygon layers from different file types - who knows.

One idea I had was to calculate the area of the polygons and simply select those with non-zero (or above any obvious peak in the distribution) values, but irritatingly the line polygons seem to have substantial areas in many cases.

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    Those 'slivers' could actually be legitimate differenced polygons and not artifacts but hard to say without looking at your original polygon layers. If you only have a handful of the solid blue polygons, you can select them manually and then right-click the layer, go to Save As... and check the Save only selected features option. Then save it as a new shapefile. Alternatively, have you tried the Eliminate sliver polygons tool or the v.clean tool with the rmdangle parameter?
    – Joseph
    Commented Feb 23, 2017 at 12:17
  • Hey, thanks for replying! The slivers are irrelevant for the purpose of the analysis, and there's around 600.000 polygons in each layer all told - the difference operation itself takes around two hours. Fun times. "Eliminate sliver polygons" seems to want to merge neighboring polygons, which is not what I want, and "v.clean" had no effect (out of the parameter settings I've tried - I'll do an exhaustive search just in case). There ought to be some easy check for whether the geometry in question is a "full" polygon or not, and then I could just iterate over the polygons with a python script
    – nkc
    Commented Feb 23, 2017 at 12:34
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    That sure is a lot of polygons for each layer...have you considered importing them to PostGIS and using the ST_Difference tool? This should reduce the processing time significantly :)
    – Joseph
    Commented Feb 23, 2017 at 12:39
  • Thanks, I'll try it out :) I'm still learning PostGIS to be honest; would the query be something like SELECT ST_Difference(layer1.geom, layer2.geom) FROM layer1 INNER JOIN layer2?
    – nkc
    Commented Feb 23, 2017 at 12:45

1 Answer 1

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If ST_Difference doesn't get you where you want try a ratio of area and perimeter by making new attribute fields and calculating. You will have to make a judgement call where to set the ratio filter and if the method is suitable enough for the analysis purpose.

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  • This ended up being the right thing to do. I computed the ratio of area to perimeter and dicked around with a threshold setting for a while. I ended up on the (mostly arbitrary) value of 2, since it seemed to capture the polygons I was interested in.
    – nkc
    Commented Feb 27, 2017 at 8:04

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