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I would like to calculate the difference of area between two polygons. I tried using the Difference tool, but all it does is create lines that are different between my two polygons. I also tried the Symmetrical difference tool, but it for some reason it did not work and just generated the input layer out instead of giving me the extra polygons of my two layers. Are there other ways for me to find the area difference between the two polygons? In my attridutes table, all I have for the two polygons are their area.

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And the screen capture of my case.

Here is what I meant. The purple and green polygon are two different layers (Which largely overlap with one another with the green layer being covered by the purple layer). I would like to know the area difference between the two, which is now the parts you can see of the green polygon.

Here is what I meant. The purple and green polygon are two different layers (Which largely overlap with one another with the green layer being covered by the purple layer). I would like to know the area difference between the two, which is now the non covered parts you can see of the green polygon.

I am new to QGIS so I do not know handy methods that can do.

  • The Difference tool seems be the good tool (input layer:green difference layer:purple)? Have you verify there isn't any error of topology? – Christophe Jun 29 '18 at 8:33
  • I tried input layer:green difference layer:purple, which just produces the green polygon with the same area to me, and the other way round, which produce which just produce the same purple polygon with the same area to me, instead of giving me only the green non-overlapped parts. – Sheena Chung Jun 29 '18 at 8:38
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    Do you mean by the same area the exact polygon in the map or the area-value in the attribute-table? Values in the attribute table are not recalculated automatically. Also commom mistake ist that both input layers have do be in the same coordinate reference. – Matte Jun 29 '18 at 8:44
  • New polygons produced by difference tool have both the same area of the polygon on the map and the area-value in the attribute table of the input layer (of the dialogue box) that I selected. And yes both input layers are of the same coordinate reference. – Sheena Chung Jun 29 '18 at 8:52
  • It seems you have to recalculate the area attribute (->field calculator with the expression $area) – Christophe Jun 29 '18 at 9:14
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Turns out something's wrong with my data file. I tried it again and again and the difference tool finally worked.

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