I am trying to overlay two polygon layers using the union tool from QGIS, to then compute the areas from the resulting polygons. However, in the resulting layer some polygons are not divided as expected. Attached you will find two figures showing the layers I want to overlay (orange and green) and the resulting one (pink). Has anyone had a similar problem? Do you know what is due to?

Two layers to overlayResulting layer

  • Welcome to GIS:SE @abo! Looks like the orientation for both polygon layers is slightly different, hence why there's a misalignment. Did you use the Vector Grid tool to create them? – Joseph Feb 12 '16 at 14:44
  • From the pink picture it seems that the union fails when a green polygon overlays 4 orange polygons, and at least at one side borders of both polygons are nearly congruent. Then the cut perpendicular to that side fails. Is that right? Maybe a precision issue? Or due to the sequence with which the algorithm processes such situations? – Detlev Feb 12 '16 at 14:55
  • Thanks for the comments. I've used the option “voronoi polygons” from the vector geometry tools to create the polygons. In my opinion, @Detlev 's description of the problem is correct. Is there a way to solve this? Thanks again. – abo Feb 16 '16 at 15:50
  • Did you check topology of your both input polygons by "Topology Checker" before apply union? – nickan Mar 2 '16 at 6:55

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