I need to create a topology where 1 and 2 touch but don't overlap, a and b touch but don't overlap, and 1 covers a and 2 covers b. Is it possible to achieve this? I've tried various snapping options, but they all seem to result in one of the inner polygons overlapping with a wrong outer polygon. I'm using QGIS.

  • What snapping options have you tried? If snapping is properly set, you shouldn't have any overlap issue (assuming your polygons are all in the same layer). – ArMoraer Apr 22 '16 at 5:25
  • I've tried basically all the snapping options i can find. I messed with allowing intersections and not allowing them, map units and pixels, etc. The outer polygons (1 and 2) are a different layer than the inner polygons (a and b). I need to make it so that none of the inner polygons can overlap with an outer polygon other than the one that covers it. – user3747260 Apr 22 '16 at 5:36
  • Ok. First of all you should use pixels and not map units. Even with polygons in different layers, snapping should work (how to snap two line vertex in two different layers used in QGIS). If this still doesn't work, maybe check if your layers have the same CRS. – ArMoraer Apr 22 '16 at 5:50
  • They're definitely snapping together, but still overlapping. The layers are the same CRS -- the default. – user3747260 Apr 22 '16 at 6:04

I seem to have found a solution that's working at least so far. I'm creating two vertices on the shared boundary of the outer polygons and then snapping the inner polygons to that.

  • That's the correct way. Without real vertices at snapping points they must be computed on-the-fly for the overlap analysis and that is sensible to rounding errors. – user30184 Apr 22 '16 at 9:23

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