I have a polygon layer that contains some polygons inside of polygons. I need to be able to identify the polygons inside of polygons and discard the polygons inside of the polygons. What is the most efficient way of doing that?

  • 1
    what's the point of the union if the polygons are contained within the bigger ones? Are there holes? Otherwise the inner ones could be simply deleted, right?
    – underdark
    Mar 15, 2013 at 17:55
  • Good point, removing that...
    – CLJ
    Mar 15, 2013 at 19:56

1 Answer 1


If the polygons need to be completely within larger polygons you can use ST_Contains, if they simply need to be touching at some point you can use ST_Intersects.

If they're completely within your union is just going to return the outer boundary of the larger polygon. Do you need to doughnut out the smaller polygons from the larger ones?

  • I see what you are saying, I don't actually need to union them, just need to take the boundary of the containing polygon and discard the polygons inside it.
    – CLJ
    Mar 15, 2013 at 18:04

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