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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?

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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 '13 at 17:55
Good point, removing that... – CLJ Mar 15 '13 at 19:56
up vote 2 down vote accepted

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?

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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 '13 at 18:04

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