I have a shapefile with many polygons. Some partly overlap with each other, some are not overlapping. I'm considering a polygon to be overlapping given a minimum area of overlap. 'Islands' of partly overlapping polygons occur. I'm interested to find out how many islands are there and which polygons are forming them.

I can use ArcGIS 10.2 and/or OGR, both with Python but I'm not sure how to proceed. I know there is an overlap function in OGR but it just finds overlapping polygons. Is there maybe functionality already available which does the above?

Example: selected polygons form one 'island'


A trick you can use is to use the Intersect tool on a single layer.

So inputs could be: Polygon layer with overlapping polygons

After the Intersect tool has been run you get this polygon layer from which you can extract centroids:

enter image description here

  • Thanks for your quick reply! Unfortunately it doesn't answer my question yet as I am interested to get a list of all the polygons overlapping in one 'island'. Your figure above would be one such an island, if the overlaps would be larger than a minimum area. – joosthoek Nov 18 '13 at 13:21
  • The resulting featureclass created by the intersection tool would contain the attributes so you can summarize and filter your data. If this does not answer your question, may be you can post up an image of what you are asking? – Hornbydd Nov 18 '13 at 14:37
  • I added an image. The selected polygons form an 'island' and I would like to automatically find these islands and the polygons who form them. – joosthoek Nov 19 '13 at 9:07

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