I have a polygon shape file that has many overlapping features. Each feature represents an area used by polar bears. So some of the polygons overlap and some don't, and there are separate clusters of overlapping areas.

I would like to get a measure of how much overlap there is in a particular area. But I have no clue how to do this. I'm pretty new to GIS and am still learning the ropes, it just seems I can't find an answer.

  • 1
    Are you interested in calculating the total area of the overlapping regions of the polygons?
    – Aaron
    Jan 23 '13 at 14:21
  • Well yes eventually, but that is not my focus.
    – Ghazam
    Feb 5 '13 at 0:00

In ArcGIS, the easiest way to create a polygon layer with the count of overlapping features is as follows:

  1. Run the Union tool on your source polygon layers. This will result in a layer with one feature for each area of overlap.

  2. Add a new field to the layer created in Step 1, called NewID or something to that effect, and use Field Calculator to set it equal to the FID field.

  3. Use the Merge tool to merge your source polygon layers into a single layer with overlapping features.

  4. Run the Union tool on the layer created in step 3. This will result in a single layer with multiple features for each area of overlap (shown below). The Union tool behaves differently (creating multiple features for each area of overlap) when run with a single input, as explained in the How Union Works help page. enter image description here

  5. Run the Spatial Join tool. Your target features will be the result of Step 4. The join features will be the result of Step 2. The match option will be ARE_IDENTICAL_TO. All other values should be left as defaults. The result of this is the same as Step 4, except with new attributes added, including the NewID.

  6. Run Summary Statistics on the output of Step 5. Choose whatever Statistic Field you want. Set your case field to NewID. The resulting table will have a column called FREQUENCY that shows how many times each NewID was found in the output of Step 5. This is equal to the number of overlapping features in the source data. You can join this table back to the output of Step 2 for visualization.

  • The problem is that I do not have multiple layers. All of my polygons are within a single layer. So when I do step one, my input is only one layer and the result is similar to step 4. I go from 135 features to a total of 755.
    – Ghazam
    Feb 5 '13 at 0:10
  • @user14496 There is no way to "force" the Union tool to not create overlapping features besides changing the number of input layers. A workaround for your situation is to split your 135 input features into two layers: one with 134 features and the other with just 1 feature.
    – dmahr
    Feb 14 '13 at 16:57

The attached image shows several polygons representing polar bear ranges (i.e. hollow red color). The areas in blue show the overlapping regions. The labels correspond to the polygon ID's. To perform the analysis, try the following:

  1. Make sure your featureclass is in a File Geodatabase for ease of area calculations
  2. Intersect the featureclass
  3. Use Dissolve based on "OBJECTID" to combine all of the overlapping areas. If you want individual overlapping regions, dissolve based on "FID_yourpolygon", which is the ID of the original polygons.
  4. Add a new field in the dissolve output attribute table "Acres". Right click the "Acres" field and calculate geometry based on area > acres

enter image description here enter image description here

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