Esri Mapping Center - Ask a Cartographer

How do I create a density-like display of overlapping polygons?

April 17 2010 | 3 comments
Categories: Map Data

I have a large data set of polygons (more specifically: ellipses) where the actual event location could statistically be anywhere within their respective ellipse. To perform a density of their center points does not accurately portray the highest concentration of events. I am trying to use these polygons to create a density-like, heat map-ish, clustering display, similar to how spatial analyst creates densities of points. Can anyone please help with this issue?

Mapping Center Answer:

You say that an event is likely to occur anywhere in an ellipse. Given that premise, here is something you could try.

Convert each ellipse to a raster filled with 1’s
Sum all the rasters
Symbolize

Note, when converting to raster use the same extent and cell size.

Here is the output when we tried it:

Ellipses

Basically you will be stacking the ellipses, then overlaying the stack with a grid, then counting the number of ellipses that underlie each grid cell. If each ellipse represents a probable area for an event, and if you want to try to find the locations where the event (based on the ellipses) is most probable, then the locations (grid cells) where the largest number of ellipses occur/coincide would be a good guesses.

Further Details Needed posted by Jason Gedney on Apr 28 2010 7:35AM

Thank you very much for the assistance and answer to my above question. I do have one more question though. How do I convert the ellipses to rasters as a group, rather than creating a new lay for each ellipse manually? For some data sets there could be hundreds of ellipses, and I hope that I don't have convert do each one independantly. I know how to do step 2 and step 3, but a little guidance on step 1 would be greatly appreciated. Thanks again for the assistance!

Yes - separate grids for each ellipse posted by Aileen Buckley on Apr 28 2010 6:02PM

To use the above-described technique you would have to create each ellipse as a separate raster because ultimately you would need to add them together. There might be a different solution, perhaps using vector data -- I need to think about this some more.

Simple visual density analysis posted by David Hollema on Aug 8 2012 7:22AM

I'm looking for a straightforward, lazy way to provide a visual display of polygon density based on overlapping (stacked) polygons. ArcMap won't seem to do it. What I was hoping for was a way to use display transparency in the arcmap layer properties to render stacked polygons darker in an additive fashion. It doesn't display this way. All polygons get the same display transparency value and there doesn't appear to be any color addition based on overlapping polygons.

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