Making a "heat map" that counts overlapping polygons

Question

Attempting to symbolize where and how many polygons overlap, making a sort of heat map.  For example, an area where 8 polygons overlap would be symbolized red, whereas an area with only one would be symbolized green.  Only the area of overlap should be considered.  Attempted advanced merges with ET Geowiz, but doesn't retain the attributes for a polygon count.  Was going to then execute a dissolve to get the individual polygons, but without the count info in the attribute table, it wouldn't work.  Ideas?  Currently using version 9.1, making the jump to 10 later this summer.

CheyenneCummings · Answer

I tried this method and feel although you get an idea of what your data is showing, its not a true value. another way is to take those same polygon and construct a raster file (and you don't need to convert polygons to raster first).

First thing you need to do is calculate the Euclidean Distance for your polygon layers (Each layer/shapefile will need its own euclidean distance calculated). This tool (ArcMap 10) can be found under spatial Analyst tools --> Distance --> Euclidean Distance. Once you have all the need layers you can use the raster calculator (Spatial Analyst tools --> Map Algebra --> Raster Calculate) to merge the rasters into one file.

The raster will be auto set to 'black to white' color ramp but one can change to "Red to green' or any other chosen ramp. I provided images before and after.

Euclidean Distance.jpg

Polygon.jpg

DonovanCameron · Answer

After the Union, I made a new field in the Union output called 'DissolveID' and calculated it as Round(Shape_Area, 4). I then ran my Union polygons through Dissolve using 'DissolveID' as the dissolve field and my unique Id as a COUNT and the value as a SUM.

I have done this a couple of times in the past, but have used a different UID which is a concatenation of the X and Y values (centroid of the polygons).

To add to the methods mentioned above here are some existing forums (lots in the arch. forums):
Note the method which you use to generate your centroids because in some instances overlaps occur where they should not.

Spaghetti and Meatball Method (haven't tried this one yet)
Sum of Overlapping Areas

JimW1 · Answer

There's probably a way to get Arc to do this more automatically, but here's how I did it:

1.) Union your polygons with the polygons as the only input - there will be multiple copies of each overlapping polygon
2.) Run Find Identical tool on unioned feature class to assign a unique number to each set of identical polygons - this is found in a new field called FEAT_SEQ
3.) Join this table back to the unioned table based on ObjectID from unioned feature class and IN_FID from find identical table
4.) Run Frequency tool on the find identical table to count the rows with the same FEAT_SEQ value
5.) Join the frequency table to the unioned table based on FEAT_SEQ

It's friday, the ArcInfo licenses are all in use... I normally do this the same way as above but today I was feeling inspired to find shorter way.

After the Union, I made a new field in the Union output called 'DissolveID' and calculated it as Round(Shape_Area, 4). I then ran my Union polygons through Dissolve using 'DissolveID' as the dissolve field and my unique Id as a COUNT and the value as a SUM. 
The resulting polygons look great (See Attached).
Make sure to unchecked 'Create Multipart..'

I first tried Dissolving on the Shape_Area field directly but ESRI and circle areas (point buffer polygons) are not consistent - the shape area migrates a few micrometres somewhere in the geoprocessing

Sure there are some data quality issues if two adjacent polygons have the same shape area down to four decimal places and carrying any fields beyond the statistics fields are almost impossible but... No joins and No ArcInfo!

I tested a more complicated market and the method looks to be okay for those without ArcInfo (see attached #2).

Hope this helps anyone sans ArcInfo and without 'permission' to load ET Geowizards.

polygonjoins.JPG

polygonjoins_complicated.JPG