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Vary the dot density value depending upon data in map extent in ArcMap

September 24 2008 | 0 comments
Categories: Symbology

I'm creating dot density maps of the US (various datasets to support this) where a user,in ArcMap, will zoom to points of interest with an extent showing approx 10-15 miles of data.

The issue I'm facing is that the a single dot density value does not really work since the density difference between places like Bozeman v. Manhattan is so great. And I either get too many dots and it obscures the map or to few to be meaningful.

Also there are thousands of points of interest (with the potential of more being added) so I don't want to have to check each point of interest and determine the 'best' dot density value for each one.

Is there any way to vary the dot density value based on data within the extent or the like? Or some other more automated way to deal with this, either out of the box or with custom code?


Mapping Center Answer:

This is one of those "double edged sword" issues.  If users have any way of comparing the maps for Bozeman, MT and Manhattan, NY then you definitely don't want to vary the density value (or dot ratio) or the size of the dots.  The result would be confusing. 

But you could use Color, so you might have three different map designs, one where 1 dot = 100 people, another where 1 dot = 2,000 people, and another where 1 dot = 25,000 people. Make the dots sufficiently different colors. 

You could approach this by using the U.S. Census tracts dataset (Data & Maps collection) and calculate three classes of density (low medium and high).  One of the very first Mapping Center blogs described a method where you set up your classification as normal and then convert the result to representations which will give you a new field (Rule_ID) that has a value of 0,1, or 2, which would represent the three classes.

Then set up three layers, with definition queries such that each layer isolates one one of the classes.  Set up the Dot Density for each layer and then no matter where you pan/zoom, you'll have a reasonable coverage of dots and no confusion over what the meaning of each dot represents.  Worst case is you may need four or five classes (colors) of dots to get the job from Bozeman to Manhattan done properly.

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