Many of you have probably seen one form or another of the U.S. election results maps in the last decade, showing red vs. blue states (or counties), with areas won by the republican candidate shaded in red and those by the democratic candidate in blue.

State by state U.S. Presidential election results, 2012
Source: http://www-personal.umich.edu/~mejn/election/2012/

It’s easy to be ‘misled’ by a map, however, in this case because the colors are applied to states that have very different characteristics (area, total population, population density, and population distribution). The red – vs – blue areas appear to be about equal (and it was a close election) but the map is not as factual as it could be. Is it more accurate to show results on a county by county basis?

County-level results for the 2012 U.S. Presidential election
Source: http://www-personal.umich.edu/~mejn/election/2012/

On this map, the red areas vastly outnumber the blue, giving the appearance of a republican victory. Clearly, looking at the results using counties instead of states does not produce a more accurate map. What’s missing? Population density.

Election results mediated by population density.
Source: http://www.theatlanticcities.com/politics/2012/11/political-map-weve-been-waiting/3908/

In the above map, the darkness of each color indicates how densely populated a county is. The vast areas of the Great Plains are very sparsely populated, appearing almost white in some parts, whereas the urban centers on the west and east coasts (look at Long Island), as well as in the Great Lakes, have much higher populations and are therefore very dark.

If you are interested in seeing more examples of these maps, I suggest you explore the article showcasing the population density map at The Atlantic Cities, Mark Newman’s election web site (U of Michigan), or Robert Vanderbei’s maps of the 2012 election (Princeton). At these sites you will find cartograms, county maps shaded to show voter split (purples where the vote was close to 50-50), animations, and more.