Choropleth Mapping In The News

Mapping displaying regional “chunks” abstract information, make certain impressions

A famous choropleth map – Electoral College results – from the 2004 US presidential elections (credit:”Maps and cartograms of the 2004 US presidential election results“, Gastner, Shalizi, and Newman, Univerisity of Michigan).

One of the ways cartographers can express the quality of statistics across regions is by means of the choropleth map.

The word choropleth is, surprisingly so given the English language’s embarrassment of glossaries and dictionaries, not well documented. Raisz’s General Cartography, however, holds that the words come from the Greek for region and measure, and this definition seems to fit the use of choropleth mapping quite well.

But what is choropleth mapping? In as few words as possible, a choropleth map interprets statistics across regions by dividing them into smaller tracts and fills those tracts with a graphical representation of the statistic within those tracts. This interpretation can be iconic, but designers usually find the fill with the greatest impact tend to be patterns, shades of gray, or colors.

What’s Red and Blue and Known All Over?

While choropleth maps sound like a tool mainly used by academics, like any other map or style of mapping, it impacts everyday life and public thought when one least expects it. An example is reporting on the US Electoral College.

As many know by know, the Electoral College is how presidential elections are won in America. Each Presidential candidate is awarded electoral ‘votes’ according to whether or not they win each state’s individual vote tally; the candidate who wins the most electoral votes rather than the most individual (”popular”) votes wins the election. The result of the last contest is depicted by the map on the right at the top of this article.

There’s Something About Abstraction

The Electoral College results map is a particularly Procustean and brutal sort of choropleth map, where the regions are obvious and the classification criteria are very coarse indeed. Filled with red and blue, two colors with high contrast, impressions generated by it have even entered the popular lexicon and have been made fashoionable by news reportage; Red for Republican support, Blue for democratic.

When viewing a choropleth map, especially one as coarse as this, requires a bit of knowledge. The overhwelming color is red, which tends to leave one with the first impression that this preponderance reflects a great deal of popular support; the blue areas seem somewhat isolated. But looking closely at the map shows that most of the blue states (California, New York, and Illinois, for example) are some of the nation’s most populous, whereas the majority of the red region are the more modestly populated American states (notable exceptions being Texas, Florida, and Ohio). Moreover, it reflects little of the actual closeness of the last American Presidential contest, which teetered on the knife edge of one hotly contested state; had the Democrats won Ohio, the contest would have worked out in favor of Gore rather than Bush, without a remarkable increase in blue coverage on the map.

A More Nuanced Map

A section of a map (credit: Pensito Review) showing recent Presidential approval ratings, using the red/blue convention

On 23 November 2005 a blog of politics and cultural news, the Pensito Review, tabluated the
Presidential approval ratings and produced a map summarizing the numbers, state by state. The results, drawn from surveys performed by the national pollster Survey USA, document the results by applying more divisions across the data, providing a more nuanced map (see illustration). By continuing the use of the red-blue scheme made fashionable by the news in 2000 and 2004, the map communicates with punch (especially when compared with the 2004 electoral map) the zeitgeist of Presidential support that the day’s news stories seem to reflect (the entire post and map is available by loading http://www.pensitore...3/america-turns-blue/

But comparing this map (or indeed any) with an Electoral College map is somewhat akin to comparing apples and oranges, as the popular trope holds. While they express basically the same assumptions they express them in different ways with different rationales.

Best Viewed while thinking

The drawbacks and strengths of choropleth mapping can be suggested by these examples but the deeper story is a great deal more complex (and, given the scope of this article, an excercise for the reader). Choropleth mapping displays the general tendencies of data but, depending on the subject, should not be taken on thier own as an indicator without knowledge or questioning; indeed, a popular quip heard these days is that America is neither red nor blue, but purple.

When viewing such a map, the viewer should always have thier thinking cap on – not necessarily due to suspicion, but more to effectively interpret the information with meaning, producing understanding.