Cartisan > Types of Maps > Choropleth Maps

Choropleth maps are ubiquitous. These are maps where enumeration units, such as states or countries, are shaded a particular color depending on that unit's data value. There is not much more to them, and it is this simplicity that makes them popular. Unlike cartograms, choropleth maps present readers with geography they are familiar with. Most people comprehend maps based upon outlines of familiar areas. Try sketching the outline of your home state and it will be recognizable. Compare that to a cartogram's outline of the same state and you'll probably scratch your head a little bit.

On a choropleth map, cartographic convention assigns the darkest color to the highest value. This is to portray the concentration of that data within the enumeration unit. Depending on the map's subject, this can often be a more complex decision to make. When producing a map of least expensive counties to live in, what gets the darkest color on the map - is it the county new home sale prices of $1 million, or the county with a $75,000 sale price? More money means a darker color, right? Not necessarily, because you're mapping the "least expensive". Since the map is about highlighting more economical counties, and research shows that our eyes are more typically drawn to darker, denser areas of a map, you'll want to make sure the least expensive counties are the darkest. The question can become deeper than visual perception. What if you are mapping illiteracy rates? Do you want to make the more illiterate areas darker? Does this bring into question mental perceptions of the relationship between illiteracy and race or ethnicity (i.e. skin tone)? Sometimes you work with cartographic convention, and sometimes you need to adjust it.