Whether you’re checking election results or checking the CDC website for health-related recommendations, chances are you’ve probably come across a choropleth. Choropleth maps are everywhere. In fact, they are probably one of the most commonly used thematic map types. This map type displays information divided into spatial units such as states, provinces, census tracts, etc., and uses varying shaded colors or pattern textures in proportion to the distribution and density of a particular variable. For example, the image above uses a yellow-red color gradient to display population density by state within the US. Here low population densities are displayed in yellow-orange colors and high population densities are displayed in hues of red.
Although these maps are popular, they are not suitable for every purpose. Choropleth maps are used to display rates like percentages (in other words, normalized data) over raw values. For raw values, proportional symbols are generally better than choropleth maps because they avoid geographic distortion. A choropleth map, like any map type, can be deceiving, as aggregating data by predefined spatial units can give a false impression of data changes at boundaries. This is more commonly known as the Modifiable Areal Unit Problem.
- Population density by county
- Per-capita income by census tract
- Unemployment rate by state
- Give an example of the type of data that is appropriate for a choropleth map.
- Why would raw values not be appropriate for this map type?