Tutorials and hands-on lessons for learning

Choropleth Maps

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.

Choropleth map examples
Youth Tobacco Survey |Mapbox Election Map

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.

Examples of data sets appropriate for this map type:

  • Population density by county
  • Per-capita income by census tract
  • Unemployment rate by state

Practice Tutorials

In the following tutorial, you will use Mapbox Studio to create a choropleth map to display population density in the US:

Make a choropleth map, part 1: create a style

Your turn

Use Mapbox Studio to create a choropleth map with any of the following datasets:

  1. US unemployment rate
  2. 2016 Election map with albers USA projection
  3. CDC 2018 opioid overdose data by state

Review questions

  1. Give an example of the type of data that is appropriate for a choropleth map.
  2. Why would raw values not be appropriate for this map type?
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