Choropleth map

A choropleth map shades or colors predefined regions, such as countries, states, counties or census tracts, according to the value of a statistic in each one, for example unemployment rate or average income. It is one of the most common ways to visualize how a measure varies across geography.

Building one requires two things: boundary data for the regions, produced in a geographic information system, and a classification scheme that bins the underlying values into color categories, using methods such as equal intervals, quantiles or natural breaks. This differs from a dot-density or point map, which shows individual located events rather than a value aggregated over an area, and from a cartogram, which resizes regions by value instead of shading them.

Choropleth maps are widely used in public health, elections, market analysis and spatial analysis generally, anywhere a rate or ratio needs to be compared across places. The best-known pitfall is that large regions dominate the eye even when their population or relevance is small, a phenomenon that made red-blue US election maps notoriously misleading versus population-weighted alternatives; showing a raw count rather than a rate per area or per capita compounds the same problem.