Choropleth map

A choropleth map is a type of statistical thematic map that uses pseudocolor to represent an aggregate summary of a geographic characteristic within predefined spatial enumeration units, such as population density or per-capita income. It is likely the most common type of thematic map because published statistical data is generally aggregated into well-known geographic units, making them relatively easy to create using GIS, spreadsheets, or other software tools.

The earliest known example of such a map was created in 1826 by Baron Pierre Charles Dupin, depicting the availability of basic education in France by department. These early "cartes teintées" were soon used in France to visualize other "moral statistics" on education, disease, crime, and living conditions.

Their popularity grew with the increasing availability of demographic data from national censuses, notably a series of choropleth maps in the official reports of the 1841 Census of Ireland. After 1850, the advent of chromolithography allowed color to be added. The term "choropleth map" was introduced in 1938 by geographer John Kirtland Wright and was in common usage by the 1940s.

A choropleth map combines spatial data (a partition of geographic space into distinct districts) with statistical data aggregated within each district. The districts are usually pre-existing governmental or administrative units, such as counties or countries, whose boundaries may not align with the geographic distribution of the variable.

This can lead to issues like the ecological fallacy and the modifiable areal unit problem, as the constant color applied to each district masks internal variation. Using smaller districts can mitigate this but may result in overly complex maps. The choice of districts depends on the map's purpose and audience.