Choropleth map

Lore & Background

More 'cartes teintées' ('tinted maps') were soon produced in France to visualize other 'moral statistics' on education, disease, crime, and living conditions. The term 'choropleth map' was introduced in 1938 by the geographer John Kirtland Wright and was in common usage among cartographers by the 1940s. Also in 1938, Glenn Trewartha reintroduced them as 'ratio maps,' but this term did not survive. The map brings together two datasets: spatial data representing a partition of geographic space into distinct districts, and statistical data representing a variable aggregated within each district. Two common conceptual models exist: 'district dominant,' where districts are the focus, and 'variable dominant,' where the variable is the focus. Using pre-defined aggregation regions has advantages, including easier compilation and mapping of the variable, recognizability of the districts, and applicability to further inquiry and policy. However, it can result in issues such as the ecological fallacy and the modifiable areal unit problem (MAUP), because the constant color applied to each district masks variation within it. These issues can be somewhat mitigated by using smaller districts, though they can make the map overly complex.

Reader's Guide

Choropleth maps are significant because they provide an easy way to visualize how a variable varies across a geographic area or show the level of variability within a region. They became the most common type of thematic map due to the widespread availability of statistical data aggregated into well-known geographic units like countries, states, provinces, and counties. Their legacy includes widespread use in demographics, economics, and agriculture, though they are generally discouraged for mapping spatially extensive variables like total population, as patterns can be easily misinterpreted. Instead, they are best suited for spatially intensive variables such as densities, proportions, and rates. The choice of aggregation districts ultimately depends on the map's intended audience and purpose, and alternative techniques like isarithmic or chorochromatic maps may be preferable when detailed data is available. Despite potential misinterpretations, choropleth maps remain a fundamental tool in cartography and spatial analysis.

Did You Know?

• The term 'choropleth map' was introduced in 1938 by geographer John Kirtland Wright.
• Choropleth maps are likely the most common type of thematic map because published statistical data is generally aggregated into well-known geographic units.
• The constant color applied to each aggregation district can mask variation within the district, leading to issues like the ecological fallacy.