Choropleth map
A thematic map using color for aggregated geographic data.
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 was created in 1826 by Baron Pierre Charles Dupin, depicting the availability of basic education in France by department. These early "tinted maps" soon visualized 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 published in the official reports of the 1841 Census of Ireland. After 1850, chromolithography allowed color to be added. The term "choropleth map" was introduced in 1938 by geographer John Kirtland Wright and was common among cartographers by the 1940s. A choropleth map combines spatial data representing 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 census tracts, whose boundaries do not necessarily align with changes in the variable being mapped. This contrasts with chorochromatic and isarithmic maps, where region boundaries are defined by the phenomenon itself. Using pre-defined regions offers advantages like easier data compilation and recognizability, but can mask internal variation, leading to issues such as the ecological fallacy and the modifiable areal unit problem. Using smaller districts can mitigate these issues by showing finer variation, though it may make the map overly complex. The dasymetric technique can sometimes refine region boundaries to better match actual changes in the subject phenomenon.
- creator
- Baron Pierre Charles Dupin
- field
- Cartography
- term_coined
- 1938 by John Kirtland Wright
- common_usage_by
- 1940s
- type
- Statistical thematic 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.
Frequently Asked Questions
When and by whom was the term 'choropleth' coined?
The word itself did not enter the lexicon until 1938, when John Kirtland Wright introduced it. By the 1940s the name had become the standard label for this mapping technique.
What exactly does a choropleth map show?
It shades predefined geographic units—like counties or states—with pseudocolor to visualize an aggregated statistic such as population density or per-capita income. Each polygon's tone encodes a single summary value for that area.
Why is the choropleth map the most common type of thematic map?
Because published census and economic data are already rolled up into familiar administrative boundaries, the data aligns naturally with the map's enumeration units. This makes it straightforward to produce in GIS packages, spreadsheets, or dedicated cartography tools without heavy preprocessing.
What field does the choropleth map belong to, and what is its core purpose?
It sits squarely within cartography as a statistical thematic map. Its core job is to let readers quickly compare magnitude of a chosen variable across regions at a glance, using color intensity as the primary visual channel.
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