Choropleth maps are based on statistical data aggregated over previously defined regions (such as counties), in contrast to area-class and isarithmic maps, in which region boundaries are defined by data patterns. Thus, where defined regions are important to a discussion (as in an election map divided by electoral regions), choropleths are preferred. Where real-world patterns may not conform to the regions discussed, issues such as the ecological fallacy and the modifiable areal unit problem (MAUP) can lead to major misinterpretations, and other techniques are prefereable. For example, a map showing population bell spread the colour symbol for Canada's population over its entire expanse, while most of the population lies along the coasts and southern border. Unfortunately, choropleth maps are frequently used in inappropriate applications due to the abundance of choropleth data and the ease of design using Geographic Information Systems. The dasymetric technique can be thought of as a compromise approach in many situations. Broadly speaking choropleths represent two types of data: Spatially Extensive or Spatially Intensive. Spatially Extensive data are things like populations. The population of the UK might be 60 million, but it would not be accurate to cut the UK into two halves of equal area and say that the population of each half of the UK is 30 million. Spatially Intensive data are things like rates, densities and proportions. These can be thought of conceptually as field data that is averaged over an area.
When producing a choropleth the cartographer must choose appropriate colours or shades of grey to represent the different classes of data being mapped. Choropleth maps are now commonly produced by Geographic Information Systems that automate many of these processes, but choices of colour can still present a challenge. Cynthia Brewer of Pennsylvania State University devised the freeware ColorBrewer, which is useful in formulating colour swatches for choropleth maps.
The earliest known choropleth map was created in 1826 by Baron Pierre Charles Dupin.