choropleth

A choropleth is a type of thematic map in which geographic areas are shaded or colored according to the value of a statistical variable for each area. It is commonly used to visualize numeric data tied to spatial units such as countries, states, counties, or census tracts, including population density, unemployment rates, election results, or disease incidence.

To construct a choropleth, data must be linked to a defined geographic boundary and typically normalized when appropriate. Normalization helps compare areas of different sizes or populations (for example, per capita rates or rates per 100,000 people). The choice of geographic units and boundary files is important, as changes can affect interpretation.

Classification and color are central design decisions. Data can be grouped into classes using schemes such as equal intervals, quantiles, natural breaks (Jenks), or standard deviation. A sequential color ramp—from light to dark—conveys increasing magnitude, while diverging schemes are used for data with a meaningful midpoint. Palettes should be colorblind-friendly and legible at the map’s scale, with an appropriate number of classes to balance detail and readability. A clear legend and labels are essential for interpretation.

Limitations include the modifiable areal unit problem (MAUP), where different boundary choices alter results, and potential bias toward larger areas. Choropleths show relative differences but not raw counts without context. They are often complemented by alternative visualizations such as dot density maps, heat maps, or cartograms to convey additional aspects of the data.