The earliest choropleth map is credited to Charles Dupin, a French mathematician who, in 1826, published a thematic map using shadings to show the distribution of illiteracy in France.
How are Choropleth Maps Organized?
Choropleth maps are based on predefined political borders such as states or counties. The color provides an easy way to visualize the distribution of a measurement across a region. However, there can be perceptual issues. For example, in the United States the Southwest takes up a large area on a map but the population is relatively low, while New England takes up a very small area on a map but has very high population numbers. For population-related information, then, New England can appear under-represented.
When using a choropleth map, it’s important to use an appropriate color scale so the map’s color legend should use a progression that mimics the data. For categorical data, five to seven color categories are generally the most that should be used so that the viewer can easily identify the color and match it with the category.
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Choropleth Map Example: