What’s a choropleth map?

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A choropleth map is a type of map that uses colors or patterns to relate data about a specific statistic to predefined regions. A common example would be a map of the United States with each state colored red or blue based on which political party candidate the state’s voters voted for in a presidential election. A choropleth map is often paired with a graduated series of shades of a particular color to show the relative frequency of the stat or feature being depicted.

Typically, a choropleth map is made using predefined regions such as states or counties, but it can use other geographic divisions. They can be used to illustrate the distribution of almost any statistically observable data, most often relating to only one variable, such as population density. By applying a pattern key or shades of a particular color to a regularly divided scale of the variable, the map provides a graphical representation of the data that is quickly and easily interpreted. Models are often used for strictly black and white choropleth maps, but more commonly shades of one color are used, with a deeper or richer hue indicating a higher density or occurrence of the variable in question. Grayscale shading is also sometimes used.

Population density is a common dataset represented in a type of choropleth map. The map is divided, based on overall scale, into recognizable geographical or political units, such as states, territories, counties, or even city districts or neighborhoods. The key divides the population density into a range from minimum to maximum for the area depicted. Each division is assigned a key or pattern based on key, with darker shades indicating greater density. The individual units on the map are then colored accordingly, so that at a glance the population density of a given region can easily be determined by comparing its color with the key.

Maps of this type can be used to show almost any variable data set, and can be drawn on almost any scale that allows division. The relative distribution of land use, population, political affiliation and rainfall are just a few examples. In some cases, different colors may be used when the map is intended to show different types of data, rather than relative degrees of a data type. For example, different colors could be used to show the most common make of car owned by residents of the various divisions on the map, with each color corresponding to a make of car. A single color system would be more appropriate for the relative distribution of a particular make of car as a percentage of all cars in each geographic division.

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