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 according to which political party's candidate that states electors voted for in a presidential election. A choropleth map is often keyed to a graded array of shades of one particular color to show relative frequency of the illustrated statistic or characteristic.
Typically, a choropleth map is made using predefined regions such as states or counties, but may use other geographical 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 key of patterns or shades of a particular color to a regularly divided scale of the variable, the map provides a graphic representation of the data that is quickly and easily interpreted. Patterns are often used for strictly black and white choropleth maps, but more commonly, shades of one color are used, with a deeper or richer shade indicating a higher density or occurrence of the variable in question. Grey scale shading is sometimes used as well.
Population density is a common data set depicted in one type of choropleth map. The map is divided, according to overall scale into discernible geographic or political units, such as states, territories, counties, or even boroughs or neighborhoods in cities. The key divides population density across a range from the minimum to the maximum for the area depicted. Each division is assigned a shade or pattern according to the 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 any given region can be easily determined by comparing its color to the key.
Maps of this type can be used to show almost any variable data set, and can be drawn to almost any scale that allows for division. Relative distribution of land use, population, political affiliation, and rainfall are just a few examples. In some cases, different colors can be used when the map is intended to show different types of data, rather than relative degrees of one type of data. For example, different colors could be used to show the most common brand of car owned by the residents of the various map divisions, with each color corresponding to one brand of car. A single color system would be more appropriate for the relative distribution of one particular brand of car as a percentage of all the cars in each geographic division.