Choropleth map
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A choropleth map (Greek χωρα, area, region + πληθαίνω multiply) is a map in which areas are shaded or patterned in proportion to the measurement of the statistical variable being displayed on the map, such as population density or per-capita income. It provides an easy way to visualize how a measurement varies across a geographic area. Choropleth is often misspelled as chloropleth -- with an extra L -- because it is mistakenly assumed that the name of the map refers to its use of color.
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 densities by nation would spread the color 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 England might be 40 million, but it would not be accurate to cut England into two halves of equal area and say that the population of each half of England is 20 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 grey-shades to represent the different classes of data being mapped. Choroplethic Maps are now commonly produced by Geographic Information Systems that automate many of these processes, but choices of color-range can still present a challenge. Cynthia Brewer of Pennsylvania State University devised the freeware ColorBrewer, which is useful in formulating color swatches for choropleth maps.
Links: ColorBrewer
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