Choropleth Map

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http://www.geog.ucsb.edu/~jeff/gis/choropleth_maps.html

A choropleth map shows areas by shading or patterning in proportion to the measurement of the statistical variable being displayed on the map. Common choropleth maps use statistical variables like population density or per-capita income. A choropleth map is a simple map type that depicts an average via a graded scale. The choropleth map provides an easy way to visualize how data varies across a geographic area or it shows the level of variability within a region or multiple regions. The choropleth map above is of crime rates in the U.S. overlayed with election results from 2004 above via colored shading.