Choropleth Maps are area maps that display areal data of some feature or phenomena under study. These areal divisions are often boundaries such as census tracts, counties, states, etc. Points, lines, and areal symbols represent features such as towns, roads, forests, etc and can also be aggregated, or averaged, by a spatial unit. The area of the spatial unit is then assumed to have a uniform value. In a classed choropleth map, areal units are combined into a smaller number of groups and portrayed in intervals. Typically, 4 to 7 intervals are used and can be divided up based on four different classification techniques: equal steps, quantiles, natural break, and minimum variance. The above map is a classed choropleth map of the unemployment rate of some area. It is classed because it is broken up into six distinct intervals designated by the legend at the right, and is divided by the classification technique of quantiles.
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