Understanding and Using Choropleths, Isopleths, and Area Maps
How do I read it?
A choropleth map will display different values or percentages of a data set in the geographical location. A map that shows the number of births per county in a state defines the boundaries of the data. Counties with a higher percentage of births could be assigned a dark color, while counties with lower birthrates could be identified by a lighter color.
Isopleth maps rely strictly on continuous data. The isolines, or boundaries, that appear on a map connect data points using the same measure. This means that isolines of equal value appear on an isopleth map with values on one side being higher than the values on the other side. The same color is used to fill an area when your ranges possess a similar value. Isopleth maps are similar to choropleth maps in that they color the areas between isoline.This type of map allows the reader to see the gradual changes occurring on a map due to the absence of strict boundaries.