Isopleth Map

http://dwb4.unl.edu/Chem/CHEM869V/CHEM869VLinks/weather.about.com/newsissues/weather/library/weekly/aa071600a.htm

This is an example of an isopleth map that uses the third dimension of color to indicate the Nitrate ion concentration of the US in 1998. Isopleth maps are used to simplify data that is continuously distributed across a region. In this case, after looking at the key located at the bottom right, one can observe that the most highly concentrated areas were located in the North eastern states and with the highest being at a 2.0 mg/L in Michigan.