Isopleth

In geography, isopleths are contour lines that depict a variable which cannot be measured at a point, but which instead must be calculated from data collected over an area, such as population density. As you can see in the above map of Jacksonville, an isopleth is being used to display a five year study to measure the average total toxic chemical emissions coming from three waste incinerators.

http://www.stonelions.com/Jacksonville_isopleth_map.jpg