Isopleth Map

 http://geographyfieldwork.com/DataPresentationMappingTechniques.htm

Isopleth maps generalize and simplify data that has a continuous distribution. Unlike choropleth maps, the data is not confined to a predefined unit, such as a county. These maps are great for showing gradual change, since there are no set boundary lines that will cause an abrupt change. The data is shown by series of lines called isopleths, that connect points of equal value. It is important to note that isopleths always form enclosed circles as well. The isopleth map above displays the precipitation that fell on June 10, 2000. As precipitation is a gradual occurrence, an isopleth map was appropriate.