Isopleths

http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Hurr_Structure_Energetics/Closed_Isobars/Closed_Isobars.html

An Isopleth is essentially an Isoline. Lines or contours connect natural occurences in equal amounts in any given area. these contours portray a 3rd dimention on a map.