Isopleths

An isopleth is also known as a contour line. It is a feature of some charts and maps, connecting points which have an equal value of some variable at a given time and spatial area. The particular variables shown may include values such as pressure, temperature, wind speed, population, etc. They are used to assist in visualizing the general features of an area. Some common types of isopleths include: isotherms (points of equal temperature); isobars (points of equal pressure); isohyets (points of equal precipitation). Below, you will see an illustration of the construction of an isopleth map, beginning with various values and ending with lines and shadings connecting those points of equal value.

http://www.fao.org/DOCREP/003/T0446E/T044648.gif