Isolines are lines that connect points of equal value on a map. They are used to describe distributions of weather elements, such as temperature or rainfall, as well as other types of data, such as population density or elevation. Isolines can be divided into two classes: those in which the values actually exist at points, such as temperature or elevation values, and those in which the values are ratios that exist over areas, such as population per square kilometer or crop yield per acre. The first type of isoline is specifically called an isometric line or isarithm, while the second type is called an isopleth. Some common examples of isolines include isotherms, which represent lines of equal temperature, and contour lines, which represent lines of equal elevation.