Isopleth

http://scrabble.wonderhowto.com/inspiration/scrabble-bingo-day-isopleth-0129881/

Isopleth maps are used for 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. Population density is an example of an isopleth because it cannot be specifically measured. This map represents the average precipitation in Eastern US.