Isopleth - Flu-like Illness (France, 2002)

http://www.femwiki.com/fem/w/fem/choosing-an-appropriate-type-of-map.aspx

Isopleths are lines of that denote constant values of whatever variable you are mapping.  These variables can be anything from wind speed to sickness.  Because isopleths have such a wide range of possible uses, they are utilized in a multitude of different sciences.  This technique of mapping a phenomena can be extremely useful in determining relationships that may exist.

In the map above, incidences of flu-like illness are plotted using isolines in France.  The areas inbetween the lines in this situation are colored in, but they don't have to be.  Through this mapping technique we can see that there is a high concentration of this flu-like illness in the southeastern part of the nation but is relatively low in other areas.