Contour line

Contour lines or isolines are used when plotting a function of 2 variables. All the points where the function has the same value are connected. Two well-known examples where such lines are common are: height lines drawn on topographical maps, and showing areas with the same pressure or temperature on weather charts. Contour lines are an application of level sets.

• 
  A map of Stowe VT with height lines
• 
  World map showing magnetic lines, for the year 2000
• 
  Weather diagram showing Meltemi winds over Greece and Turkey
• 
  Temperature diagram of Norway: areas with the same temperature range have the same colour.
• 
  The +10 degree celsius isotherm (July) is commonly used to define the Arctic region.

The bottom part of the diagram shows some contour lines with a straight line running through the location of the maximum value. The curve at the top represents the values along that straight line.

A two-dimensional contour graph of the three-dimensional surface in the above picture.