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.

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