Contours

A contour is a line connecting points of equal surface value. Contour lines reveal the rate of change in values across an area for spatially continuous phenomena. Where the lines are closer together, the change in values is more rapid. Elevation and barometric pressure are commonly mapped using contours.

 

Contour lines can be used to represent surfaces. A contour line is a line following an equal value.

 

Contour lines are drawn at an interval that you specify. The interval is simply the change in z value between the contour lines. For example, a contour map of precipitation with a contour interval of 10 inches would have contour lines at 10, 20, 30, and so on. Each point on a particular contour line has the same value, while a point between two contour lines has a value that is between the values of the lines on either side of it. The interval determines the number of lines that will be on a map and the distance between them. The smaller the interval, the more lines will be created on the map.

You can also specify a base contour, which is a starting location. A base contour is not the minimum contour, but refers to a starting point from which contours go both above and below, based on the contour interval. For example, the base contour may be set to 0 and the interval may be set to 10. The resulting contour values would be -20, -10, 0, 10, 20, and 30.

Contours can represent many types of data. Lines connecting surface or sample points of equal value are known as isolines. The following are all examples of different types of isolines: