Contour line is a line which makes a closure. It has one and only one value associated with each line. The value may be written along the path of the line. Everywhere along a contour line, the value remains the same.

Each contour line is different from the adjacent line by a constant value called contour interval. In this illustration, the value is 10. Notice that this value has nothing to do with how wide or how narrow contour lines are separated on a map.

In a topographic map, contour lines are used to represent elevation above sea level. Not only the elevation along a contour line is shown, elevation everywhere else can also be estimated .

There are several important rules about the use of contour line. These rules regulate how do we read a topographic map and how do we use contour lines to make a topographic map.