What Is a Contour Line on a Topographic Map?

A contour line is a line drawn on a topographic map to indicate ground elevation or depression. A contour interval is the vertical distance or difference in elevation between contour lines. Index contours are bold or thicker lines that appear at every fifth contour line.

What Is a Contour Line on a Topographic Map?

A contour line on a topographic map is an imaginary line that connects points of equal elevation above a reference surface, such as mean sea level.

The reference surface may be a geoid, an ellipsoid of revolution, or a horizontal plane. The elevation of a point on the Earth‘s surface is its height above or below the reference surface.

In the case of a topographic map, the reference surface is usually mean sea level. Contour lines are a useful tool for visualizing and understanding the shape of the Earth‘s surface.

They can be used to identify features such as mountains, valleys, and plains, and to measure the height of these features. Contour lines can also be used to estimate the size and shape of features that are not visible on the surface, such as underground caverns or hidden lakes.

The spacing of contour lines on a map is known as the contour interval. The contour interval is the difference in elevation between two adjacent contour lines. It is usually expressed in units of feet or meters.

A contour interval of 20 feet means that each contour line is 20 feet higher or lower than the contour line next to it. The contour interval is an important consideration when interpreting a