Contour Intervals

What Do Contour Intervals on a Topographic Map Show?

The most distinctive feature of a topographic map is its contour lines. Contour lines are lines drawn on topographic maps connecting points of equal elevation. This means that if you follow a contour line, the elevation will remain constant. Contour lines indicate the elevation and shape of the terrain.

These lines are useful because they show the topography of the land, or the surface of the earth. This is a great way to learn how to interpret contour lines. Take a small object, such as a ball or pile of laundry and shine a red laser beam along its side. This will create a contour line that looks like a topographic map.

Topographic maps only show lines at certain elevations to simplify things. These lines are equally spaced. This spacing is called the contour interval.

If your map has a 10-foot contour interval you will see contour lines at every 10 feet (3 m) of elevation -- lines at 0, 10, 20, 30, 40, and so forth.

Different maps may use different intervals depending on topography. For example, if the terrain is very high, the map may be drawn at intervals of 80-100 feet (24-to-30 meters).

This makes it easier for you to read the map. Too many contour lines could make it difficult. To find its contour interval, look in the margin of your cart.

Every fifth contour line on topographic maps is an index contour. This makes them easier to read. It is impossible to label every contour line on a map with the elevation, so the index contour lines will be the only ones. Index contours are darker brown lines than regular contour lines.

Only the index contour lines will show the elevations. For elevations to be determined, you need to pay attention how much space is between the lines.

A steep slope is one where the contours are very close together. The terrain will be relatively flat if the contours are not well defined or have large gaps between them.

Do you see contour lines, mountains, or depressions?

If you see a closed contour, such as the one below it means that there is either a mountain/mountain/depression.

Without a label at the contour line, you don't know what it is. We have added labels to the following example, and it should be obvious that it is a depression.

This is not a normal depression. This is Arizona's massive meteor crater from a long, long time ago.

Cartographers may use teeth marks (or hachures) to mark depressions at craters and volcanoes on some maps. This marks the elevation going up/down.

How do Mount Fuji's contour lines look?

Closed contour lines signify a mountain in this instance. Mount Fuji, 3,776m high above sea level, is the most beautiful mountain. Here's what the contour lines look like at 250 meters apart:

Mount Fuji can be seen in 3D because the contours are very closely placed together.

We've seen two types of closed contours so far. But what about contours in valleys?

The rule of U-shaped and V-shaped valleys

Valleys are low-lying, elongated depressions with rivers running through them. When contour lines are U-shaped or V-shaped, you know you are looking at a valley's bottom.

This is a 3D view showing the Collayomi Valley, California. All rivers flow downwards from higher to lower elevations perpendicularly to the contour line. The V-shaped contour is the opposite direction to the flow of a stream.

The contours of the "V" shape indicate drainage and streams. As you can see the "V" indicates an elevation higher than the "V". To delineate a stream, connect the intersections of the contour lines with a V-shaped shape. You can also estimate the stream gradient by counting how many contours cross a stream using the contour interval.

The slope is always perpendicular with contour lines. A steeper slope is determined by the distance between contour lines. The slope will be uniform if there are equally spaced contours.

Contoured lines create a "M" or 'W" shape at a stream junction. This could be read as two intersecting "V-shaped contours."