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Definition of Contour Interval

The contour interval is the vertical difference between two consecutive contours. The contour interval is defined as the vertical difference between two consecutive contours that are 100 m long, 98m long, 96m long, etc.

The nature of the ground, scale of the map and survey objective will all affect the interval. For flat country, contour intervals are usually small. 0.25 m, 0.5 m, 0.75 m, etc. It is possible to have a steep slope in a hilly region by using the following: 5 m, 10 m, 15 m, etc.

Similar to a small-scale map's interval, it can be either 1 m or 2 m for 3 m and so on. For large-scale maps, the interval may be 0.25m, 0.50m, 0.75m, etc. The contour interval for a particular map is always constant.

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What is a Contour Interval?

The Contour Interval refers to the vertical distance or elevation difference between contour lines. Index contours are thicker, bolder lines that reach every fifth contour line.

The elevation of the terrain will rise if the numbers within certain contour lines are increasing. A decrease in the number of contour lines indicates an elevation loss.

The contour intervals will be used to plot the area. These intervals can be found on the right-handside. If the contour interval is not shown on the map, it can be calculated within the following sections. The most commonly used contour interval for a 1 to 24,000 map scale is 20 feet.

How to Calculate Contour Interval on Maps

The contour interval is usually indicated on a map legend. Sometimes, however, only a portion of the map is available. It is important to know how to calculate the contour interval.

Each fifth contour line on most maps is either an index line or an index contour.

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Calculation of Contour Interval

These index lines will be marked with their elevation. Find the elevations of adjacent index lines. The largest number is indicative of an uphill rise. Find the difference between these elevations.

A contour map includes contour lines that represent a particular geographical area. The contour map is easy to understand and not marked with the elevation reading. These lines, also known as index contour lines, are stored and remembered.

The dark lines in the image above are index contour lines.

How to Read Contour Line

The contour lines indicate the earth's shape. One contour line indicates an equal elevation line. This means that every point along the contour line is 1000 feet above the average sea level.

As the contour lines do not cross, a point on a map cannot have two points at the same time. The earth's slope will be smoother the closer the contour lines are to the map. The more inclined the terrain will become the closer the contour lines are to the map.

The contour lines are very close to the edge. The contour lines that run parallel to a vertical cliff can appear almost as if they are merging.

One line can cross over another on pending cliffs (this is the only time they can cross), and the one line will appear as dotted. However, it is important to remember that even in areas of smooth slopes, smaller cliffs may occur between contour lines.

For example, a cliff 15 feet high along a flow channel, or because of slight faults would not indicate whether the cliff lies between two contour lines. This is especially true if they have a larger contour gap.

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