What is Contour Interval? Its Calculation, Uses and When it is Required

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What is Contour Interval?

A contour interval in surveying is the vertical distance or the difference in the elevation between the two contour lines in a topographical map. Usually there are different contour intervals for the different maps. Considering the size of the area to be mapped, contour intervals are assumed. In every map, on the right-hand bottom side, the contour interval is specified. When the contour interval is not specified in the map, it can be calculated as explained in the following sections. The commonly used contour interval is 20 feet for a 1:24,000 map scale.

Factors Affecting the Selection of Contour Interval

The selection of the contour interval is decided by the survey leader before the start of the mapping process depending upon the ground factors.

Sl. No	Factors	Select High CI like 1m, 2m, 5m or more	Select Low CI like 0.5m, 0.25m, 0.1m or less
1	Scale of the map	For small scale maps covering a wide area of varying terrain	For large maps showing details of a small area
2	Extent of survey	For rough topographical map meant for initial assessment only	If a detailed map is to be prepared for execution work
3	Nature of ground	If the ground has large variation in levels, for instance, hills and ponds	If the terrain is comparatively level
4	Time and resources available	If less time and resources are available	If time and resources abundant

How to Calculate Contour Interval from Maps?

A contour map consists of contour lines of a given geographical region. To keep the contour map simple and easy to read, not every contour line is marked with its elevation reading. These marked or labeled lines are known or termed as Index Contour Lines. In the above figure, the dark lines with reading are index contour lines. The calculation of the contour intervals is as below:

Step 1:

Firstly locate 2 index contour lines that are labeled with a specific elevation.

Step 2:

Now calculate the difference between the two-selected index contour line selected from a map. To take the difference, subtract the higher elevated line with the lower elevated line reading.

Step 3:

Now count the number of non-index lines contour lines between the 2 index contour lines selected for the contour interval calculating in the 1^st step.

Step 4:

The number of lines obtained in the above step is taken and added with 1. For Ex: If the number of lines between 2 index lines are 5. Then add 1 to 5 that becomes 6.

Step 5:

the final step is the quotient of the difference between 2 index lines (step 2) and the number of lines in between two index lines plus 1 (step 5).

Step 6:

The final answer we get after dividing is the contour interval of the specific topographical map

Example Calculation of Contour Intervals:

Considering the above map, the steps involved in contour interval calculation are, Let’s assume, 7000 and 7100 and calculate the interval between it. Now difference between 7100 and 7000 is 7100 - 7000 = 100 The number of contour lines in-between 7000 and 7100 are 4. Adding 1 to 4, 4 + 1 = 5 Now dividing 100 by 5, 100/5 = 20 units The Contour Interval of the above map us 20 Units.

Uses of Contour Intervals in Surveying

1. When a large area is to be mapped in small piece of paper contour intervals are used. A higher contour interval is used for a large area and small contour interval for small area.
2. In a large map, index contour lines are less to keep it simple to read the map easily. In this case, to find out the intermediate points elevation, contour intervals are used.
3. Earthwork estimations for any type of structure like bridges, dams or roads can be found out with the help of contour intervals in a map.

As contour intervals are for the calculation of vertical elevation of an area, same way to calculate the horizontal distance it is termed as Horizontal Equivalent. The horizontal distance between two points on two consecutive contour lines for a given slope is known as horizontal equivalent. The difference between Contour intervals and Horizontal Equivalent are tabulated below:

S. No	Contour Interval	Horizontal Equivalent
1	It is based on vertical levels	Represents horizontal distance
2	No measurement or scaling is required since the contour levels are indicated on the contour lines	The distance must be measured on the map and converted to actual distance by multiplying with the scale of the map
3	In a given map the contour interval is a constant	The horizontal equivalent varies with slope. Closer distance indicates steep slope and wider distance gentle slope

Read More: Contour Lines and Its Types, Characteristics and Uses in Surveying Contour Maps and Their Uses Methods of Contouring