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.
Select High CI like 1m, 2m, 5m or more
Select Low CI like 0.5m, 0.25m, 0.1m or less
Scale of the map
For small scale maps covering a wide area of varying terrain
For large maps
showing details of a small area
Extent of survey
For rough topographical map meant for initial assessment only
If a detailed map is to be prepared for execution work
Nature of ground
If the ground has large variation in levels, for instance, hills and ponds
If the terrain is comparatively level
Time and resources
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:
Firstly locate 2 index contour lines that are labeled with a specific elevation.
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.
Now count the number of non-index lines contour lines between the 2 index contour lines selected for the contour interval calculating in the 1st step.
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.
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).
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
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.
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.
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:
It is based on vertical levels
Represents horizontal distance
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
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