Aquaculture Engineering (2+1)

Contour surveying - Contour interval

Unit 1- Surveying

Contour surveying

Contour interval
The vertical distance between any two consecutive contours is called the contour interval. The contour interval is kept constant for a contour plan; otherwise the general appearance of the map will be misleading. The horizontal distance between two points on two consecutive contours is known as the horizontal equivalent and depends upon the steepness of the ground.

The contour intervals depend upon the following considerations.

• Nature of ground
• Scale of the map
• The purpose and extent of the survey
• Time and expense of field and office work

Nature of ground
The contour interval depends upon whether the country in flat or highly undulated. For very flat ground, a small interval is necessary. If the ground is more broken, greater interval should be adopted, otherwise the contours will become too close to each other.

Scale of the map
The contour interval should be inversely proportional to the scale. If the scale is small, the contour interval should be large. If the scale is large the contour intervals should be small.

The purpose and extent of the survey
The contour interval largely depends upon the purpose and the extent of the survey. For example, if the survey is intended for detailed design work or for accurate earth work calculations small contour interval is to be used. In this case extent of survey will be generally small. In the case of location survey, for lines of communication for reservoir and drainage areas, where the extent of survey is large, a large contour interval is to be used.

Time and expense of field and office work
If the time available is less, greater contour interval should be used. If the contour interval is small greater time will be taken in the field survey in reductions and in plotting the map.