Contour Interval:

•    A contour interval is a vertical distance or difference in elevation between contour lines.

•    It is always kept the same or constant for a map.

The contour interval depends upon the following factors:

1. The scale of the map: The contour interval normally varies inversely to the scale of the map i.e, if the scale of the map is large, the contour interval is considered to be small and vice versa. 

2. Nature of ground: For flat ground, a small contour interval is chosen whereas, for undulating and broken ground, a greater contour interval is adopted.

3. Purpose and Extent of survey: Purpose and extent of survey affects the choice of contour interval, e.g. small contour interval is used for a survey intended for detailed design work and for accurate earthwork calculations. A large contour interval is used when the extent of survey is large, e.g. location surveys for communication lines, highways and railways.

4. Time and Cost:  If the contour interval is small, greater time and funds will be required in the field survey, in reduction, and in plotting the map. If the time and funds available are limited, the contour interval may be kept large.