1) Scale of the map
The contour interval is kept inversely proportional to the scale of the map. Smaller the scale of the map, larger the contour interval. On the other hand, if the scale of the map is large, the contour interval should be small. If, on a small scale map, a small contour interval is adopted the horizontal distance between two consecutive contours i.e. horizontal equivalent, is also small and when plotted on the scale of the map, the two contours might unite together. It necessitates to increase the contour interval on small scale maps.
2) Purpose of the map
The contour interval on a map also depends upon the purpose for which the map is to be utilized. If the map is prepared for setting out a high-way on hills slopes, a large contour interval might suffice. But, if the map is required for the construction of an university campus, a small contour interval will be required for accurate work.
3) Nature of the ground
The contour interval depends upon the general topography of the terrain. In flat ground, contours at small intervals are surveyed to depict the general slope of the ground whereas high hills can only be depicted with contours at larger contour interval. In other words, we may say that the contour interval is inversely proportional to the flatness of the ground i.e. steeper the terrain, larger the contour interval.
4) Availability of time and funds
If the time available is less, greater contour interval is adopted to complete the project in the specified time. On the other hand, if sufficient time is at the disposal, a smaller contour interval might be decided, keeping in view all the other factors already described.