Contour interval factors and characteristics

Factors affecting contour interval

1. Purpose of the map: a small CI is used for detailed engineering design (dam, building site); a large CI for a general reconnaissance map or a route survey.
2. Scale of the map: a large-scale map (1:500) has a small CI (0.25 m to 0.5 m); a small-scale map has a large CI (5 m to 20 m). A rough guide is CI (m) ≈ scale denominator / 2000 (0.25 m at 1:500, 5 m at 1:10 000), so that the lines do not crowd.
3. Nature of the ground: flat ground needs a small CI (0.25 to 1 m) to show the relief; steep or hilly ground needs a large CI (5 to 10 m or more), or the contours crowd together.
4. Time and money available: a small CI needs more field points and more time, so the cost increases.
5. Accuracy required: if the work needs high accuracy (for example, earthwork volumes), a small CI is chosen.
6. Size of the area: for a large area the CI is large; for a small area it is small.

The contour interval should be kept constant over one map, so that the slope can be judged from the spacing of the contours.

Characteristics of contours with sketches

1. Contours are closed curves. Every contour must close on itself, either inside the map or outside it. It cannot start or end abruptly.
2. All points on a contour have the same elevation.
3. Contours never cross each other, except at an overhanging cliff or a cave. Two contours of different elevation cannot meet or cross, because one point cannot have two elevations.
4. Contours of different elevations do not unite to form one line, except at a vertical cliff, where they coincide and appear as one line.
5. Closely spaced contours indicate a steep slope; widely spaced contours indicate a gentle slope. Equally spaced contours indicate a uniform slope, and straight, equally spaced, parallel contours indicate an inclined plane.
6. Contours cross ridge lines and valley lines at right angles. The direction of the steepest slope is at right angles to the contour.
7. A set of concentric closed contours with the higher values inside represents a hill; with the lower values inside it represents a depression (pond or pit).
8. Contours bend in V or U shapes across a ridge and a valley. For a ridge (spur) the bend of the contour points towards the lower ground; for a valley the bend points towards the higher ground.
9. A contour of the same elevation may reappear on the other side of a ridge or a valley, so one contour value can appear twice on a map.
10. A single contour cannot lie alone between two contours of higher (or lower) value. If the ground dips (or rises) between them, it crosses that level twice, so the value appears as two lines or as one line closing on itself.

        HILL                    DEPRESSION
      .--100--.                .--100--.
     / .-110-. \              / .--90-. \
    | / (120) \ |            | /  (80)  \ |
     \ '-----' /              \ '-----' /
      '-------'                '-------'
   RL rises to the centre     RL falls to the centre

   RIDGE (v points downhill)     VALLEY (^ points uphill)
      130  \   /                    120     /\
            \ /                           /  \
      120  \   /                    110   /    \
            \_/                          /      \
      110  \     /                  100 /        \
            \___/
   high ground at top            high ground at top

   Steep slope      Gentle slope      Uniform slope
   (close)          (wide apart)      (equal spacing)
   ----100          ----100           ----100
   ----101                            ----101
   ----102                            ----102
   ----103          ----101           ----103