Characteristics of contours with sketches

A contour is an imaginary line joining points of equal elevation on the ground. Every point on a contour has the same RL (reduced level), and a contour is the line in which a level surface cuts the ground.

Characteristics of contours

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, equal and 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 value inside represents a hill; with the lower value inside it represents a depression (pond or pit).
8. Contours bend as 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. Contours 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 contour cannot lie between two contours of higher (or lower) values unless it closes on itself (hill or depression).
11. A saddle (col) is shown by the closed contours of two hills on opposite sides, with lower ground (valleys) on the other two sides; the saddle point lies at the neck between the hills.

        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