Contour terms and characteristics with sketch

Definitions

• Contour interval (CI) is the constant vertical distance between two consecutive contours, for example 1 m or 5 m. It is the same over one map.
• Contour gradient is the line of constant slope (uniform gradient) that cuts the contours at the same horizontal equivalent. It is laid on the ground to fix the route of a road, railway, canal or pipe line. The gradient = CI / HE = 1 in (HE/CI).
• Index contour is every fifth contour (for example every 5th line when CI = 2 m, giving 10 m, 20 m, 30 m ...) drawn as a thicker line and labelled with its RL, so that the map can be read quickly. The other contours are called intermediate (minor) contours.

Main 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).

        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