Contour characteristics and uses of contour map

Principal 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

Uses of a contour map

1. To find the elevation of any point and the slope between two points from the map (without going to the field).
2. To draw the longitudinal section and cross-sections of a road, railway, canal or pipe line on paper.
3. To fix the best alignment of a road, railway or canal with a given gradient (contour gradient).
4. To find the intervisibility of two points (to decide if a signal station can be seen from another).
5. To calculate the capacity of a reservoir, the catchment area and the volume of earthwork (area between contours).
6. To select suitable sites for a dam, building, bridge, airport or a water tank, and to mark the drainage lines (watershed).
7. To find the quantity of cutting and filling for levelling a site and to plan irrigation and drainage layout.
8. To study the general topography (hills, valleys, ridges) of a large area, and for military purposes and town planning.