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Direct contouring method, uses of contour map

Short answer

In direct method, staff readings are set to HI minus desired contour elevation; staff man moves until reading matches, then points pegged and located by plane table/theodolite. Contour maps are used to find elevations, design alignments with given gradient, compute earthwork and reservoir capacity, assess intervisibility, select sites, and plan drainage.

Explain the direct method of contouring and write the uses of contour map.

Direct method of contouring

In the direct method the contours are located and marked directly on the ground, and the points are then surveyed and plotted. It is accurate but slow and costly, so it is used for small areas and large-scale maps.

Steps

  1. Establish control (traverse stations) and a benchmark in the area.
  2. Set up a level at a convenient point, take a back-sight on the BM and find the height of instrument: HI=RLBM+BSHI = RL_{BM} + BS.
  3. For a contour of RL hh, the staff reading required is r=HI−hr = HI - h. A staff man moves up or down the slope until the staff reading equals rr; that point lies on the contour. Mark it with a peg.
  4. Find several points of the same contour, then move to the next contour (staff reading changed by the CI).
  5. When the level is shifted, a new HI is found by sighting a fixed point (change point).
  6. The pegs are then located by the plane table, theodolite (tacheometer), chain or compass from the control stations and plotted on the sheet. The points of equal RL are joined by a smooth curve.

For steep ground a clinometer or a hand level may be used with a ranging rod.

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.

✓ Answer checked by Claude Opus 5.5 (high effort)

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