Theory
The location of a point in topographic survey in values both horizontal and vertical control. The methods of locating contours depends upon the instrument used. In the direct method of locating contour, the contour to be plotted actually traced on the ground only those points are surveyed which happen to be plotted. After having surveyed those points they are plotted and contour are drawn through them. The method is slow and tedious and is used for small area and where great accuracy is required.
Instrument required
1. Dumpy level
2. Ranging rods
3. Levelling staff
4. Chain
5. Tape
6. Tripod
Diagram
Procedure
- Before bearing the work the instrument level is setup and flying level are taken from the nearest permanent bench mark near the survey site.
- The level then set in a commonding position and accurately levelled. The height of instrument is determined by taking a back site on the bench mark and adding it to the reduce level of the bench mark from known elevation of the contours and the height of instrument the required staff reading to fire points on the various contour lines contour points are obtained to the first place of a decimal by subtracing the elevation of each of the contours from the height of instrument (H.I.).
- The staff is held an estimated position of a point. It is then moved up down the slope until the desired reading is obtained.
- The point so determine is marked by whites or by fixing the ranging rod.
- Several points are thus determine and mark on each contour in the length of 100 m.
- Only arc contour is located within the desired.
- Having fixed the contour within the desired length the level is moved and setup in a new position. The new height of instrument(H.I.) and the required staff reading are then calculated in a similar manner and the process is repeated until the whole area is surveyed.
- The position of the contour points are located by suitable methods of surveying.
- The point are plotted on the plan and contour are drawn by joining the points by plotted curve line.
