The purpose of topographic survey is to get the necessary data to produce a topographic map of the earth’s surface. This map will include contour lines, location of natural features, such as streams, gullies, and ditches and man-made features like bridges, culverts, roads, fences, etc. which are needful for detailed planning. The best practical method of presenting topography is by means of contour maps.
Contour or contour lines:
“A contour is an imaginary line of constant elevation on the surface of the ground”. Contours are represented on the map by contour lines. The contour and contour lines are often used inter-changeably.
“The vertical distance between any two successive contours on a given map is called the contour interval”. Contour intervals usually vary from 25 to 250 cm in engineering work. In rough country, the vertical distance between contours is kept greater while in flat areas 25 to 50 cm contour intervals are used.
Characteristic of Contour Lines:
1] All points on a contour line have same elevation
2] Contour line close to each other on s plan view; represent very steep ground. Contour lines for apart indicate relatively flat land
3] On uniform slopes the contour lines are spaced uniformly .along plane surfaces these lines are straight and parallel to one another.
4] Contour lines Crosse ridge lines or valley lines at right angles valley contour are convex towards the stream.
5] Contour lines can not and anywhere, but close on themselves. Either within or outside the limits of map they can not merge or cross one another.
6] A series of closed contour on the map indicate a depression or a summit, depending whether the successes contour have lower or higher values inside
7] At ride line the contour lines form carves of U shape .At Valley lines they farm sharp curves of shape
USES OF CONTOUR:
1] Information regarding character of a tract of a country (such as flat undulating, Mountainous, etc) is abstained.
2] In agricultural work, contours maps are useful as guide lines in planning land improvement project .the tile drainage system can be conveniently planned whit contour maps
3] Cost estimates can be made with the help of the contour maps.
4. Maps which show both topography and land use capability classification are important in conversation of farm land.
5. The most economical and suitable site for engineering works such as reservoir, canal, road, waterways, .etc. can be selected.
6. Quantities of earthwork and runoff from watershed can be computed.
7. Contours may be used to determine area of the catchments and the capacity of the reservoir.
8. A suitable route of a given gradient can be marked on the map.
9. The possible route of communication between different places can be determined from contour map.
Survey for Contour Map [Grid Survey Method]
The area is divided into a series of square. The size of these squares depends upon the nature and extent of the ground. Generally, they have sides verifying from 5 to 20m or 5 to 30m. The corner of the squares are numbered serially, as 1, 2, 3 … A temporary bench mark is set up near the site. The elevations of the ground at the corners of the squares are taken and contour lines are drawn by interpolation, “the process of spacing the contour proportionately between the plotted points is turned interpolation”. For precise work, the proportional spacing of the corresponding contour between any two points is calculated and measured.