- map is prepared according to a scale of 1 cm to 1 km (i.e. 1/100,000).
The topographic survey involves determining the horizontal and vertical locations of objects on the surface of the earth. Horizontal location entails locating objects like roads, railways, ponds, houses, boundaries of properties etc. by measuring horizontal distances the objects are indicated by symbols.
Vertical location includes the location of hills, valleys, depressions, benchmarks, etc. by measuring vertical distances the objects, in this case are represented in relief.
- Thus, a topographic map shows the nature of the earth surface along with the positions of different objects.
- Such a map is essential for the engineering projects involving roads, railways, irrigation, reservoirs, townships etc.
- The scale of a topographic map depends on the extent of area it covers, and the purpose for which it is to be prepared.
Representation of Relief
From the general map of a country, only the positions of objects can be found out, angles of the traverse. The lengths of traverse legs are measured accurately by using the total station if the area is extensive; it is divided into a number of sectors. Each sector is enclosed by a closed traverse having a proper connection with the other sectors (it is ensured sectors have that the common sides).
Location of objects
In the case of a triangulation survey, the objects are located by taking offsets with respect to the survey lines. The offsets are noted in a field book, maintaining a proper sequence of survey lines (AB, BC, CD). Then a map is prepared by plotting field records according to a suitable scale.
In the case of total station traversing, first, the traverse is plotted to a suitable scale by the coordinate method (on the basis of latitude and departure) and then the objects are located on the map by plane table by radial and intersection methods or by the total station. If the objects are located by the transit-and-tape method, a field book has to be entered while measuring the traverse legs. In this case, the plotting is done afterward.