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, RLs of points, 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.
Generally topographic map is prepared according to a scale of 1 cm to 1 km (i.e. 1/100,000).
Representation of ReliefFrom 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 proper connection with the other sectors (i.e., it is ensured sectors have that the common sides).
Location of objectsIn case of 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 (i.e. AB, BC, CD, ….). Then a map is prepared by plotting field records according to a suitable scale.
In case of total station traversing, first the traverse is plotted to a suitable scale by the coordinate method (i.e.. 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, plotting is done afterwards.
Location of contourDuring the process of locating objects, the contours are located on the map. This may be done directly by plane table/ total station. However, this method is very laborious. These contours may also be located indirectly by dividing the area into squares or by taking cross-sections are noted on the map, and the contour lines are drawn by interpolation. Also, rather than by drawing contour lines, the nature of the ground may be indicated in relief as described previously. The purpose of a TOPOGRAPHIC SURVEY is to gather survey data about the natural and man-made features of the land, as well as its elevations. From this information a three- dimensional map may be prepared. You may prepare the topographic map in theoffice after collecting the field data or prepare it right away in the field by plane table. The work usually consists of the following:
- Establishing horizontal and vertical control that will serve as the framework of the survey.
- Determining enough horizontal location and elevation (usually called side shots) of ground points to provide enough data for plotting when the map is prepared.
- Locating natural and man-made features that may be required by the purpose of the survey. Computing distances, angles, and elevations.
- Topographic surveys are commonly identified with horizontal and/or vertical control of third- and lower-order accuracies.