Topographic Surveying – Topographic surveying is the process of determining the positions of natural and artificial features in a location, both on plan and in elevation, for the purpose of delineating them with traditional signs on a topographic map. The shape or configuration of the earth’s surface is referred to as topography.
The primary goal of a topographic map is to depict the three-dimensional relationships between the terrain and the surrounding environment. The relative positions of points are thus represented both horizontally and vertically on a topographic map. The relief is a visual representation of the difference in elevation.
On a plan, shading hachures, form lines, or contour lines can be used to represent the relative altitudes of the points. The topographic map depicts natural features such as streams, rivers, lakes, and trees, as well as man-made features such as highways, railroads, canals, towns, houses, fences, and property lines, in addition to relief.
Most engineering projects, such as the location of railways and highways, the design of irrigation and drainage systems, the development of water power, the layout of industrial plants, and city planning, rely heavily on topographic maps. During a war, topographic maps are also very useful for directing military operations.
Methods of Representing Relief
The system for depicting relief on a topographic map must accomplish two goals -:
- The map’s user should be able to interpret it as a representation of the ground, and
- It should also provide specific information about the elevations of points on the map. On a map, relief can be depicted using hachures, form lines, tinting, or contour lines.
Hachures are a set of short lines drawn in the slope’s direction. The lines are heavy and closely spaced on a steep slope, but fine and widely spaced on a gentle slope. While hachures depict the surface form, they do not provide precise height information. A contour line is a ground-based imaginary line that connects points of equal elevation. It is a line where the ground’s surface is intersected by a level surface.
Contours are the most common and accurate way to represent relief on a topographic map. Form lines are similar to contours, but they are not drawn with the same precision. Each form line represents an elevation, but it has not been determined by enough points to meet the accuracy standards demanded by contours. On maps intended for navigation, form lines are sometimes used to show peaks and hilltops along the coast.
Tinting can also be used to indicate relief or elevations. The area between two selected contours is tinted one way, the area between two others is tinted another way, and so on.
Topographic Surveying Procedure
- establishing horizontal control as well as a vertical control,
- locating the contours, and
- locating the details such as rivers, streams, lakes, roads, railways, houses, trees, etc.
Because the three coordinates of a point (i.e. two co-ordinates in the horizontal plane to locate it horizontally, and one co-ordinate in the vertical plane to locate it vertically with respect to the datum) can only be established or measured with respect to well connected horizontal and vertical control systems, the establishment of the horizontal and vertical control system is the most important part and is the first step in the topographic survey.
Vertical control creates a framework against which elevation differences can be calculated. This is crucial because the relief, or third dimension, must be indicated on the topographic map.
The vertical control’s goal is to determine the elevations of the primary control stations or to set up bench _narks near them at a convenient interval. To accurately define the position of all the control points, high order spirit level circuits are used to determine elevations.
Trigonometric levelling is frequently used to transfer elevations from precise levelling circuits to triangulation stations, which are typically located on high, commanding points, with the levels being run in as far as possible over level or gently sloping terrain. The elevations of traverse stations or benchmarks near them are then used to establish the secondary vertical control. The tacheometric method or spirit levelling can be used to determine this.
When using the level, control point elevations can be determined by running level circuits, or benchmarks can be placed in such a way that they can be seen from nearby horizontal-control points. Barometric levelling can be used for rough work.
The horizontal control serves as the survey’s skeleton, allowing contours and other details to be located. When the survey area is small, the horizontal control may consist of just one station, and each point’s distance and direction can be measured in relation to this station. When the area is relatively large, a traverse or a series of connected traverses may be used as the horizontal control.
Depending on the size of the area, the traverses can be done with a tape-compass, plane table, or tape-transit. Stadia measurements are sometimes used to determine the length of the traverse sides, especially when the land is uneven.
In the survey of an uneven area of moderate size, a stadia traverse is used. On large surveys, the horizontal control may be a simple or complex triangulation system, with traverses connecting the triangulation stations providing additional control.
The plane table is sometimes used to perform secondary traverses. Precision traversing can be used to establish primary horizontal control in flat and densely wooded countries where triangulation is impractical or prohibitively expensive.
Use of Topographic Surveying
The topographic surveying is used for :
- Preparing topographic maps.
- Topographic Surveying is also used in establishing vertical and horizontal control for accurately defining locations.
- Topographic Surveying is used in constructing topographic (cross-sectional) profiles.