Topographic surveying is the method of determining the positions, on the surface of the earth, of human made and natural features. It also is used to determine the configuration of the terrain. The purpose of a topographic survey is to find the data necessary for the construction of a graphocal portrayal of topographic features. The graphical portrayal from the gathered data forms a topographic map. A topographic map will show the character of the vegetation by conventional signs, as well as, the horizontal distances between features and their elevation above a given datum.
Ground methods are needed in topographic surveying. The tools are the transit, level, plane table alidade, and tape. Hand levels are also often used i contouring . Aerial photography is now used for making most of the small-scale topographic maps; the perocess is known as photogrametry. Even with the photogrametric methods, however, a certain amount of the work must be done in the field.
A contour line is a line passing through points of equal elevation. A level plane that intersects the ground surface would show on the map as a contour line. In nature you may think of the short lineof a still lake as a contour line. The contour interval for contour line is the constant vertical distance between adjacent contour lines. Contour lines on the maps are drawn in true horizontal positions with respect to the ground surface. Topographic maps with contour lines show the slope of topographic features. - hlls, valleys and ridges- and the lines give the elevations of these features.
A topographic map represents in a small area upon a drafting medium a portion of surface of the earth. For this reason the distance between any teo points on the map must have a know ratio to the distance between the same two points on the ground. The ratio of these points is the scale of the map. The scale is given in terms of map distance in a given number of units, which corresponds to a certain distance on the ground. Scales may be expressed by direct correspondence or by a ratio. For example, a typical scale is 1 in = 100 ft. Expressed as a ratio, this is 1:1200, or 1/1200. Ascale of 1:1200, or 1/1200, indicates that 1 unit on the map correspondes to 2000 units on the ground. Some typical scales are as follows:
1 in= 200 ft for highway reconnaissance
1 in= 2000ft for US. Geological survey topographic maps
1 in= 50 ft for plot plan for buildings
Topography on the map may be represented by contour lines or by hill shading. Hill shading is accomplished by means of hachures, a series of lines drawn in the direction of slope. For a steep slope the lines are heavy and spaced closed together, For a gentle slope the lines are widely spaced and of a light weight. The hachures give a general impression of the configuration of the ground. They are used to give actual elevation of the ground.
Representing terrain by contour lines