The horizontal positions of these points are measured or computed and the points are plotted on the plan. In this method, the spot levels of selected guide points are taken with a level and their levels are computed. The horizontal control is generally provided by method of plane table surveying or locating the positions of points. This point acts as a point of known elevation, for shifting the position of the instrument to another position, from where the work proceeds in the similar manner till the entire area is contoured. Therefore, locating contours of higher value, the instrument has to be shifted to some other suitable position.Įstablish a forward station on a firm ground and take fore sight on it. The contour of 101.5 m cannot be set from this setting of the instrument because the height of instrument for this setting of the instrument is only 101.485 m. Similarly locate the points where the staff reading would be 101.485 -101 = 0.485 m for 101m contour. Mark all such positions on the ground where the staff reading would be 0.985 m by inserting pegs. To locate the contour of 100.5 m value, the staff man is directed to occupy the position on the ground where the staff reading is 101.485 -100.500 = 0.985 m. If the reduced level of the bench mark is 100 m, the height of instrument would be 100 + 1.485 = 101.485 m. Let the back sight reading on the bench mark be 1.485 m. The level is set up on any commanding position and back sight is taken on the bench mark. In this method, a benchmark is required in the project area. This method is slow and tedious and thus used for large scale maps, small contour interval and at high degree of precision. Points which happen to fall on a desired contour are only surveyed, plotted and finally joined to obtain the particular contour. In the direct method, the contour to be plotted is actually traced on the ground. In general, the field methods of contouring may be divided into two classes: Thus, contouring depends upon the instruments used (to determine the horizontal as well as vertical position of points). Contouring involves providing of vertical control for location of points on the contours and horizontal control for planimetric plotting of points. It requires planimetric position of the points and drawing of contours from elevations of the plotted points. The method of establishing / plotting contours in a plan or map is known as contouring. The process of locating these contour lines on the surface of the earth is known as contouring. It should be remembered that the contour interval for a particular map is constant. and for large scale map,it may be of 0.25m,0.50m,0.75m etc. 5m, 10m, 15 m etc.Īgain ,for a small-scale map, the interval may be of 1m,2m,3m etc. For a steep slope in hilly area is greater, eg. This interval depends upon,Ĭontour intervals for flat country are generally small, eg. For example, if the various consecutive contours are 100m, 98m,96 m etc., then the contour interval is 2m. The vertical distance between any two consecutive contours is known as a contour interval. The concavity in contour lines is towards higher ground in the case of ridge and towards lower ground in the case of valley . Contour lines in U-shape cross a ridge and in V-shape cross a valley at right angles. A set of ring contours with higher values inside, depicts a hill whereas the lower value inside, depicts a depression (without an outlet).Ĭontours deflect uphill at valley lines and downhill at ridge lines. They are perpendicular to ridge and valley lines where they cross such lines.Ĭontours do not pass through permanent structures such as buildings.Ĭontours of different elevations cannot cross each other (caves and overhanging cliffs are the exceptions).Ĭontours of different elevations cannot unite to form one contour (vertical cliff is an exception).Ĭontour lines cannot begin or end on the plan.Ī contour line must close itself but need not be necessarily within the limits of the map.Ī closed contour line on a map represents either depression or hill . The steepest slope of terrain at any point on a contour is represented along the normal of the contour at that point. Thus, contours are spaced equally for uniform slope The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope. The variation of vertical distance between any two contour lines is assumed to be uniform. The principal characteristics of contour lines which help in plotting or reading a contour map are as follows: It facilitates depiction of the relief of terrain in a two dimensional plan or map. A line joining points of equal elevations is called a contour line.
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