Similarly the contour points along any line connecting two neighbouring points may be obtained and the points pricked. Then the intersection of dark lines on AB represents the points on 98 m and 99 m contours. Rotate the tracing sheet so that 100.2 the parallel line passes through point B. Assume the bottom most dark line represents 97 m RL and every parallel line is at 0.2 m intervals. If RL of A is 97.4 and that of B is 99.2 m. Every 10th or 5th line is made darker for easy counting. On a tracing sheet several parallel lines are drawn at regular interval. Mechanical or graphical method of interpolation consist in linearly interpolating contour points using tracing sheet: In this survey theodolite with tacheometry facility is commonly used.įor interpolating contour points between the two points any one of the following method may be used: This method is ideally suited for hilly areas. On these lines at selected distances points are marked and levels determined. In this method several radial lines are taken from a point in the area. This method is ideally suited for road and railway projects. The cross- sectional line need not be always be at right angles to the main line. Closer intervals are required if ground level varies abruptly. The spacing of cross-section depends upon the nature of the ground, scale of the map and the contour interval required. The points are marked on the drawing sheets, their reduced levels (RL) are marked and contour lines interpolated.įigure 2 shows a typical planning of this work. By levelling the reduced level of all those points are established. In this method cross-sectional points are taken at regular interval. Reduced levels of grid points marked and contour lines are drawn by interpolation. Then grid square is plotted on the drawing sheet. Levels of all grid points are established by levelling. 1).Ĭommonly used size of square varies from 5 m × 5 m to 20 m × 20 m. In this method area is divided into a number of squares and all grid points are marked (Ref. Thus in this method horizontal control is established first and then the levels of those points found.Īfter locating the points on the plan, reduced levels are marked and contour lines are interpolated between the selected points.įor selecting points any of the following methods can be used: In this method, levels are taken at some selected points and their levels are reduced. This method is slow, tedious but accurate. It is better if both are nearby to communicate easily.įor getting speed in levelling some times hand level and Abney levels are also used. The level and survey instrument need not be shifted at the same time. Plane table survey is ideally suited for this work.Īfter required points are established from the instrument setting, the instrument is shifted to another point to cover more area.
Thus, several points are established on a contour line on one or two contour lines and suitably noted down. It is followed by establishing horizontal control. A surveyor establishes the horizontal control of that point using his instruments.Īfter that instrument man directs the staff man to another point where the same staff reading can be found. The instrument man asks staff man to move up and down in the area till the required staff reading is found. The plane of collimation/height of instrument is found and the required staff reading for a contour line is calculated. A level is set on a commanding position in the area after taking fly levels from the nearby bench mark.
It consists in finding vertical and horizontal controls of the points which lie on the selected contour line.įor vertical control levelling instrument is commonly used. There are two methods of contour surveying:
Based on the instruments used one can classify the contouring in different groups. If the theodolite is used, both horizontal and vertical controls can be achieved from the same instrument. To exercise vertical control leveling work is carried out and simultaneously to exercise horizontal control chain survey or compass survey or plane table survey is to be carried out. Contouring in surveying is the determination of elevation of various points on the ground and fixing these points of same horizontal positions in the contour map.
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