Write note on direct and indirect method of contouring?
1 Answer

There are two methods of locating contours:

1. Direct method

2. Indirect method

Direct method- In direct method contour to be plotted is actually traced on the ground. These points are plotted on ground and contours are marked through them. This method is fallowed where great accuracy is required.

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Procedure- The work is started from BM and the level is set up at the centre of the area. Suppose it is required to find out the contour of 90.000m then the staff should be a move to various positions on the plot where the reading on staff should give RL OF 90.000m. On ground. When all the point is located they mark on the ground directly. The similar process is followed for locating 95.000m and other contours.

Indirect method- In this method, spot level is taken at the regular interval along predetermine line on the ground the work is then plotted on plan and then the required contour line is drawn by the process of interpolation. The indirect method is less tedious and speedy as compared to the direct method. The method fallow in the indirect method of contouring is.

  • By cross-section- This method is suitable for road, railway and canal survey.. consider X, Y, Z as the centre line of the road or railway or canal as shown in fig. the cross section is set at every 10m on the centre line whereas the other dimension to complete a rectangle may be 5m.

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  • By squares (block contouring)- In this method the area to be surveyed is dividing into no of the square of size 5 to 20m.depending upon the nature of ground and contour interval required. The elevation of the corner of the square is determined by the mean of the process of levelling. The calculated RL of this nodal point is them written on the respective noddle points. And contours are interpolated between them.

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  • By tachometric method- In the case of hilly area tachometric contouring method is used. Here the instrument known as tachometer which is theodolite is utilised which determine horizontal distance and elevation of the point.

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