Procedure |
Let us consider a station B whose elevation is to be established with reference to a B.M station A, quite a distant apart. In establishing the station B as B.M., differential leveling is carried out starting from A and terminating at B. In order to carry out the leveling, first the instrument is set up at some location, say I1 (Figure 13.2), in such a way that backsight reading taken on A can be read clearly. The staffman is then directed to move forward towards B and choose a point, say S1 which is firm and stable. It is preferable that the distance of S1 from I1be the same as that of station A from I1. After proper selection of the point S1, staff is held to take the foresight reading for this instrument set up. The instrument is then shifted to some other position in forward direction, say I2 towards B and take the backsight reading on S1. Thus, point S1 is used as a turning point. From I2 foresight reading is taken to another well chosen (as followed in S1) turning point S2. Finally, from I3 backsight is taken on S2 and last sight at the terminal point B.
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