Posted October 25, 201410 yr Procedure Suppose A and B are two distance points whose difference of level is to be determined as in the following figure, Place the staff over point A and set up the instrument at suitable distance towards B. The suitable distance between instrument and staff can be about 25 meters. Take a sight on the staff, that reading is called Back Sight (B.S). Now, move the staff to a new position towards B and take the reading, this will be a Fore Sight (F.S). Care should be taken to make the F.S equal to preceding B.Sm, by doing this we will eliminate the effect of curvature and refraction. Now, move the instrument to a new position and take the reading on the previous position of the staff. This position of staff is known as Change Point (C.P) and this reading will be Back Sight (B.S). Now, shift the staff to the point B and take reading, this reading will be Fore Sight. Setting up of instrument means careful leveling of instrument so, correct reading can be taken. It is not necessary to move in the straight line from A to B. If due to any reason F.S is not equal B.S then this error due to curvature and refraction can still be eliminated by making, ∑ (Back Sight distances) = ∑ (Fore Sight distances)
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