Prolonging A Straight Line
There are two methods of prolonging a given line such as AB
(1) Fore sight method ,and (2) Back Sight Method
(1)Fore Sight Method. As shown in the fig. below
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| Fore Sight Method |
i) Set up the theodolite at A and level it accurately .Bisect the point b correctly. Establish a point C in the line beyond B approximately by looking over the top of the telescope and accurately by sighting through the telescope.
ii) Shift the instrument to B ,take a fore sight on C and establish a point D in line beyond C.
iii) Repeat the process until the last point Z is reached.
(2) Back Sight Method. As shown in the fig. Below
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| Back Sight Method |
i) Set up the instrument at B and level it accurately .
ii) Take a back sight on A.
iii) Tighten the upper and lower clamps, transit the telescope and establish a point C in the line beyond B.
iv) Shift the theodolite to C ,back sight on B transit the telescope and establish a point D in line beyond C. Repeat the process until the last point ( Z) is established.
Now if the instrument is in adjustment, the points A,B,C,D and Z will be in one line, which is straight but if it is not in adjustment i.e. line of collimation is not perpendicular to the horizontal axis ,then C’, D’ and Z’ will not be in a straight line.
Double reversing Method:
When the line is to be prolonged with high precision or when the instrument is in imperfect adjustment, the process of double sighting or double reversing, is used. Suppose the line AB is to be prolonged to a point Z.
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Double reversing Method |
Procedure: As shown below:
i) Set up the theodolite at B and level it accurately.
ii) With the face of instrument left, back sight on A and clamp both the upper and lower motions.
iii) Transit the telescope and set a point C1 ahead in line.
iv) Loosen the lower clamp ,revolve the telescope in the horizontal plane and back sight on A .Bisect A exactly by using the lower clamp and its tangent screw. Now the face of instrument is right.
v) Transit the telescope and establish a point C2 in line beside the point C1.
vi) The exact position of the true point C must be mid-way between C1 and C2.
vii) Measure C1 C2 and establish a point C exactly mid-way, which lies on the true prolongation of AB.
viii) Shift the instrument to C, double sight on B ,establish the point D1 and D2 and locate the true point D as before .
ix) Continue the process until the last point Z is established.
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