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# DETERMINATION OF TACHEOMETRIC CONSTANTS

Aim: To measure the horizontal angles between the given stations about the instrument station as the vertex by the method of reiteration.

Instruments and Accessories Required:
Theodolite, Pegs, Arrows,Tripod stand,Levelling staff, Metric chain,etc.

Procedure:
1. The theodolite is mounted on the tripod stand.
2. A metric chain of 20m or 30m length is unfolded and streched straight.
3. The theodolite is centered over an arbitrarily selected station from where a chain line is run along a straight course and from where the staff station selected at uniform intervals on the chain line can be sighted without any obstruction and the instrument is levelled using the foot screws and both the plate bubble and the altitude bubble are brought to the centre of the run.
4. The line of sight is made horizontal by setting both the vernier C and D on the vertical circle at 0°00’00” .
5. The levelling staff is held vertically on the stations selected at uniform intervals on the chain line and the stations are sighted in sequence through the telescope.
6. The stadia hair readings are observed for every staff station and are recorded along with the respective horizontal distance of the station from the instrument station
7. The staff intercept is calculated for every observations.
8. The horizontal distance and the corresponding staff intercepts are substituted in the distance equation for horizontal line of sight and as many equations as the number of observations are formulated.
9. These equations are solved for the values of tacheometric constants.
Tabulation

 S.No. Distance D(m) Stadia Hair Reading (m) Staff Intercept S  (m) Top Middle Bottom 1 20 2 40 3 60 4 80

D1=KS1+C ---------------- (1)
D2=KS2+C ..................... (2)
D3=KS3+C ..................... (3)
D4=KS4+C ..................... (4)

Result: The tacheometric constant of the given instrument are determined
1. Multiplying Constant K =