Temporary adjustment of theodolite

 The adjustment which is required to be made at every instrument station before making observations, is known as temporary adjustments.

 The temporary adjustment of theodolite includes following operations :-

1) Setting up and centering the theodolite - In this operation, the centering of theodolite over the ground mark and approximate leveling is done.

Procedure -

• A plumb bob is suspended with a string attached to the bottom of the instrument.
• The theodolite is place over the station mark by spreading the legs well apart so that telescope is at a convenient height.
• The tripod legs are moved radially and circumferentially till the plum bob hangs within 1 cm horizontally of the station mark.

2) Leveling of a theodolite - The operation of making the vertical axis of theodolite truly vertical, is known as leveling of the theodolite.

Procedure -

• The level tube is made parallel to the line joining any two foot-screws by turning the horizontal plate.
• The bubble is brought at the centre by turning the both foot-screws in either inwards or outwards direction.
• After bringing the bubble at centre, the theodolite is turned though 90 degree so that the plate level becomes perpendicular to the previous position.
• With the help of the third foot-screw the bubble3 is again brought to the centre.
• Now, again level tube is rotated in first position to check that bubble is at centre or not. If it is not at the centre, the same process is repeated till the bubble comes at centre in all position.

3) Elimination of parallax - An apparent change in the position of the object caused by the change in the position of the observer's eye is known as parallax. Elimination of parallax is done by -

• Focusing the eye-piece - To focus the eye-piece for distinct vision of cross hairs, a white paper is hold in front of the objective or the telescope is sighted towards the sky. Then, the eye-piece is moved in or out till the cross hairs are seen sharp and distinct.
• Focusing the objective - After cross hairs have been properly focused, the telescope is directed on a well defined distant object and that is intersected with vertical wire. The objective is focused still a sharp image is seen. Removal of the parallax may be checked by moving the eye slowly to one side. If the object still appears intersected, there is no parallax.

Thank you.

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