Upendra Kumar
This setting completed by two operations, centring a theodolite over a station & approximately level of tripod legs.
The temporary adjustment of all types of theodolite approximately are the same, and it is required for every set up of the instrument. It includes instrument is exactly over the station. Levelling of plate level means the verticle axis of the instrument is truly verticle. Accurate work is not possible without eliminated of parallax.
1. Setting the theodolite over the station. 
This setting completed by two operations, centring a theodolite over a station & approximately level of tripod legs.
Procedure: After placed the instrument over the station following operations should be done.
- the first step is to spread the legs of the tripod well apart
- Keep the telescope of the theodolite at convenient height.
- plumb bob suspended approximately over the station.
- The Leveling head approximately level. After complete, the above operations lift the instrument from the tripod. A plumb bob suspended above about 2cm from stations. Lift the instrument from tripod without disturbing the relative positions of the legs. suspended position of the plumb bob should within 1cm from the station horizontally. Move the instrument circumferentially so as to bring the plumb bob exactly without disturbing plate level over the station. Press the legs firmly into the ground. After completed the centring process by legs. The plumb bob lifted, final centring complete by optical centring device attached in an above picture.
After setting, the instrument over station levelling of the instrument is required, for the level of the instrument. Turn the instrument until Longer plate level parallel to any pair of foot screw.
Other plate levels will then be parallel to the line joining the third screw and the midpoint of the line joining the first pair.
Bring the bubble to the centre of it's run by turning both screws uniformly.Bring the other bubble to the centre of it's run by turning the third levelling screw.
Repeat the process until finally both bubbles are exactly centred. Rotate the instrument about the vertical axis through a complete revolution. Each bubble will now remain in the centre of it's run. Forgreater accuracy, the instrument should be levelled with reference to the altitude level.
Elimination of parallax:- Accurate wok is not possible if parallax is not eliminated.Due to this reason . The image formed by the objective does not lie in the plane of the crosshairs.it is eliminated by.
(A) Focussing the eye-piece:-point the telescope towards the sky or hold a sheet of white paper in front of the object-glass, move the eyepiece in and out until the crosshairs are seen distinctly and clearly.
(B) Focussing the object glass:- Due to this reason, the image formed by object-glass does not plane of the crosshairs. apparent movement of an object with the eye is called parallax. Turn the telescope toward the object and turn the focusing screw until the image appears clear and sharp.
