Temporary adjustment of theodolite is a process of making the thoedolite ready for taking observation. Temporary adjustment of theodolite consists of following steps
- Setting theodolite over the station
- Approximate levelling using a tripod stand
- Centering
- Levelling
- Focussing the eyepiece
- Focussing the object-glass
- Setting the vernier
Setting Theodolite over the Station
The tripod stand is placed over the station where theodolite is to be set. The theodolite is then placed over the tripod stand and tightened using wing nuts.
Approximate leveling using a tripod stand
The legs of the tripod stand are fixed firmly in the ground and kept well apart. Two legs are pressed in the ground and the third leg is adjusted up and down or in and out so that bubble is approximately at the center of its run.
Care should be taken that the plumb bob remains approximately over the station.
Centering
It is the process of bringing the theodolite exactly over the station. Centering is done using a shifting head (movable capstan nut). It is so adjusted that the plumb bob comes exactly over the nail fitted at the station peg.
Levelling
All the levelling screws should be at centre before starting the levelling operation.
Three foot Screws Theodolite
- The plate bubble is placed parallel to any pair of foot screws say foot screws 1 and 2. Then these foot screws are turned either inward or outward so that the bubble comes to the centre.
- The plate bubble is then kept at 90 degrees to the line joining the foot screws 1 and 2. Then third foot screw is turned either inward or outward so that the plate bubble comes to the centre.
- Some theodolites have two plate bubbles perpendicular to each other. In that case, there is no need to rotate the theodolite, When one plate bubble is kept parallel to the two-foot screws other plate bubbles will be perpendicular to these foot screws.
- This process is again repeated till the plate bubble comes at the centre in a direction parallel to the two-foot screws as well as perpendicular to two-foot screws.
- At last, the theodolite is rotated 360 degrees to check whether the bubble remains at the centre. If it remains in the centre then levelling is perfect if not the above process is repeated again.
Four foot screws theodolite
- Place the plate bubble parallel to any pair of diagonally opposite foot screws say foot screw A and B. turn these foot screws inward or outward to bring the bubble in the centre.
- Now swing the theodolite and bring the plate bubble parallel to another set of diagonally opposite foot screws say C and D. Turn these foot screws in or out and bring the bubble in the centre.
- Repeat the process until the bubble is perfectly at the centre.
- Turn the theodolite at 360 degrees if the bubble remains at the centre then levelling is perfect.
Focussing the eyepiece.
Focusing of the eyepiece is done so that the cross-hair can be seen clearly. The telescope is directed towards the sky or a white piece of paper is kept in front of the telescope. The eyepiece is rotated till the crosshairs appear distinct and sharp.
Focussing the object glass.
Focusing of the object-glass is done so that the sharp image of the object appears. The telescope is directed towards the object (e.g. ranging rod or staff), the focussing screws are then turned clockwise or anticlockwise until the clear image of the object appears free of parallax.
Parallex can be checked by moving the eye up and down if there is no relative movement between image and crosshairs then focussing is perfect.
Setting the vernier
The vernier A is set to 0 degrees and vernier B at 180 degrees. To do this lower clamp is tightened and the upper clamp is loosened. The theodolite is rotated till it comes approximately near to 0 degrees.
Now upper clamp is tightened and the upper tangent screw is turned until the vernier A coincides exactly with 0 degrees.