Theory
At every set up of the instrument certain temporary operation where carried out before the observation were made. These operation are known as temporary adjustment or station adjustment and they are made to achieve the following two conditions
a) The plane of collimations is horizontal
b) The vertical axis passes through the centre of the leg.
Following are the three temporary adjustment of a theodolite
1. Setting up
2. Levelling up
3. Elimination
Diagram
Setting up
a) In setting the instrument was removed from the box and centered over the station mark for the purpose a plumb bob was suspended from the hook.
b) The instrument was roughly levelled by tripod legs. The tripod was placed in such a way that an equilateral with station point at its centre of gravity was formed. Each leg was then moved radially as well as circumferancally so as to bring the plumb bob exactly over the station mark.
Levelling up
It was levelled using levelling screws for this operations one of the plate level was placed parallel to line joining any two screw at the same time. The second plate level was parallel to the line joining the third screw to the centre of the line connecting the other two screw. Now the two screws were grasped between thumb and finger and the bubble was brought at the centre by moving the screws in words or out words similarly the bubble of the other plate level was brought at centre by turning the third screw. The process was repeated till bubble of both the plate level were brought in centre position.
Elimination of parallax
The situation in which the plane of image does not coinside with the plane of cross hairs is called parallax. It was carried out in two steps
a) The eye piece was focused so that cross hair were seen clearly and distinctly. The telescope was pointed towards a white paper held in front of object glass. The eyepiece was moved in and out till the cross hair were seen sharp and black.
b) The object glass was focused so that the image of object was brought in plane of cross hairs and there was no apparent movement of the image for this purpose the telescope was directed towards the object and the focusing screw was turned until the image was sharp and clear.
