What is an Adjustment?
Adjustment of a theodolite means the operation of tightening or loosening of moveable parts to prepare the instrument for accurate measurement. It also includes other operations meant for this purpose. There are two types of adjustments for a theodolite - Temporary Adjustment & Permanent Adjustment.
1 - Temporary Adjustments
These are required for each setting up of the instrument and includes following,
a - Centering
This is to center the instrument exactly over the ground station which is indicated by optical plummet.
b - Leveling
It means to make the horizontal and vertical axes in their true position. It is indicated by the central position of plate level.
c - Removal of parallax
That is to bring the cross hairs and the object in focus simultaneously
2 - Permanent Adjustments
These are to be tested after a long interval or at the beginning of an important project. The field party is only expected to carry out the test and adjustment, if required will be done by the trained for this purpose in a workshop. Permanent Adjustments for a theodolite have discussed below,
a - Plate level Adjustment
Its purpose is that bubble should remain central in all positions after the adjustment. Its procedure is as follows,
After making the circular bubble central bring the plate level parallel to any two foot screws.
Move the two foot screws inward or outward till the bubble is centeral.
Rotate the instrument through 90° and with the help of third foot screw bring the plate bubble in the central position.
Repeat the process at least two times so that bubble is central at the end of each step.
Now, rotate the instrument through 180°, if the bubble remains central then Adjustment is correct otherwise it is to be done.
b - Horizontal axis Adjustment
Its purpose is that the horizontal axis should be remain truly horizontal after the instrument has been carefully leveled. Its procedure is as follows,
Setup and level the instrument at a position where highly inclined sight is available.
Move the telescope in upward direction to bisect the a well defined elevated point.
Now, depress the telescope and take a staff reading on the horizontally placed staff below the elevated point. This observation is taken with face left.
Now, change the face and again bisect the same elevated point.
Depress the telescope and take the staff reading, if the two reading are same then Adjustment is correct.
c - Line of collimation Adjustment
The line of collimation must pass through the point of intersection of cross hairs and optical center of the object glass (also the geometric center) and line joining the intersection of cross hairs. Its procedure is as follows,
Setup and level the instrument carefully at a position from where about 100 ft long sights are available on the opposite sides.
Fix the horizontal movement and fix an arrow at a distance of 100 ft and bisect it by vertical hair with face left.
Transit the telescope and take the staff reading on the horizontally lying staff at a distance of about 100 ft.
Rotate the instrument though 180° and again bisect the arrow.
Now, again transit the telescope and take the staff reading, if the two readings are the same then the Adjustment is correct.