Which of the following actions is NOT a permanent adjustment in case of transit theodolite?
The theodolite is quite likely the most versatile and intricate surveying instrument for the measurement of horizontal and vertical angles. Theodolites enable the surveyor to measure angles to an accuracy ranging from 1’ to 0.1".
Temporary Adjustment of Theodolite
Theodolites must be adjusted at every set-up of the instrument. These adjustments are therefore temporary; they include
(i) setting up and centering,
(iii) elimination of parallax, for both the eyepiece and the objective.
Setting up and centering:- The operation of setting up and centering includes setting up the instrument after fixing it to the tripod head, such that the vertical axis passes exactly through the station mark, and then approximate leveling by manipulating the tripod. The accuracy of the centering to be aimed at depends on the circumstances and the nature of the survey. With a self-centering theodolite and a plumb line, it should not be difficult to center the instrument over the station mark to a millimeter or better.
Leveling:-Levelling is required to ensure that the vertical axis of the instrument is truly vertical. The plates are then truly horizontal. Leveling a three-foot-screw instrument is simple and rapid if done systematically. Assuming that there is only one bubble on the horizontal plate, the leveling process is the same as that used with a similar leveling instrument: bringing the bubble to the center of the run in two perpendicular positions, parallel to two-foot screws and over the third.
Elimination of parallax:- Eliminating eyepiece parallax follows the same procedure for a level. It involves making the cross-hairs on the diaphragm distinct and clear against a white background like a sheet of paper, by turning the eyepiece lens in or out. It depends on the observer’s eyesight; it is usually sufficient to make it once. Eliminating objective parallax involves focusing the object to be viewed using the focusing screw, such that it is seen distinctly against the cross-hairs. Internal focusing telescopes are invariably used in theodolites.