STUDY OF THEODOLITE
Aim:
To study about the Temporary and Permanent adjustments of a Theodolite.
Instrument used:
Theodolite
ADJUSTMENTS OF THEODOLITE
The Theodolite should be properly adjusted to obtain accurate observations. The adjustments are
mainly of two types. They are as follows:
1. Permanent adjustments and
2. Temporary adjustments.
1. Permanent adjustments
The permanent adjustments are to be done to maintain the required standard relationship between
the fundamental lines (axes) of a Theodolite. The fundamental lines are as follows:
a. Vertical axis
b. Horizontal axis or trunnion axis
c. Line of collimation or line of sight
d. Axis of plate level
e. Axis of altitude level.
Required relations between the fundamental lines (axes)
i) The axis of plate level must be perpendicular to the vertical axis.
ii) The line of collimation must be perpendicular to the horizontal axis
iii) The horizontal axis must be perpendicular to the vertical axis.
iv) The axis of the altitude level must be parallel to the line of collimation.
v) The vernier reading of vertical circle must read zero when the line of collimation is horizontal.
The permanent adjustments of a Theodolite are:
Adjustment of plate level.
Adjustment of line of sight
Adjustment of horizontal axis
Adjustment of altitude bubble and vertical index frame.
2. Temporary adjustments
The adjustments which are carried out at every setting of the instrument before the observations
are referred as temporary adjustments. There are three types of temporary adjustments as
follows.
a. Setting up
b. Levelling up
c. Elimination of parallax.
a) Setting up
This adjustment includes the following two operations.
i. Centering the Theodolite over the instrument station.
ii. Approximate leveling of Theodolite with the help of the tripod legs only.
Centering
It is the operation by which the vertical axis of the theodolite represented by a plumb line is
made to pass through the mark of instrument station on the ground.
Approximate levelling
The approximate leveling may be done with the reference to a small circular bubble provided on
the tribrach or by eye judgements.
b) Levelling up
The operation of making the vertical axis truly vertical is known as leveling of the Theodolite.
After the centering and approximate leveling an accurate leveling is to be done with the help of
foot screws.
i) First the telescope is to be kept parallel to any of the two foot screws as in the figure.
ii) The bubble of plate level is to be brought to the centre of its run by turning the foot screws
either inwards or outwards simultaneously.
iii) Then the telescope is to be turned through 90°, so that it lies over the third foot screw (i.e
perpendicular to the first position)
iv) The bubble is to be brought to the centre of its run by turning the third foot screw either
clockwise or anticlockwise.
v) Then the telescope is brought back to its original position (position at (i)) and the position of
bubble is checked whether it remains in the center or not.
vi) If the bubble is not in centre the above operations are repeated till the bubble retain at centre
in both the positions.
c) Elimination of parallax.
An apparent change in the position of an object caused by the change in position of the
observer’s eye is known as parallax. This can be eliminated in two steps.
i) Focusing the eye piece for distinct vision of the cross hairs.
ii) Focusing the objective to bring the image of the object in the plane of cross hairs.
i) Focusing the eye piece
The telescope is to be pointed towards the sky or a sheet of white paper is to be hold in front of
the objective.
The eye piece is to be moved in or out by rotating it gradually until the appearance of cross hairs
becomes sharp and distinct.
ii) Focusing the objective
Telescope is to be directed towards the object. Focusing screw is to be turned until the
appearance of the object becomes sharp and clear.
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