TEMPORARY
ADJUSTMENT OF THEODOLITE
ADJUSTMENT OF THEODOLITE
Temporary adjustment are the adjustment which are required to be made at
each setting of the instrument before taking observation. These adjustments are
also known as station adjustments.
each setting of the instrument before taking observation. These adjustments are
also known as station adjustments.
The following
three adjustments are required:
three adjustments are required:
1. Setting up
and centring,
and centring,
2.Levlling
3.Elimination of
parallax.
parallax.
Setting up and Centring
This involves setting the theodolite exactly over
the station mark or on the station peg. It is done by the following steps:
the station mark or on the station peg. It is done by the following steps:
- The
plumb bob is suspended from a small hook attached to the vertical axis of the
theodolite - The
instrument is placed over the station mark with the telescope at a convenient
height and with the tripod legs set well apart. - Two
legs of the tripod are set frimly into the ground and the third leg is moved
radially to bring plumb bob exactly over the station mark. Then the third leg
is also pushed into the ground. - The
instrument is roughly centered over the station mark and then by means of the
shifting head, the plumb bob is brought excatly over the station mark.
Levelling
The procedure of levelling with three foot
screws is as follows:-
screws is as follows:-
- Turn the upper plate
untill the longitudinal axis of the plate level is roughly parallel to a
line joining any two of the levelling screws. - Hold these two levelling screws between the thumb and
the first finger and turn them uniformly so that the thumbs move either towards
each other or away from each other until the bubble is central. - Turn the upper palte 90, i.e untill the axis of the
level passes over the position of the third levelling screw C. - Turn this levelling screw C untill the bubble is
central. - Return the upper plate through 90 to its original
position. Rotate screws A and B inwards or outwards till the bubble is central. - Repeat the above steps,till the bubble is central in
both the positions.
Elimination of parallax
It consists of
focusing of eyepiece and the objective.
focusing of eyepiece and the objective.
Focussing
the eyepiece This
operation is done to make the cross-hairs appear clearly visible.
the eyepiece This
operation is done to make the cross-hairs appear clearly visible.
The following
steps are involved:-
steps are involved:-
1.
The
telescope is directed towards the sky or a sheet of white paper held in front
of the objective.
The
telescope is directed towards the sky or a sheet of white paper held in front
of the objective.
2.
The
eyepiece is moved in or out until the cross-hairs appears clear and distinct.
The
eyepiece is moved in or out until the cross-hairs appears clear and distinct.
Focussing
the objective This
operation is done to bring the image of the object in the plane of the
cross-hairs. The following steps are involved:-
the objective This
operation is done to bring the image of the object in the plane of the
cross-hairs. The following steps are involved:-
1. The telescope is directed towards the object.
2. The focusing screw is turned until the image appear
clear and sharp.
clear and sharp.
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