Short note on Theodolite and important parts of a Theodolite

It is an instrument which is used mainly for accurate measurement of horizontal and vertical angles up to 10′ or 20′.

Theodolite is used for following purposes: to measure horizontal angle and vertical angles.

Theodolite is of two types:

(1) Transit theodolite (2) Non-transit theodolite.

(1) Transit Theodolite: In this type of theodolite, the telescope can
be revolved through 180° in a vertical plane about its horizontal axis.
(2) Non-transit Theodolite: In this type of theodolite, the telescope
can not be revolved through 180° in a vertical plane about its horizontal axis.

A line diagram of a transit theodolite is shown in figure. It consists of
alidade assembly at the top, the horizontal circle assembly in the middle and the levelling head assembly at the bottom.
(i) Alidade Assembly: It is the topmost assembly, which includes a
telescope supported by two standards of the shape of letter A. An altitude
bubble is attached to the standards.

(ii) Horizontal Circle Assembly: It consists of two plates, the lower and
the upper. The lower plate carries the main scale and the upper plate carries verniers.
(iii) Levelling Head Assembly: it is the bottom most assembly which
is screwed on the top of the tripod. It contains tribrach, foot screws and trivet.

The following are the essential parts of a theodolite:

(i) Trivet: It is a circular plate having a central hole for fixing the theodolite on the tripod. It is also called as base plate. Three foot screws are secured to this plate.
(ii) Foor Screw: These are used for levelling the theodolite. The upper
part of these are inserted into tribrach and lower part is inserted into trivet.
(iii) Tribrach: It is the triangular plate carrying three foot screws at its ends.
(iv) Spindles: The theodolite consists of two spindles or axes-one is inner and other outer. The inner axis is solid and conical and the outer is hollow.
(v) Lower Plate: It is attached to the outer axis. The scale is graduated
from 0° to 360° in a clockwise direction. Each degree is subdivided into two, three or four divisions, that means one small division may be 30°, 20°, or 15° respectively.
A clamp screw and a tangent screw is also attached to the lower plate.
(vi) Upper Plate: It contains the vernier scale A and B. It is attached
to the inner axis. The motion of upper plate is controlled, by the upper clamp screw and the upper tangent screw.
(vii) Standard or A Frame: Two frames shaped like the letter A are
provided on the upper plate to support the telescope.
(viii) The Telescope: The telescope is provided between the standards
at right angles to the horizontal axis. It can be rotated about its horizontal axis in a vertical plane
(ix) Vertical Circle: It is fixed with the telescope and moves with it. It is divided into four quadrants. Each quadrant is graduated from 0° to 90°.
(x) Altitude Bubble: It is provided to level the instrument.

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