Theodolite Transit theodolite

Theodolite Transit theodolite

What is Theodolite

A theodolite is an important instrument used for measuring horizontal and vertical angles in surveying.

It can also be used for a number of surveying operations such as prolonging a line, measuring distances indirectly and levelling.

Theodolite can be classified into transit and non-transit theodolite.

Transit theodolite

A theodolite is said to be transit when its telescope can be rotated through 180° in a vertical plane about its horizontal axis, thus directing the telescope in exactly opposite direction.

Non-transit theodolite

A theodolite is said to be non-transit when its telescope cannot be rotated through 180° in a vertical plane about its horizontal axis. Such theodolites are obsolete nowadays.

 Theodolite can also be classified into two types as follows

1- Vernier theodolite

2- Precise optical theodolite

In vernier theodolites, verniers are used for taking the readings. These theodolites are most commonly used in general work. Most of the vernier theodolite can read angles up to 20". [The least count of theodolite is 20".]

Precise optical theodolite is fitted with an optical system which is used to read both horizontal and vertical angle precisely. These theodolites are having a micrometre for taking readings and are also called as microptic theodolite.

These theodolites are used for precise work. Most of these theodolites can read angles up to 1 second or less.

The size of a theodolite is defined by the size of its lower graduated circle. Generally the size of the theodolite varies from 8 to 25 CM.

Main parts of vernier theodolite


The telescope is mounted on a horizontal spindle called the horizontal axis or the trunnion axis.

The telescope can be rotated in the vertical plane about the horizontal Axis for the purpose of sighting the objects.

The telescope is internal focusing type. In this the objective lens is fixed in position.

Vernier frame

The vernier frame also called T-frame or index frame, consists of a vertical leg called clipping arm and a horizontal bar called the index arm. Index arm is horizontal and becomes reference for measuring Vertical angle. When line of sight is horizontal reading as measured by index Aam on vertical circle is 0°.

Vertical clamp screw

The vertical circle and the telescope can be clamped at any desired Vertical angle by means of the vertical clamp screw. Thus the rotation of the telescope about the horizontal axis is prevented.

Altitude bubble

Altitude bubble is attached to index arm. Hence to make the index Aam truly horizontal during temporary adjustment altitude bubble is used.

Note- In some theodolites, the altitude bubble tube is attached to the telescope.

Striding Level

A very sensitive level is sometimes mounted at right angle to the telescope axis is called striding level.

Upper Plate

The upper plate is also called vernier plate, support the standard at its upper surface.

At its lower face, the upper plate is attached to the vertical spindle, known as the inner spindle or the inner axis.

Lower Plate

The lower plate is also called as the main scale plate for the horizontal circle. It it is mounted on a hollow tapered spindle, also called the outer spindle or the outer centre.

The lower plate is graduated in degrees from 0° to 360° with least count of 20'. This graduations increase in a clockwise direction. The least count of the veneers is 20".

Plate Level

A level tube called plate level is mounted on the upper plate. The bubble is centred with the help of levelling screws. In some instruments there are 2 plate levels. These plate levels are mounted horizontally at right angles to each other on the upper plate. One of the plate levels is parallel to the trunnion axis and the other at right angles to it.

These plate levels are used to make the vertical axis of the instrument truly vertical.

Some basic definitions

Centring of theodolite

It is the process of setting up the instrument exactly over the station mark. A plumb Bob suspended from a small hook attached to the underside of the inner spindle is used for precise centring.

Horizontal axis

It is also called the trunnion axis or transverse axis. It is the axis about which the telescope can be rotated in a vertical plane.

Vertical axis

It is the axis about which the telescope can be rotated in a horizontal plane. It is also known as azimuth axis.

Line of sight

It is the imaginary line passing through the intersection of the cross-hairs of the diaphragm and the optical centre of the objective. It is also known as line of collimation.

When the line of sight comes in horizontal plane it is called as line of collimation.

Axis of Level tube

It is a line tangential to the longitudinal curve of the level tube at its centre. Axis of plate level is horizontal when bubble is centred.

Face right

When the vertical circle of a theodolite is on the right hand side of the observer, the position is called face right and the observation made is called face right observation.

Face left

When the vertical circle of a theodolite is on the left hand side of the observer, the position is called face left and the observation made is called face left observation.

Note- By taking the mean of both face readings, the collimation error is eliminated.

Telescope normal