Axis of the Altitude Level Tube : It is the straight line tangential to the longitudinal curve of the altitude level tube at its centre. The axis of the altitude level tube is horizontal when the bubble is centred.

Axis of the Plate Level : It is the straight line tangential to the longitudinal curve of the plate level tube at its centre. The axis of the plate level is horizontal when the bubble is centred.

Centring : It is the process of setting up the instrument exactly over the station mark. The plumb bob suspended from a small hook attached to the underside of the inner spindle is used for centring.

Changing Face : It is the operation of bringing the telescope from the face left condition to the face right condition and vice versa. The face is changed by plunging the telescope and swinging it by 180°.

Compass : an instrument used to find the direction of a line with reference to magnetic meridian.

Convergence of meridian : Meridians on the surface of the earth converge towards each other as the distance from the equator towards either of the poles increases.

Double Sighting : It is the process of measurement of a horizontal angle or a vertical angle twice; once with the telescope in the normal condition and once with the telescope in the inverted condition. Double sighting is also called double centring.

Face Left Condition : If the vertical circle is on the left side of t,he observer, the theodolite is in the face left condition. Normally, the face left condition is used in practice. The face left condition is, therefore, also known as the normal condition. The telescope is in the normal position. It is also called the direct condition 

Face Right Condition : If the vertical circle is on the right side of the observer, the theodolite is in the face right condition. The telescope is in the inverted position. It is also called the reverse condition.

Horizontal Axis : It is the line passing through journals which fit into the bearings at the top of the standards. The telescope is turned about the horizontal axis when measuring vertical angles. The horizontal axis is also known as the trunnion axis, elevation axis or transverse axis.

Line of collimation : Imaginary line passing through the intersection of the cross-hairs of the diaphragm and the optical centre of the objective lens.

Plunging the Telescope : It is the process of rotating the telescope over the herizontal axis through 180 in the vertical plane. After plunging the telescope, the directions of the objective end and eyepiece end are reversed, and the telescope points exactly in the opposite direction. Plunging is also known as transiting or reversing.

Swinging the Telescope : It is the process of turning the telescope about the vertical axis in a horizontal plane. The swing is called right swin1!; when the telescope is turned clockwise from the previous position. The swing is called left swing when the telescope

Theodolite : is an instrument used to find horizontal and vertical direction by measurement of horizontal and vertical angles.

Transiting the telescope : The operations consisting of revolving the telescope through 180° in a vertical plane about its horizontal axis.

Vertical axis : It is the direction of gravity and is indicated by a freely suspended plumb bob. In case of perfectly adjusted theodolite, it is the line passing through the centre of the inner spindle, outer spindle and the bearing in the tribach.

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