Theodolite are primarily classified as (1) Transit (2) Non-transit.
A theodolite is called to be transit when its telescope can be transited i.e., revolved through a complete revolution about its horizontal axis in the vertical plane. Whereas in non transit type, the telescope can not be transited. The non transit theodolite are inferior in utility and have now become obsolete.
Terms Used in Transit Theodolite
Following terms are used in transit theodolite are given below:
1. Centering: It means setting the theodolite exactly over an instrument-station so that its vertical axis lies immediately of the station mark. It can be done by means of plumb-bob suspended from a small hook attached to the vertical axis of the theodolite.
The centering shifting arrangement if provided with the instrument helps in easy and rapid performance of the centering.
2. Transiting: It also known as plunging or reversing. It is the process of turning the telescope about its horizontal axis thought 180 degree in the vertical plane thus bringing it upside down and making it point exactly in opposite direction.
Swinging the Telescope: It means turning the telescope about its vertical axis in the horizontal plane. A swing is called right or left according as the telescope is rotated clockwise or counter clockwise.
3. Face Left: If the vertical circle of the instrument is on the left of the observer while taking a reading, the position is called the face left and the observation taken on the horizontal or the vertical circle in this position, is also known as the face left observation.
4. Face Right: If the vertical circle of the instrument is on the right of the observer while taking a reading, the position is called the face right, and the observation taken on the horizontal or the vertical circle in this position, is also known as the face right observation.
5. Changing Face: It is the operation of bringing the vertical circle to the right of the observer, if originally it is to the left, and vice versa. It is done in two steps: Firstly revolve the telescope through 180 degree in a vertical plane and then rotate it through 180 degree in the horizontal plane i.e., first transit the telescope and then swing it through 180 degree.
6. Line of Collimation: It is also known as the line of sight. It is the imaginary line joining the intersection of the cross-hairs of the diaphragm to the optical center of the object glass and inits continuation.
Axis of the Telescope: It is also an imaginary line joining the optical center of object glass to center of the eye piece.
7. Axis of the level Tube: It is also called the bubble line. It is a straight line tangential to the longitudinal curve of the level tube at the center of the tube. It is horizontal when the bubble is central.
8. Vertical Axis: It is axis about which the telescope can be rotated in the horizontal plane.
9. Horizontal Axis: It is also called the trunion axis or the transverse axis. It is the axis about which the telescope can be revolved in the vertical plane.