Terminology of Theodolite
It is important to clearly understand the terms associated with the theodolite and its use and meaning. The following are some important terms and their definitions.
Vertical axis It is a line passing through the centre of the horizontal circle and perpendicular to it. The vertical axis is perpendicular to the line of sight and the trunnion axis or the horizontal axis. The instrument is rotated about this axis for sighting different points.
Horizontal axis It is the axis about which the telescope rotates when rotated in a vertical plane. This axis is perpendicular to the line of collimation and the verti-cal axis.
Telescope axis It is the line joining the optical centre of the object glass to the centre of the eyepiece.
Line of collimation It is the line joining the intersection of the cross hairs to the optical centre of the object glass and its continuation. This is also called the line of sight.
Axis of the bubble tube It is the line tangential to the longitudinal curve of the bubble tube at its centre.
Centring Centring the theodolite means setting up the theodolite exactly over the station mark. At this position the plumb bob attached to the base of the instrument lies exactly over the station mark.
Transiting It is the process of rotating the telescope about the horizontal axis through 180 o . The telescope points in the opposite direction after transiting. This process is also known as plunging or reversing.
Swinging It is the process of rotating the telescope about the vertical axis for the purpose of pointing the telescope in different directions. The right swing is a rotation in the clockwise direction and the left swing is a rotation in the counter-clockwise direction.
Face-left or normal position This is the position in which as the sighting is done, the vertical circle is to the left of the observer.
Face-right or inverted position This is the position in which as the sighting is done, the vertical circle is to the right of the observer.
Changing face It is the operation of changing from face left to face right and vice versa. This is done by transiting the telescope and swinging it through 180 o .
Face-left observation It is the reading taken when the instrument is in the normal or face-left position.
Face-right observation It is the reading taken when the instrument is in the inverted or face-right position.