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.
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