Geodetic Data Collection Techniques - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 22

Fig. 2.2 Basic structure of
a theodolite
The Theodolite
There are many types of theodolite, from the classical optical theodolite to the
modern electronic theodolite. Theodolites are classified into J 07 ,J 1 ,J 2 , and J 6 in
China according to their precision. “J” is the initial letter of Chinese Pinyin (Jing
Wei Yi) for theodolites and the subscript indicates their accuracy for angle mea-
surement (mean square error). J 07 and J 1 are of high accuracy and are used for the
first- and second-order national control surveys, while J 2 is of medium-level
accuracy for third- and fourth-order surveys (see, e.g., Xu et al. 1991).
A theodolite consists of the principal components shown in Fig. 2.2 . These
components are related as follows:
1. The vertical axis must be perpendicular to the axis of the plate bubble. When the
plate bubble is centered, the vertical axis coincides with the plumb line.
2. The vertical axis must be at right angles to the horizontal circle and pass through
its center. When the vertical axis is perpendicular, the horizontal circle is parallel
to the horizontal plane through the point of observation and the angle measured
in such cases will be the true horizontal angle.
3. The horizontal and vertical axes must be perpendicular and the collimation axis
must be at right angles to the horizontal axis. So, when the vertical axis is
perpendicular and the telescope is elevated or depressed, the plane formed by the
collimation axis will be the vertical plane of sighting.