A theodolite (or "transit") is an instrument for measuring
both horizontal and vertical angles. It consists of a telescope mounted
movably within two perpendicular axes, the horizontal or trunnion axis,
and the vertical axis. These must be mutually perpendicular. The condition
where they deviate from perpendicularity (and the amount by which) is
referred to as horizontal axis error.
The optical axis of the telescope, called sight axis and defined by
the optical center of the objective and the center of the cross-hairs
in its focal plane, must similarly be perpendicular to the horizontal
axis. If not, we call the deviation from perpendicularity collimation
Both axes of a theodolite are equipped with graduated circles that
can be read out through microscopes. The vertical circle (the one associated
with the horizontal axis!) should read 90° or 100 gon when the sight
axis is horizontal (or 270°, 300 gon, when the instrument is in
its second position, "turned over"). If not, we call half
of the difference with 300 gon index error.
Horizontal axis error, collimation error and index error are regularly
determined by calibration, and removed by mechanical adjustment at the
factory in case they grow overly large. Their existence is taken into
account in the choice of measurement procedure in order to eliminate
their effect on the measurement results.