Concept of operation Theodolite

The axes and circles of a theodolite.

Both axes of a theodolite are equipped with graduated circles that can be read out through magnifying lenses. The vertical circle (the one associated with the horizontal axis) should read 90° or 100 grad when the sight axis is horizontal (or 270°, 300 grad, when the instrument is in its second position, "turned over" or "plunged"). Half of the difference with 300 grad is called the "index error".

The horizontal and vertical axes of a theodolite 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 the "sight axis" and defined by the optical center of the objective and the center of the crosshairs in its focal plane, must similarly be perpendicular to the horizontal axis. Any deviation from perpendicularity is the "collimation 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.

A theodolite is mounted on the tripod head by means of a forced centering plate or tribrach, containing four thumbscrews (or in some modern theodolites three thumbscrews) for rapid levelling. Before use, a theodolite must be placed precisely and vertically over the point to be measured — centering — and its vertical axis aligned with local gravity — leveling. The former is done using a plumb bob, spirit level, optical or laser plummet.

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