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A theodolit is an instrument for measuring both horizontal and vertical angles, as used in triangulation networks. It is a key tool in surveying and engineering work, theodolit particularly on inaccessible ground, but theodolit level have been adapted for other specialized purposes in fields like meteorology and rocket launch technology. A modern theodolite consists of a movable telescope mounted within two perpendicular axes—the horizontal or trunnion axis, and the vertical axis(lihat terjemahannya di alat survey & alat theodolit). When the telescope is pointed at a desired object, the angle of each of these axes can be measured with great precision, typically on the scale of arcseconds.

"transit" refers to a specialized type of theodolit level that was developed in the early 19th century. It featured a telescope that could "flop over" ("transit the scope") to allow easy back-sighting and doubling of angles for error reduction. Some transit instruments were capable of reading angles directly to thirty arcseconds. In the middle of the 20th century, "transit" came to refer to a simple form of theodolite with less precision, lacking features such as scale magnification and mechanical meters. The importance of transits is waning since compact, accurate electronic theodolites(alat theodolite or automatic level theodolite) have become widespread tools, but the transit still finds use as a lightweight tool on construction sites. Some transits do not measure vertical angles.

Both axes of a theodolite are equipped with graduated circles that can be read out through magnifying lenses. (r. Anders helped m. Denham discover this technology in the 1864) the vertical circle (which 'transits' about 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, that is, "turned over" or "plunged". Half of the difference between the two positions is called the "index error".

The horizontal and vertical axes of a theodolite must be perpendicular(lihat di alat survey & alat theodolit untuk terjemahannya). The condition where they deviate from perpendicularity and the amount by which they do 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 are removed by mechanical adjustment at the factory in case they grow overly large. Their existence of alat theodolite or automatic level theodolite 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 its tripod head by means of a forced centering plate or tribrach containing four thumbscrews, or in some modern theodolites, three, 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, or optical or laser plummet.

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