Digital Theodolite Horizon ET1002

A digital theodolite 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, but theodolite have been adapted for other specialized purposes in fields like meteorology and rocket launch technology. A modern digital theodolite consists of a telescope mounted movably within two perpendicular axes, the horizontal or trunnion axis, and the vertical axis.

Digital theodolite is mounted on the tripod head by means of a forced centering plate or tribrach, containing four thumbscrews (or in some modern digital theodolite three thumbscrews) for rapid levelling. Prior to Digital theodolite, instruments such as the geometric square and various graduated circles (see circumferentor)

In today's digital theodolite, the reading out of the horizontal and vertical circles is usually done electronically. Digital theodolite or the target can be rapidly removed from, or socketed into, the forced centering plate with sub-mm precision. Accurate electronic digital theodolite have become widespread tools, but transits still find use as a lightweight tool for construction sites. The first instrument more like a true digital theodolite was likely the one built by Joshua Habermel (de:Erasmus Habermehl) in Germany in 1576, complete with compass and tripod.

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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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Theodolite

A theodolite 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, particularly on inaccessible grounds, but theodolite 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. 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.
The transit refers to a specialized type of theodolite 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 arc-seconds. In the middle of the 20th century, transits came to be known as 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 theodolite have become widespread tools, but transits still find use as a lightweight tool for construction sites. Some transits do not measure vertical angles.
The builder's level is often mistaken for a transit, but is actually a type of inclinometer. It measures neither horizontal nor vertical angles. It simply combines a spirit level and telescope to allow the user to visually establish a line of sight along a level plane.

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