An optical surveying instrument for measuring vertical or (more importantly) horizontal angles. It is a small telescope, with cross wires, movable over horizontal and vertical graduated circular scales. It is usually seen mounted on a stable tripod.
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, but theodolites have been adapted for other specialized purposes in fields like meteorology and rocket launch technology. A theodolite consists of a telescope mounted movably within two perpendicular axes, the horizontal or trunnion axis, and the vertical axis.
The transit refers to a specialized type of theodolite that was developed in the early 19th century. 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 theodolites have become widespread tools, but transits still find use as a lightweight tool for construction sites.
The builder's level is often mistaken for a transit, but is actually a type of inclinometer.
Concept of operation
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").
The horizontal and vertical axes of a theodolite must be mutually perpendicular. 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.
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.
A theodolite is a mounted tripod by means of a forced centering plate or tribrach, containing four 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 -- levelling.
History
The history of theodolites goes back to so-called plane table alhidades, devices allowing the graphical mapping of the terrain.
The first description of a theodolite, or 'theodelitus', is found in the surveying textbook Pantometria (1571) by Thomas Digges, son of Leonard Digges who is widely credited with the invention.
Using theodolites in surveying
Triangulation, as invented by Gemma Frisius around 1533, consists of making such direction plots of the surrounding landscape from two separate standpoints.
Modern triangulation as, e.g., practiced by Snellius, is the same procedure executed by numerical means.
In the late 1780s Jesse Ramsden, a Yorkshireman from Halifax, England who had developed the technique of dividing angular scales accurately to within a second of arc, was commissioned to build a new instrument for the British Ordnance Survey. The Ramsden theodolite was used over the next few years to map the whole of southern Britain by triangulation.
In network measurement, the use of forced centering speeds up operations while maintaining the highest precision. The theodolite or the target can be rapidly removed from, or socketed into, the forced centering plate with sub-mm precision. The height of the reference point of the theodolite -- or the target -- above the ground bench mark must be measured precisely.
The American transit gained popularity during the 19th century with American railroad engineers pushing west.
Modern theodolites
In today's theodolites, the reading out of the horizontal and vertical circles is usually done electronically. In the latter case the circles spin rapidly, reducing angle measurement to electronic measurement of time differences.
Today's theodolites are usually equipped with integrated electro-optical distance measuring devices, allowing the measurement in one go of complete three-dimensional vectors -- albeit in instrument-defined polar co-ordinates -- which can then be transformed to a pre-existing co-ordinate system in the area by means of a sufficient number of control points. The instruments, "intelligent" theodolites called self-registering tachometers or "total stations", perform the necessary operations, saving data into internal registering units, or into external data storage devices.
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