Theodolite
A theodolite (/θiˈɒdəˌlaɪt/)[1] is a precision optical instrument for measuring angles between designated visible points in the horizontal and vertical planes.
The traditional use has been for land surveying, but it is also used extensively for building and infrastructure construction, and some specialized applications such as meteorology and rocket launching.
[2] It consists of a moveable telescope mounted so it can rotate around horizontal and vertical axes and provide angular readouts.
The modern theodolite has evolved into what is known as a total station where angles and distances are measured electronically, and are read directly to computer memory.
Temporary adjustments are a set of operations necessary in order to make a theodolite ready for taking observations at a station.
Finally, angle readings became an indirect optical readout, with convoluted light paths to bring them to a convenient place on the instrument for viewing.
Prior to the theodolite, instruments such as the groma, geometric square and the dioptra, and various other graduated circles (see circumferentor) and semicircles (see graphometer) were used to obtain either vertical or horizontal angle measurements.
The first occurrence of the word "theodolite" is found in the surveying textbook A geometric practice named Pantometria (1571) by Leonard Digges.
[11] Possibly the first instrument approximating to a true theodolite was the built by Josua Habemel in 1576, complete with compass and tripod.
This type of theodolite was developed from 18th century astronomical Transit instruments used to measure accurate star positions.
The Survey of India at this time produced a requirement for more rugged and stable instruments such as the Everest pattern theodolite with its lower center of gravity.
In the 1870s, an interesting waterborne version of the theodolite (using a pendulum device to counteract wave movement) was invented by Edward Samuel Ritchie.
The Wild instruments were not only smaller, easier to use and more accurate than contemporary rivals but also sealed from rain and dust.
Triangulation, as invented by Gemma Frisius around 1533, consists of making such direction plots of the surrounding landscape from two separate standpoints.
The theodolite or the target can be rapidly removed from, or socketed into, the forced centering plate with sub-millimeter precision.
It features a vertical circle which is graduated through the full 360 degrees and a telescope that could "flip over" ("transit the scope").
By measuring the same horizontal and vertical angles in these two modes and then averaging the results, centering and collimating errors in the instrument can be eliminated.
This method was extensively used in World War II and thereafter, and was gradually replaced by radio and GPS measuring systems from the 1980s onward.
The theodolite is typically mounted on a rugged steel stand, set up so it is level and pointed north, with the altitude and azimuth scales reading zero degrees.
CCD sensors have been added to the focal plane of the telescope allowing both auto-targeting and the automated measurement of residual target offset.
Many modern theodolites are equipped with integrated electro-optical distance measuring devices, generally infrared based, allowing the measurement in one step of complete three-dimensional vectors—albeit in instrument-defined polar coordinates, which can then be transformed to a preexisting coordinate system in the area by means of a sufficient number of control points.
The gyrotheodolite comprises a normal theodolite with an attachment that contains a gyrocompass, a device which senses the rotation of the Earth in order to find true north and thus, in conjunction with the direction of gravity, the plane of the meridian.
