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(thēŏd`əlīt'), calibrated optical instrument used to determine relative position in surveyingsurveying,
method of determining accurately points and lines of direction (bearings) on the earth's surface and preparing from them maps or plans. Boundaries, areas, elevations, construction lines, and geographical or artificial features are determined by the measurement of
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, navigation, and meteorology. It is similar in construction to a surveyor's transit, consisting of a telescope fitted with a spirit level and mounted on a tripod so that it is free to rotate about its vertical and horizontal axes. (Sometimes two or more telescopes of different magnifications are used.) Graduated scales are used to measure the amount of rotation about the axes. Measurements of the altitude and azimuth of a weather balloonweather balloon,
balloon used in the measurement and evaluation of mostly upper atmospheric conditions (see atmosphere). Information may be gathered during the vertical ascent of the balloon through the atmosphere or during its motions once it has reached a predetermined maximum
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 at precise time intervals are used to compute the estimated wind velocity of the atmosphere.



(or transit), a geodetic instrument for determining directions and measuring horizontal and vertical angles during geodetic work and topographic and mine surveying, in construction, and in other applications. Horizontal and vertical circles calibrated in degrees and smaller units are the main measuring devices in the theodolite.

Figure 1. Schematic diagram of an optical theodolite: (1) tripod, (2) vertical axis system, (3) horizontal circle, (4) alidade control, (5) alidade of horizontal circle with reading device, (6) switch for reading of horizontal and vertical circles, (7) level for alidade (5), (8) telescope, (9) reading microscope, (10) horizontal axis system, (11) control device for telescope (8). (12) level for alidade of vertical circle, (13) light mirror, (14) setting device of level (12)

Until the mid-20th century, theodolites with metal circles read by means of verniers or micrometric microscopes were used. In the 1920’s, theodolites with glass circles equipped with optical reading devices appeared, and they came to be called optical theodolites. Schematic and optical diagrams of a theodolite are given in Figures 1 and 2, respectively; the devices for the vertical circle analogous to those for the horizontal circle are not shown.

In the USSR, GOST (the All-Union State Standard) permits only the manufacture of optical theodolites, whose main specifications are given in Table 1 (the number included in the type name is the permissible mean square error of measurement of the horizontal angle in seconds of arc).

Theodolites often have various attachments, such as a declinometer, sighting marks, or an optical range finder.

Table 1. Specifications of main Soviet theodolites
 Diameter of circles (mm)Scale divisionsMagnification of telescopeMaximum measurement of Vertical anglesWeight in case (kg)
 verticalHorizontalCirclesReading device   
Note: The reading devices of the T05, T1, and T2 are optical micrometers, that of the T5 and T15 are scale microscopes, and that of the T30 is an index
T05 ...............18013010’1”35 ×50°21 + 15
     50 × two
     60 × pieces
T1 ...............1359010’1”30 ×65°13.5
     40 ×  
T2 ...............906520’1”25 ×75°95
T5 ...............957028 ×65°6 5
T15 ...............72722’25 ×60°40
T30 ...............727210’20 ×55°3.2

Various kinds of specialized theodolites exist. Among them are astronomical theodolites, which allow sighting on the zenith and have ocular micrometers; tachymeters, which automatically give the difference in elevation of points according to readings on a scale; mine surveying theodolites, for work in mine shafts; gyroscopic theodolites, for determining the direction of the meridian; and theodolites that automatically record the results on punched tape for feeding to a computer.

Figure 2. Optical diagram of a T2 theodolite: (1) optical parts of telescope, (2) scale and separating block of optical micrometer, (3) movable wedges of optical micrometer, (4) ocular and objective of reading microscope, (5) fixed wedges of optical micrometer, (6) prism for switching readings by circles, (7) objective of horizontal circle, (8) horizontal circle, (9) objective for combining images of lines of horizontal circle, (10) collective of light system, (11) parts of optical centering device, (12) objective of vertical circle, (13) light mirror, (14) protective glass, (15) objective for combining images of lines of vertical circle, (16) magnifying prism of reading system of level (17), (17) level for alidade of vertical circle

The theodolite has a number of typical instrument errors, whose effect is diminished by well-thought-out design, careful manufacture and testing, and appropriate measurement techniques.


GOST 10529-70: Teodolity. Tipy. Osnovnye parametry i teknicheskie trebovaniia.
GOST 20063-74: Teodolity. Metody ispytanii i proverki.
Eliseev, S. V. Geodezicheskie instrumenty ipribory, 3rd ed. Moscow, 1973.
Deimlich, F. Geodezicheskoe instrumentovedenie. Moscow, 1970. (Translated from German.)
Zakharov, A. I. Novye teodolity i opticheskie dal’nomery. Moscow, 1970.



An optical instrument used in surveying which consists of a sighting telescope mounted so that it is free to rotate around horizontal and vertical axes, and graduated scales so that the angles of rotation may be measured; the telescope is usually fitted with a right-angle prism so that the observer continues to look horizontally into the eyepiece, whatever the variation of the elevation angle; in meteorology, it is used principally to observe the motion of a pilot balloon.


Shakespearian theater: Fortune Theater, London, mid-17th cent.
A precision instrument used in surveying; consists of an alidade which is equipped with a telescope, a leveling device, and an accurately graduated horizontal circle; also may carry an accurately graduated vertical circle.


An optical instrument consisting of a sighting telescope—mounted, so that it is free to rotate around the horizontal and vertical axes—and graduated scales, so that the angle of the azimuth and the elevation can be measured. The telescope is usually fitted with a right-angle prism, so that the observer continues to look horizontally into the eyepiece whatever the variation of the elevation angle. The device tracks pilot balloons used to determine the wind velocity in the upper air. See also pilot balloon.