theodolite—A precision surveying instrument consisting of an alidade with a telescope. It is mounted on an accurately graduated circle and equipped with levels and reading devices. Sometimes, the alidade carries a graduated vertical circle. See also theodolite, digital; theodolite, direction instrument; theodolite, repeating.

theodolite, digital—A theodolite that records and stores data electronically. See also total station instrument.

theodolite, direction instrument—A theodolite in which the graduated horizontal circle remains fixed during a series of observations. The telescope is pointed on a number of signals or objects in succession, and the direction of each is read on the circle, usually by the use of micrometer microscopes. In measuring horizontal angles with a direction instrument, angles are not repeated (accumulated) on the circle, but precision and accuracy are obtained by having a circle of high quality, by using precision methods of reading the circle, and by shifting the circle between sets so that each direction is measured on a number of different parts of the circle. Direction instruments are used almost exclusively in first- and second-order triangulation.

theodolite, gyro—A theodolite so designed that a north-seeking gyro attachment can be mounted directly over the vertical axis. The spin axis of the gyro, which is horizontal, seeks a position in the meridian plane. Hence the theodolite can be oriented with respect to North.

theodolite, repeating—A theodolite so designed that successive measures of an angle may be accumulated on the graduated circle, and a final reading of the circle which represents the sum of the repetitions may be made; repeating instrument. The observed value of the angle is obtained by dividing the total arc passed through in making a series of observations by the number of times the angle has been observed. The total arc passed through may include several complete circuits of the circle, which must be added to the circle reading before making the division. Theoretically, the repeating theodolite is an instrument of great precision, but in its mechanical operation it does not give as satisfactory results as does the direction instrument.

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Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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