Theodolite is an instrument surveyors use to measure angles and directions. Most theodolites are mounted on a tripod (three-legged stand). Modern theodolites have an optical plummet, a special telescope device that enables a surveyor to position the instrument precisely over a selected point. A theodolite also has a telescope that pivots both vertically and horizontally, permitting accurate sighting in any direction. A horizontal plate below the telescope provides readings around the horizon in degrees, minutes, and seconds of arc. A minute is equal to 1/60 of 1 degree, and a second of arc is equal to 1/3,600 of 1 degree. A vertical plate and scale, mounted to the left of the telescope, permit vertical readings.
