This instrument is aimed for topographic use and conceived to measure position angles and calculate distances. These operations must be performed by means of a horizontal graduated disc, equipped with a compass, on which a vertical graduated semicircle with eyepiece is free to rotate. When the eyepiece is directed to a given point, the meter can plot the azimuth (or position) angle on the horizontal graduated disc as against the magnetic north direction, and the zenith angle on the vertical semicircle. The theodolite was distributed by Leonard (1520-1559) and Thomas Digges (1546-1595) in 1571. In modern theodolites the eyepiece has been replaced by a telescope.