Tachymeter(redirected from Tachymetre)
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(or tacheometer), a geodetic instrument used in tachymetric surveying to measure the position of a point relative to the instrument station. The tachymeter permits determination of the horizontal angle β, the vertical angle v, the distance s, and the difference h between the elevation of the point and the elevation of the instrument station.
Several types of tachymeters exist. The rotary tachymeter is a repeating transit that has a vertical circle with a cylindrical level; the telescope is provided with stadia hairs. Nomographic and autoreducing tachymeters permit h to be determined from a vertical rod and a nomograph visible in the field of view of the telescope; such tachymeters make use of a double-image range finder to determine horizontal s from a horizontal rod. In another type of tachymeter, the base line is an integral part of the instrument. Such a tachymeter can be used to determine horizontal s directly and to find h from the measured angle v.