Trade name of an electronic optical device that measures ground distances precisely by electronic timing and phase comparison of modulated light waves that travel from a master unit to a reflector and return to a light-sensitive tube where an electric current is set up. It is normally used at night and is effective with first-order accuracy up to 1345 distances of 3 to 25 miles (5 to 40 km). Etymol: acronym for geodetic-distance meter. Compare: tellurometer