using a parallactic triangle with fixed angles and a base of variable length in the observation station, e.g. in the instrument

Full Title

Measuring distances in line of sight; Optical rangefinders (tapes, chains or wheels for measuring length G01B3/00; active triangulation systems, i.e. using the transmission and reflection of electromagnetic waves other than radio waves, G01S17/48) > using a parallactic triangle with fixed angles and a base of variable length in the observation station, e.g. in the instrument