ABSTRACT
Theodolites have, for more than 200 years, been a fundamental instrument of surveying – improving to become a one second of arc or better instrument – along with the chain/tape and level. In the mid 1960s a new unit, the EDM (electronic distance meter) became available and revolutionised distance measurement by utilising the phase difference of infrared light transmitted and received from a reflector at the monitored location. The emission accuracy is now to within 0.5 mm.
