It is used for measuring angles between points to an accuracy of an arc-second. Measuring two arcs, one clockwise the other anti-clockwise, is called an arc of observations. The innovation of Electronic Distance Measuring made it possible to measure distances electronically. The time taken for the light to travel to the reflector and back is accurately measured. The time is halved and multiplied by the speed of light to give as the distance to the reflector.
Various instrument are available today.
Generally distances accurate to millimetre level, up to 5 kilometres away, can be obtained. This Office primarily uses the instrument for fourth order work, such as Town Survey schemes. Doppler The Doppler system was the first artificial satellite-based navigation system introduced to the geodetic fraternity. The system consisted of a receiver which tracked and recorded data from the satellites in the system constellation. The receiver records the frequency of the received message.