Principle of EDM:- The fig shows a survey line AB, the length D of which is to be measured using EDM equipment placed at ends A. Let a transmitter be placed at A to propagate electromagnetic waves towards B, and let a receiver B placed at B, along with a timer. If the timer at B starts at instant of transmission of wave from A, and stops at the instant of reception of incoming wave at B, the transit time for the wave from A and B in known.
From this transit time, and from the known velocity of propagation of the wave, the distance D between A and B can be easily computed. However this transit time is of the order of 1 × 10-6 which requires varying advanced electronics. Also it is extremely difficult to start the timer at B when the wave is transmitted at A. Hence a reflector is placed at B instead of a receiver. This reflector reflects the waves back towards A, where they are received as shown in the fig. Thus the equipment at A acts both as a transmitter as well as receiver. The double transit time can be easily measured at A. This will require EDM timing devices with an accuracy of ± 1 × 10-9s.