Electronic Distance Measuring (EDM) utilises the fact that the speed of electromagnetic energy of any particular wavelength e.g. light or a radar beam, is known and is constant.

Hence, you can accurately calculate how far a pulse of electromagnetic energy travels  in a given time.

To find the distance between two points using EDM equipment, an instrument is  placed on the first point and a pulse of electromagnetic energy transmitted to a second instrument positioned over the other point.

This second instrument contains a reflector that ”bounces” the energy back to the source. The time taken for the energy to return is recorded and the distance between the two instruments then calculated using the known speed and the time taken to cover the return distance.