Explain the principles of Electronic Distance Measurement (EDM).

EDM measures distance using electromagnetic waves. Phase comparison method: modulated infrared/laser beam travels to reflector and returns, instrument measures phase difference between transmitted and received signals. Distance = (n x wavelength + delta phase x wavelength) / 2. Multiple modulation frequencies resolve ambiguities. Pulsed method: measures time for pulse round trip, distance = time x speed of light / 2. Corrections required for atmospheric conditions (temperature, pressure, humidity) using refractive index. Typical accuracy: 2mm + 2ppm. Reflectorless EDM uses diffuse reflection from surfaces up to 200m.