EDM instruments are equipments used in surveying to determine the straight-line distance between two points or stations and this instrument is capable of sending an electronic impulse with a particular velocity and it measures the time required for the impulse to travel from one point to another. Then using the equation of distance=rate x time, interval length is calculated. EDMs are of two types 1) Electromagnetic EDM instruments & 2) Electro optical EDM instruments. Before using EDMs it is very important to gather information’s regarding the operations of the instrument as described by the manufacturer.
Electromagnetic (Microwave) EDM instruments
High-frequency radio waves are used in Electromagnetic EDMs .It was first developed in 1950s.This instrument is very precise for measuring long distances. This instrument had undergone several improvements over the years and now electromagnetic EDMs are available in smaller and portable size, and they have a special feature of direct measurement readout. While using this instrument, it is necessary to fix at the both ends of the line that you are going to measure, two identical and interchangeable instruments for transmitting and receiving. Transmitting instrument will send a series of modulated radio waves and this will be captured by the receiving instrument. The receiving instrument will resend the waves received to transmitting instrument after interpreting the waves and the transmitter unit finally calculates the time required for radio waves to travel from transmitter to receiver and again to transmitter. Atmospheric condition affects the velocity, which means the rate of travel of these radio waves. Based on the instructions for operations, corrections for temperature and barometric pressure are applied.
Electro Optical (Light Wave) EDM instruments
Instruments utilizing the velocity of light waves to determine the distance between two points are Electro Optical or Light wave EDM instruments. Geodimeter was the earlier form of Electro Magnetic EDM instruments’. The instrument should be fixed at one end of the line to be measured and at the other end reflector is fixed. Electromagnetic EDM can give accurate results if the line to be measured is free without any obstacles. Unlike Electromagnetic EDMs, the Electro optical EDMs need both ends of the measuring lines to be intervisible.EDM sends a beam of light to the reflector, which acts as a mirror and it redirects the light pulse to the EDM.As soon as the EDM receives the reflected light flash, the readings registered in it are converted to linear distance between EDM and reflector. Corrections have to be made based on atmospheric conditions.