Electronic Distance Measurement EDM

EDM instruments determine lengths based on the time that it takes for an energy wave to travel from one end of a line to the other and return. They have replaced the difficult and painstaking task of taping for accurate measurement of horizontal distances. They also can measure distance over bodies of water or other inaccessible terrain as long as they have a clear line of sight. Some require a reflective surface to bounce the beam back, some are reflector less. Others use a second unit to return the signal or a reflector, Figure 12.14.

EDM's use an energy beam to send a signal to a reflector or a reflecting object and measure the time elapse from when the beam was sent and when it returns to the instrument. Distance is determined by rearranging the velocity equation.

ft min

Distance (ft) Time (min)

Distance = V

A big advantage of EDM's is that the operator does not need to walk the path, all they need is a line of sight. They can also be used when the line of sight is obscured by fog or light rain.

EDM transmitter and receiver

FIGURE 12.14. An example of an EDM.

EDM transmitter and receiver

FIGURE 12.14. An example of an EDM.

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