Electromagnetic waves traveling from transmitter and back to receiver usually have strong reflections from the intervening terrain specifically if it is smooth and free of coarse vegetation. The reflection is more from water surface along the path, if any. It is more prevalent in microwave EDM. As reflected waves travel along longer path, it causes error in the measurement of observed distance. In order to eliminate/reduce this error, a series of readings are taken, each at different frequency, from both ends of the line at the beginning and end of each measurement. The average of all the readings provides the measurement of the distance.

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