| Frequency Offset |
The tuned frequency of the electromagnetic waves may not be exact as required. An error in the frequency of modulated waves produces an error in the scale of electronic measurement of distance. For example, if the correct modulated frequency is 10 MHz and if the actual frequency departs from this by 10 Hz, a relative error of 1 ppm affects each measurement. The frequencies can be checked by using a frequency counter. A frequency check is required to be performed at regular intervals, particularly for high-order surveys or when surveys with very long lines are being performed. Otherwise, if the EDM is checked against a known distance, applying corrections for instrument and reflector offsets, meteorological conditions, and slope, a frequency shift can be detected. |