| Instrumental Errors |
Systematic instrumental errors occurring in electro-optical systems include uncertainties in the position of the electrical centre of the transmitter, uncertainties in the effective centre of the reflector, frequency drift, and instrument nonlinearity. The first two sources of errors must be taken into account in all survey measurements, the third requires monitoring and the fourth is critical only for measurements of high precision. Microwave systems are affected by uncertainties in the electrical centres of the master and remote units and by a phenomenon called ground swing or reflection. The EDM systems properly adjusted at the factory, the errors noted above will be very small and in a practical sense may be insignificant. However, it is important that users of these systems carry out periodic calibration of the instruments against a known distance and is absolutely necessary to ensure consistent results. |