Irregular fluctuations superimposed on the normal secular variation of the geomagnetic elements and related to solar activity, are discussed on a qualitative basis. Since a field survey may extend over a considerable period of time these fluctuations may affect the computed values of secular variation differently at the various stations. It is therefore advisable to reduce all the observations taken during a particular field survey, as well as the corresponding values at the control observatory, to a common epoch. It is suggested that smoothed curves of the observatory minus field station differences be used for this purpose.