The separation and delineation of external and internal current sources can be achieved by detemining intervals of very quiet magnetic activity to ascertain the main field, which can then be used as a reference to obtain the magnetic variations of the sources. A new method is proposed and justified for determining periods of very quiet magnetic activity. The very quiet dark nighttime interval, when known magnetic activity and hence external ionospheric and magnetospheric sources are minimal, can be used to establish the baseline or reference level for the analysis of the sources. Magnetic fields were calculated at these times from typical very quiet nighttime ionospheric parameters and found to be a negligible 3 nT for the mid latitude and polar cap regions, and 10--20 nT for the auroral zone. At mid and low latitudes very quiet nighttime intervals often occur, however, at high latitudes significant magnetic activity is almost always present and it is difficult to obtain quiet intervals. Baselines determined from very quiet nighttime intervals were obtained from the Canadian magnetic observations digital data base over the period from 1973 to 1980 by special selection and filtering techniques. The data were filtered to remove short period disturbances, and only intervals for which Dst was less than 10 nT and the regional and local activity were minimal were selected. The undisturbed field and its secular variation were determined and the standard deviation of the quiet levels about a quadratic function was 4--10 nT. An analysis of the residuals indicated little or no annual variation. This is a significant improvement in the delineation of the main field secular variation in high latitude regions. For some stations the secular variation determined from these undisturbed levels was systematically different from that obtained from the annual means, the difference being due to an equivalent net westward auroral current system of ~40 kA. It is recommended that these undisturbed levels be systematically determined at observatories and used for a common reference level for the reduction of magnetic and repeated surveys as well as for the analysis of internal and external sources.