4.7. Baseline Calculation

The magnetic field will vary during an absolute observation. This is unavoidable since an absolute observation may take several minutes, and the magnetic field may vary tens of nT or more during that time, especially at high latitudes. To achieve sub-nT estimation of the instantaneous vector baseline:

  • the timing accuracy of the clock used for the absolute measurements should be 1 second and each single observation with e.g. the DIM and PPM should be reported with 1 second timing accuracy

Generally, use variometer spot-readings or one-second means or means over a few seconds (and not minute means) for reducing the absolute measurements.

Accurate formula (as those presented in Section 6.5) should be used. If approximate formulas are used, then one has to be clear about the assumptions they are based on (some formula currently used at observatories only work under certain conditions, e.g. for constant geomagnetic field during the absolute measurement or for variations of the H component being small compared to the size of the H component, or assume a certain sign of the vertical component (i.e. depend on hemisphere)).

Careful synchronization between the absolute measurements and variometer data, becomes more important as the magnetic field activity increases.

Ideally all component measurements are made on the same pier. For various reasons, some observatories measure components on different piers. For example, D and I may be made on a primary pier, and total field may be measured on an auxiliary pier; possibly the variometer F may be used, or a dedicated scalar F may be used on a separate pier. Some instruments such as a proton vector magnetometer cannot be moved and the use of an auxiliary pier is unavoidable.

If more than one pier is used, vector pier correction(s) should be measured and applied to reduce all measurements to a primary pier. It may be sufficient to treat scalar component pier corrections, such as F, as a simple scalar correction, but this is not always the case as described in Section 4.8.