  One of the best tools we have in geophysical modelling is the ability to
interpolate the horizontal geomagnetic field at the surface of the Earth. This
is especially useful in regions, such as southern Africa, where there is a
sparse array of absolute magnetometers available for geomagnetic field
measurements. In terms of geomagnetic field interpolation, the spherical
elementary current systems (SECS) spatial interpolation scheme has shown to be
very successful, and the planar approximation of this method adequate for
modelling at mid-latitudes. The SECS interpolation scheme is physics based,
making use of the Biot-Savart law and equivalent ionospheric currents to
interpolate measured geomagnetic field data. As with most interpolation
methods, more data points result in lower error. Therefore, we adapt the SECS
method to work with variometers. These instruments measure variations in
magnetic field and are more abundant in southern Africa. Merging the two
resulting interpolated datasets, the initial absolute geomagnetic field
interpolation can be significantly improved. This improved interpolation scheme
is not only incredibly useful locally, where a sparse magnetometer array is a
challenge, but can also be applied just as effectively in other cases across
the globe where there are numerous magnetometers and variometers available.
