  Space weather and its impact on infrastructure presents a clear risk in the
modern era, as evidenced by the adverse effects of geomagnetically induced
currents (GICs) in power networks. To model GICs, ground-based geomagnetic
field (B-field) measurements are critical and need to be available in the
region of interest. A challenge globally lies in the sparse distribution of
magnetometer arrays, which are seldom located near critical power network
nodes. Interpolation of the geomagnetic field (B-field) is often needed, with
the spherical elementary current system (SECS) approach developed for
high-latitude regions favoured. We adapt this interpolation scheme to include
low-cost variometers to interpolate dB/dt directly and increase interpolation
accuracy. A further adaptation to the scheme is to physically represent the
mid-latitude context where most power networks and pipelines lie. The driving
current systems in these regions differ from their high-latitude counterparts.
Using a physics-consistent mid-latitude version of SECS, we show why previous
implementations in Southern Africa are incorrect but still result in useful
interpolation. The scope of these adaptations not only has direct application
to research in general, but also to utilities, where effective low-cost
instrumentation can be used to improve GIC modelling accuracy.
