Fig. 1From: Geomagnetic core field models and secular variation forecasts for the 13th International Geomagnetic Reference Field (IGRF-13)The distribution of ground-based and satellite virtual geomagnetic observatories. The ground-based observatory data (blue circles) cover the period 1957 to 2018, whereas the VO data (red stars) are derived from Swarm and cover the period 2015 to 2019.5Back to article page