Geomagnetic storms pose a hazard to many modern technologies. Therefore understanding how severe such storms could be is important to a wide range of space weather data and forecast end-users. Extreme value statistical (EVS) methods are therefore applied to a global set of geomagnetic observatory data to determine the one in 100 and one in 200 year extreme values in the north, east and horizontal field strengths and
their time rates-of-change. We use one-minute digital data from geographically widely distributed observatories with typically a few decades of digital operations. Individual generalised Pareto distribution
functions are fitted to the tail of each observatory data distribution, above some threshold marking the onset of extreme activity for that location.
We discuss the return levels, for the one in 100 and one in 200 year events, with respect to the geographical distribution of the observatories, the proximity to auroral and equatorial electrojets and compare results with a separate EVS study of European-only magnetic observatory dat
