Abstract
This work brings together the contents of previously disparate databases of absolute ground geomagnetic observations, both historic and ongoing, to process the data consistently into a coherent and accessible format for researchers. This includes converting reported time resolutions and coordinate systems into a uniform format, and applying all documented observatory baseline changes to the data records, to create a single unified vector time series of monthly mean values for each geomagnetic observatory. The data are made available freely and anonymously to researchers via a web service, updated on a monthly basis with the latest definitive and non-definitive observations reported to the World Data Centre for Geomagnetism (Edinburgh) and INTERMAGNET or Tromsø Geophysical Observatory, respectively. The current database covers 327 observatories worldwide, with observations from 1883 up until present. A separate effort involved digitising tables of monthly means in the 1841-1925 yearbooks of the Royal Observatory, Greenwich, United Kingdom and converting to homogeneous units. The resulting data represent some of the earliest magnetic data at monthly time resolution and may be of use to future studies. However, they cannot be incorporated into the monthly means database as the field vector information is incomplete; we publicise them here instead.