U.S. Geomagnetic Observatory Data

Preliminary data are available in near-real time, but they are unprocessed, they have not been explicitly adjusted for long-term baseline drift, and they have not been cleaned of occasional spikes and step offsets. Having said that, preliminary data are reasonably good representations of field variation occurring over timescales shorter than a few days.

On the other hand, definitive data are fully processed, they have been adjusted for baseline drift, and they can, therefore, be considered to be 'absolute'; definitive data have also been cleaned for spikes, step offsets, and other artifacts and contamination. Insofar as they better represent the actual time-dependent behavior of the Earth's magnetic field, definitive data are superior to preliminary data. Definitive data can be used for either short-term or long-term studies of the geomagnetic field.