United States Magnetic Declination

 

Magnetometer data from the USGS observatories are available in two
forms, 'preliminary' and 'definitive'. 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. But since producing definitive data requires the
careful inspection of the data and the application of specialized
software, they are not made available until some time after their
initial collection.