This site gives a historical perspective, dating
from William Gilbert’s study De Magnete in 1600, on
the changing understanding of the earth’s magnetic
field.
INTERMAGNET is the International Real-time
Magnetic Observatory Network, which monitors the
earth's magnetic field. It presents data and
information from geomagnetic observatories around
the world.
More suggestions and sources
McKnight, D. ‘Centenary of New Zealand’s
magnetic observatory.’ Newsletter of the New
Zealand Geophysical Society 59 (2001):
20–24.
Sutherland, R. Magnetic anomalies in the New
Zealand region, 1:4,000,000. Geophysical Map
9. Lower Hutt: Institute of Geological &
Nuclear Sciences, 1996.
Wellman, H. W. ‘The Stokes Magnetic Anomaly.’
Geological Magazine 110, no. 5 (1973):
419–429.