| Information on the past and present orientation of the Earth?s magnetic field is available
from the National Geophysical Data Center (NGDC) which serves as the national data
bank for geomagnetic data. The Earth?s magnetic field has been measured by land surveys,
ships, aircraft and satellites at hundreds of locations resulting in millions of observations.
Many of the observations are of poor quality or are unevenly distributed around the
globe. Because of these limiting factors and the variation of the main magnetic field
with time, the Earth?s field is usually approximated by mathematical models. These
models, adjusted to fit selected available observations, have wide application in
geophysical and space sciences. Surveyors can obtain data and information for individual
specific locations worldwide. Past values of the magnetic field are available from
models for areas within the conterminous U.S. from 1750 or from 1945 worldwide. These
mathematical models must be periodically updated due to the changing nature of the
Earth?s magnetic field. NGDC maintains the models and data base of observations to
track secular variation of the Earth?s magnetic field. Models and software are available
on-line. The global geomagnetic observations data bases for secular change studies
include high-quality magnetic observations made at observatories worldwide since 1800.
Other geomagnetic data bases are: IAGA Repeat Station data base which contains observations
from exact reoccupations, Other land Surveys containing miscellaneous land observations
worldwide since 1800 including a few thousand observations made at sea by the non-magnetic
ships, Carnegie and Zarya, Vector airborne surveys from the Project Magnetic aircraft,
satellite and selected marine observations. Most of the surface data are available
on-line. At most locations, three magnetic elements were observed: Either declination
(D), Inclination (I), and horizontal intensity (H), or declination, horizontal intensity,
and vertical intensity (Z), or total intensity (F). Where possible, the values for
other magnetic elements, usually the north component (X) and east component (Y), were
derived by the computer. The altitude, latitude, and longitude are recorded for each
observation. The observatory annual means file contains the results of more of the
observatories operating since 1800. Files are revised as new data become available.
Searches and plots of data from selected stations can be made. These secular change
files are the principal source for tracking the long-period changes in the direction
and intensity of the Earth?s magnetic field. Other data include a digital bibliography
of historical magnetic survey publications at NGDC and the co-located World Data Center-A.
Over 750 reports published from the 1830?s to 1980?s were inventoried in a digital
bibliography and digitally imaged in .PCX format. The digital images of these reports
have been written to magnetic tape. |