The International Geomagnetic Reference Field (IGRF) model is the empirical representation of the Earth's magnetic field recommended for scientific use by a special Working Group of the International Association of Geomagnetism and Aeronomy (IAGA). The IGRF model represents the main (core) field without external sources. The model employs the usual spherical harmonics expansion of the scalar potential in geocentric coordinates. The IGRF model coefficients are based on all available data sources including geomagnetic measurements from observatories, ships, aircrafts and satellites.

The IGRF model consists of sets of coefficients for a global representation of the Earth magnetic field for the years 1945, 1950, 1955, etc. There are definitive coefficient sets (DGRF##.DAT) for which no further revisions are anticipated and IGRF##.DAT and IGRF##S.DAT for which future updates are expected. IGRF##S.DAT provides the first time derivatives of the coefficients for extrapolation into the future. The 10th generation of the IGRF model (IGRF10) consists of definitive coefficients sets for 1945 thru 2000 and prelimenary sets for 2005 and for extrapolating from 2005 to 2010.

In combination with the IGRF coefficient sets different subroutines have been used to determine the components of the magnetic field vector and the L-value at a given location. The NSSDC version uses the subroutines FELDG (magnetic field vector) and SHELLG (L shell) developed by G. Kluge at the European Space Operations Center (ESOC). His use of inverse cartesian co- ordinates simplifies the computation. The IGRF subroutines were developed by A. Zunde of the U.S. Geological Survey (USGS). The program BILCAL produces tables of the geomagnetic field strength, vector components (B-abs., B-north, B-east, B-down, declination, inclination), equatorial/minimum field strength (B0), dipole moment, and L-value in latitude, longitude (geodetic), altitude, or year (decimal).

Before using the IGRF model please read the 'Health Warning'.