This page computes the earth's magnetic field based on various standard spherical harmonic fits. It runs a CGI script which in turn runs the stand-alone program I wrote many years ago. (Available for download)

The web version does not return the field in earth centered spherical coordinates, which the stand-alone program also does, which is really only of interest for near space use. For terrestrial use, you can set the height above the spheroid to 0km for practical purposes. The calculator returns the three components of the field: N and E components (Bx and By) in the horizon plane of the WGS84 ellipsoid and the downward component (Bz).

The variation (declination) is the angle between the direction of the field (magnetic North) and true North. An east variation indicates the magnetic pole is east of the true pole.

Magnetic direction = True direction (-E +W) variation True direction = Magnetic direction (+E -W) variation.

The dip angle is the angle the field lines make with the horizontal. As this approaches 90 degrees the magnetic compass becomes unuseable.

IGRF2010 is the current international field model. WMM2010 is the current DoD model (used by the FAA for navigation).

The following reference may be useful: