The Earth acts like a great spherical magnet, in that it is surrounded by a magnetic field. This magnetic field changes both with time and with location on the Earth and resembles, in general, the field generated by a dipole magnet (i.e., a straight magnet with a north and south pole) located at the center of the Earth. The axis of the dipole is offset from the axis of the Earth's rotation by approximately 11 degrees. This means that the north and south geographic poles and the north and south magnetic poles are not located in the same place. At any point and time, the Earth's magnetic field is characterized by a direction and intensity which can be measured. Often the parameters measured are the magnetic declination, D, the horizontal intensity, H, and the vertical intensity, Z.
From these elements, all other parameters of the magnetic field can be calculated using
Geomagnetic Field Calculator.