English: The diagram shows a magnetic meridian of the Earth's magnetic field. It is centered on the point at latitude 30° and longitude -133.16°.

The magnetic meridians (not to be confused with magnetic field lines) follow the directions shown by a magnetic compass, that is, they follow the direction of the horizontal component of the field lines in the vicinity of the Earth's surface and thus show the shape of the field. The angle between the magnetic meridians (red) and the geographic meridians (blue, running precisely north-south) is the magnetic declination. The diagram demonstrates the irregularities of the Earth's magnetic field and the fact that in general compasses point neither towards the geographic north pole nor towards the magnetic north pole.

Currently (2010) the longitude of the arctic magnetic pole is -133.16°. So for all points on the geographic meridian -133.16° (shown as a red-blue line) the magnetic and the geographic pole lie in precisely the same direction. If compasses were always pointing precisely towards the magnetic pole, a compass sitting on that particular meridian would at the same time point due north, and the declination would be zero. Instead, as the diagram shows, the declination at the compass location is positive and a traveller following the compass would travel in a curved line before arriving at the magnetic pole.

The magnetic field was computed using the coefficients of the International Geomagnetic Reference Field for the epoch 2010.