Magnetic Declination

At most locations, a compass does not point toward geographic (true) north. The angular difference between true north and magnetic north is called magnetic declination or magnetic variation. It is conventionally described as east or west of true north; NOAA's convention assigns positive values to east declination and negative values to west declination. Because the Earth's magnetic field is complex, declination is not the same everywhere and its spatial pattern does not simply follow latitude or longitude lines.

A compass responds to the local magnetic field, whose direction is shaped by the Earth's field rather than by the rotational axis alone. The geomagnetic field is not a perfectly aligned dipole, and its configuration changes. Consequently, declination depends on both location and date. NOAA's World Magnetic Model represents the main geomagnetic field and its gradual change, known as secular variation, and is periodically updated for navigation and other applications.

Maps are generally oriented to true north, whereas a magnetic compass gives a magnetic bearing. To convert a magnetic bearing into a true bearing under the east-positive convention, add declination to the magnetic bearing, then express the result in the appropriate 0–360° range. For example, with a 10° east declination, a magnetic bearing of 45° corresponds to a true bearing of 55°. A 10° west declination is −10°, so the same magnetic bearing corresponds to 35° true. Users should use a value appropriate to their location and date and follow the convention supplied by their map or instrument.

A compass needle aligns with the horizontal component of the magnetic field at its location; it does not point to one fixed magnetic pole. This distinction matters because the field direction at the surface is influenced by the full geomagnetic field. Near very high latitudes, a horizontally held compass can become difficult to use because the field is steeply inclined.