Declination, Magnetic

(D), the angle between the magnetic and the true (geographic) meridians at a given point on the earth’s surface. The direction of the true meridian is usually determined with a gyroscopic theodolite or by astronomical observations. The direction of the magnetic meridian is found with the aid of instruments, such as magnetic declinometers and magnetometers, that make it possible to determine the position of a magnetic needle at the measurement site. If the north end of the magnetic needle is deflected to the east of the direction of the true meridian, the magnetic declination is called east, or positive; if it is deflected to the west, the magnetic declination is called west, or negative.

Magnetic declination is widely used in navigation, in geodetic and mine surveying, and in military work. Lines on a map connecting points with equal values of magnetic declination are called isoclinic lines. Since magnetic declination does not remain constant over long periods of time but undergoes slow changes, maps of magnetic declination are periodically revised based on measurements repeated every five to six years.