Magnetic Declination

Magnetic declination concept

The magnetic field generated by the Earth is not immutable, on the contrary, it constantly varies both in its position and in its intensity and also periodically (in cycles of thousands of years) it is reversed. Currently, he travels about 100 meters a day.

Since a compass will always indicate the magnetic north pole, it is convenient to know the relationship or difference between the north indicated by the compass and the true or geographic north. This concept is magnetic declination.

Magnetic declination (“variation” in aeronautical navigation) is the angle between true (geographic) north and magnetic north, that is, it is the difference angle between geographic north and the north that a compass will mark.

The magnetic declination will depend on the position on earth from where it is measured, being zero at the point on earth where they are aligned.

The following figure shows the declination for three points A, B and C located in different positions on the earth's surface. For point C the declination is zero since it is aligned with the geographic north pole and the magnetic north pole.

Declination is considered positive if magnetic north is east of true north and negative if magnetic north is west of true north.

Magnetic declination maps

In order to know the magnetic variation at any point on the planet, maps are made where the magnetic declination at each location is indicated by means of isolines that represent a specific angle, since they are isolines that represent angles, they are called isogonas. Once our position or the position of the route is known, the closest isogonic line to our position will be taken as a reference. 

The special isogonic line that joins points of variation “0º” is called agonal line (without angle). A compass located in a position corresponding to an agonal curve points exactly to true north, since its magnetic declination will be zero.

Map developed by NOAA/NCEI and CIRES
https://ngdc.noaa.gov/geomag/WMM
Published December