Magnetic declination

Magnetic declination, sometimes called magnetic variation, is the angle between magnetic north and true north. Declination is considered positive east of true north and negative when west.  This article contains more information about magnetic declination and some maps to see the variation in your area or interest.

 

 

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 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 these elements, all other parameters of the magnetic field can be calculated.

 

The magnetic field is different in different places. In fact, the magnetic field changes with both location and time and changes the way it is changing. It is so irregular that it must be measured in many places to get a satisfactory picture of its distribution. This is done using satellites and about 200 magnetic observatories worldwide. However, there are some regular features of the magnetic field. At the magnetic poles, a dip needle stands vertical, the horizontal intensity is zero, and a compass does not show direction. At the north magnetic pole, the north end of the dip needle is down; at the south magnetic pole, the north end is up. At the magnetic equator the dip or inclination is zero. Unlike the Earth’s geographic equator, the magnetic equator is not fixed, but slowly changes.

 

The compass points in the directions of the horizontal component of the magnetic field where the compass is located, and not to any single point. Knowing the magnetic declination angle between true north and the horizontal trace of the magnetic field) for your location allows you to correct your compass for the magnetic field in your area. A few kilometers away the magnetic declination may be considerably different, requiring a different correction. 

 

You can calculate your true bearing from a magnetic bearing by adding the magnetic declination to the magnetic bearing. This works so long as you follow the convention of degrees west are negative (i.e. a magnetic declination of 10 degrees west is -10 and bearing of 45 degrees west is -45).  

 

Remember: east declination is positive, west negative.

 

So here are some maps to give you a feel for how magnetic declination varies over Australia and over the world.