Understanding Magnetic Declination

Magnetic declination is the difference between the direction your compass is pointing, called magnetic north, and the direction that your map is oriented, which is true north.  The Earth’s magnetic field lines up roughly, but not exactly with the true poles—those points on the surface that correspond with the Earth’s axis. Check out the “grapefruit example” in the Magnetic Declination video.

In some parts of the world the two norths appear to line up perfectly, so declination is 0 degrees, but in other places it is 25 degrees or more. The difference between declination from location to location can add up quickly.  For example, the magnetic north pole lies 15.5 degrees west of the true north pole from a position in Portland, Maine.  If you are standing in Portland, Oregon, your compass points 16 degrees east of True North.  If you travel between these two locations and forget to set your compass declination, your compass bearing could be off by over 30 degrees!

The magnetic declination for a given area is printed right on some maps (see the USGS topographic map image below), and there is a website that will determine your declination based on your website or rough latitude and longitude coordinates (see Finding Declination for Your Area (coming soon))

Setting the declination on hand held compasses with an adjustable declination option makes it easy to “set and forget” about declination while you are navigating in a single general location.  When you move more than a hundred miles or so, and every few years (the magnetic pole wanders a bit) you’ll need to reset the magnetic declination adjustment. See  Setting Declination on Your Compass (coming soon) for more ils.

Making a declination correction on a non-adjustable compass requires a little arithmetic, but don’t panic, it’s not as hard as it sounds.  See Making a Declination Correction on Your Non-adjustable Compass.

What Causes Magnetic Declination?

Our planet’s magnetic field is created by the movement of liquid iron in the Earth’s outer core. The iron moves because heat causes a convective current and because the Earth is rotating.  This energy creates an electrical current that interacts with smaller magnetic fields to create Earth’s strong magnetic field.

All the movement down below the planet means that the field is constantly changing in intensity and intensity.  In fact, the field even reverses every so often—on average, every 250,000 years, but sometimes a lot more frequently and sometimes there are millions of years between flips.  These reversals, once they get going, probably take hundreds or thousands of years to complete, so don’t worry too much about having to buy a new compass should one start next year.

A somewhat shorter-term concern for compass users is the year-to-year change in the location of the magnetic north pole.  Even this is not a huge concern because the movement is slow enough, on a human time-scale, that you could go backpacking for a few years and not have to worry about it.  Observations of magnetism going back to the 16^th century show that the declination in London, England has changed from 10 degrees east to 25 degrees west, and now back to about 3 degrees west.  Currently, the magnetic pole is moving at approximately ½ degree per year.