Map and compass navigation works on the principle that you know one thing at all times: the direction of "north". To find north, you simply look at where the red end of your compass needle is pointing. Right?
Well, not exactly. The problem is that navigation is based on knowing the location of true north , i.e. the North Pole. And unfortunately, that's not where compass needles point. Compass needles actually point toward magnetic north a point that is close to true north, but not right on top of it. This is where "magnetic declination" comes into play.
Magnetic Declination Basics
Declination is basically an angle—an angle that measures the difference between a compass needle pointing to true north and one pointing to magnetic north. The tricky thing about declination is that this angle differs depending upon where you are standing in the world.
Let's go over a few real-life examples.
Picture in your mind the globe. The North Pole is at the top, and that is the point (true north) we are using to orient our maps and measure bearings. The magnetic needle of the compass, however, is pointing to a spot off the coast of Greenland (magnetic north).
So if you are in San Francisco and are facing true north, magnetic north would be slightly to your right (east) by about 14 degrees. This is where your compass needle would be pointing.
However, if you are in Pensacola, Fla., and facing true north, you'd discover that magnetic north would be along the same line (known as the Agonic Line, where declination is 0 degrees). No adjustment to your compass reading would be required at this location.
Conversely, if you were in New York City and facing true north, then magnetic north would be to your left (west) by about 14 degrees.
People navigate successfully with maps and compasses all the time, even though magnetic north and true north don't always line up. How? They simply learn the angle of declination in their general area, then make sure that they take that angle into account when they make their navigation calculations (basically, by adding or subtracting the angle of declination from the compass bearing numbers that they read off their compasses). Some compasses can be set so that they remain adjusted for an entire trip.
How Declination Changes Over Time
The earth's magnetic fields are constantly changing, a process known as secular variation. As explained on solarpathfinder.com, our planet's outer core contains complex fluid currents of iron, nickel and cobalt. These elements generate a magnetic field whose poles do not coincide with true north or true south—the earth's axis of rotation.
For the compass user, the practical effect of secular variation is simple. It means you should always use the latest declination information for your destination. Printed topo maps often list declination info in a lower-corner of the map, but this info can quickly become outdated.
To get current declination figures, use the declination finder on NOAA's National Geophysical Data Center website. Just input the zip code or coordinates of your destination to get the latest estimate.
To find out more about magnetic declination and other aspects of map and compass navigation, shop REI's selection of navigation books, or check the schedule for the a hands-on basic navigation class offered by the REI Outdoor School or other reputable source.