A Note About Declination

Map and compass navigation works on the principle that you know one thing at all times: where "North" is. To find North, you simply look at where the red end of your compass needle is pointing.

The problem is that navigation is based on knowing where "true north" is, i.e. the north pole. And unfortunately, that's not where compass needles really point. Compass needles actually point toward "magnetic north," a point that is close to true north, but not right on top of it. And this is where "declination" comes in.

Declination is basically an angle -- an angle measuring 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 is different depending upon where you are standing in the world.

For example, imagine that magnetic north is a point at the very tip of Maine (it's not, but just imagine). If you look at your compass in Florida, the needle would point to that tip of Maine. Since the true north pole lies roughly in line with you and the tip of Maine, your needle would also be pointing very close to the true north pole. Thus your angle of declination would be very small, and it wouldn't affect your navigation very much.

Now, imagine that magnetic north is still at the tip of Maine, but you are looking at your compass in Seattle. Your compass needle would still be pointing toward the tip of Maine, but it would not be pointing anywhere near the true north pole. In this case, your angle of declination would be very large, and if you didn't account for it somehow while you navigated around Seattle, you would get lost very quickly. Luckily, magnetic north is far closer to the true north pole than the tip of Maine.

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 figure out what the angle of declination is 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.

To find out more about declination and other aspects of map and compass navigation, check REI's great selection of basic navigation books, or call your local REI and ask about their hands-on basic navigation clinics.