Getting a Fix on Declination

“You can’t get there from here” is a classic Yankee punchline, usually delivered at the end of a joke involving an old-timer, a bewildered tourist, and a location somewhere over hill and yon. But it could serve just as well to describe what happens when someone sets off in the woods with map, compass, and an incomplete understanding of magnetic declination. Or, to put it bluntly, which way north is.

The easy part to understand about north is true north, which is to say, the location of the North Pole. Most maps are drawn to true north, meaning that if you were to follow the little north arrow on the corner of the map, you’d eventually find yourself at the North Pole – on the axis of rotation of the spinning Earth.

If you were to hold up your compass and decide to follow the little red arrow instead, you wouldn’t make it to the North Pole at all. Instead, you’d wind up on an ice floe in the Arctic Ocean, about 200 miles west of Canada’s Ellesmere Island. That’s the location of the magnetic North Pole. (Presently, at least – more on that in a minute.)

This distinction between true north and magnetic north doesn’t matter at all if you live in the vicinity of, say, Kansas City, because from there, the North Pole is located due north of the magnetic pole. Follow either the map or the compass, and you’ll head in the same direction. But if you happen to live in our neck of the woods, this distinction is crucial.

In the Northeast, the magnetic pole is to the left, or west, of the North Pole as you face north. If you point your right arm to true north and your left arm to magnetic north, you’ll end up with a narrow angle between your arms. This angle is called the magnetic declination. If you are standing at Niagara Falls, the declination is roughly 11 degrees. At the three-way intersection of New Hampshire, Vermont, and Massachusetts, it’s 15 degrees. In Down East Maine near Eastport, the declination is 18 degrees.

In order to use a compass effectively with a map, you need to take declination into account. There are three ways to do this. If your map is inexpensive and disposable, you can draw a series of declination lines right on the map itself (most maps have a declination key somewhere on the edge); when you put your compass on the map, line up the magnetic needle with the lines you’ve drawn, rather than with the map’s compass rose. Alternatively, if your compass is expensive, as many are that are designed for orienteering or geologic exploration, you can adjust the compass rose by turning a screw so that the declination is taken into account. When the compass needle points to zero degrees on the rose, the compass’s case will point to true north instead of magnetic north. Finally, if your compass is relatively inexpensive and you haven’t drawn declination lines on your map, you’ll just have to remember that true north is to the right of where your compass is pointing.

Regardless of which strategy you employ, keep in mind that, as described above, declinations vary dramatically across the width of the Northern Forest. If you set your compass or draw your map lines at home near Acadia National Park, you’ll be out of luck once your canoe trip starts in the Adirondacks. And it isn’t just north that will be out of whack, it’ll be every direction. Declination error applies equally across the entire 360 degrees of the compass.

Here’s the final kicker: magnetic north isn’t stationary. The pole wanders around as the magnetic elements in the Earth’s liquid core slosh about. In 2006, magnetic north is moving northwest at a rate of about 25 miles per year. This means that declination changes over time. A century ago, our declination here in the Northeast was about five degrees less than it is now. A century from now, should you happen to be reading this article, you’d be well advised to consult some current source for the latest declination. This is assuming, of course, that your solar-powered, satellite-synched, Internet-linked, GPS-cum-chronometer has somehow failed you.

 
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