Magnetic declination is an essential principle to understand when navigating your way through the wilds with map and compass.
Yet it's a tricky thing to remember, at least the way it has traditionally been taught, using an addition / subtraction method. Just when you think you've grasped it, the concept floats away, like fog in the morning light.
Well there is a simple, practical approach to adjusting for magnetic declination when finding your bearings. The whole explanation begins with a definition of ‘north.”
There are 2 Norths
A lot of people know that there are 2 norths in terms of maps and compasses. A map shows true north, or the Geographic North Pole where all lines of longitude meet. The earth rotates around an imaginary axis that runs through the North and South Poles.
A compass needle points to magnetic north, which is determined by the earth's magnetic field. The location of magnetic north moves over time, at about 5 miles per hour. Right now it is slowly creeping around somewhere NW of Hudson's Bay in Canada, about 450 miles away from true north.
The angular difference between true north and magnetic north is known as “declination,” or “variation” in the aviation world. Declination is different for different parts of the globe. In Washington State, the angle of declination is 20°east. This means that magnetic north is 20° east of true north. In Tennessee the angle of declination is 0°, and in Maine, it's 20° west.
Declination values can be found in the margins or legend on topographic maps. Because the point of magnetic north is constantly moving, it is important to have a recent topo map for finding your way in the woods.
A similar point to remember is that compasses are calibrated for different parts of the world. So when you purchase a compass, be sure that it is intended to be used in the area of your big hiking vacations.
How to Find Your Bearing
Follow these steps to find your bearing:
1. Place the compass on the map with the arrow on the base plate pointing in the direction you want to go.
2. Turn the dial face of the compass so that north on the compass is parallel with north on the map.
3. Place your compass level in the palm of your hand and turn yourself until the needle in the compass aligns with the North marking on the face dial.
4. The big arrow on the base plate is now aligned with your bearing.
Declination Value and Bearings – A Trick to Make it Stick
If you walked 10 miles through the woods in Washington State, without factoring declination value in your bearings, you would finish over 3 miles off target. That's too much!
To avoid confusion in finding bearings, keep the central principle in mind:
Make map bearing = magnetic bearing.
You will achieve this in the field by adjusting your bearings to compensate for magnetic declination. There are 2 ways to account for magnetic declination in finding your bearings: an adjustable compass and marking your non-adjustable compass.
The easiest way is with an adjustable compass. Simply turn the declination adjustment screw on the compass to the correct value and all readings are automatically converted to true north. If it's available, it's always best to have an adjustable compass on your walk.
The next best way to account for declination is to mark the declination value on your non-adjustable compass at the beginning of your journey.
That's simple to do. For example, when the map indicates a declination value of 20° west, you will mark your compass dial at 340°. If the map indicates a declination value of 10° east, you will mark your compass at 10°. Make the mark with a permanent marker and erase later with rubbing alcohol.
Now when you take your bearings in the field, place your compass level in the palm of your hand and turn yourself until the needle in the compass aligns with the declination marking on the face dial. Then the big arrow on the base plate will point toward your bearing and automatically compensate for magnetic declination.