Magnetic declination is an essential principle to understand when navigating through nature with a map and compass.
However, it is somewhat difficult to remember, at least in the way it has traditionally been taught, using an addition/subtraction method. Just when you think you’ve got it, the concept floats away, like mist in the morning light.
Well, there is a simple and practical approach to adjusting the magnetic declination when orienting yourself. The whole explanation starts with a definition of ‘north’.
there are 2 norths
Many 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 revolves around an imaginary axis that passes 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 with time, at about 5 miles per hour. Right now, it’s slowly creeping somewhere NW of Hudson Bay in Canada, about 450 miles from true north.
Magnetic Declination
The angular difference between true north and magnetic north is known as “declination” or “variance” in the aviation world. The declination is different for different parts of the world. In the state of Washington, 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 is 20° west.
Declination values can be found in the margins or legends on topographic maps. Because the point of magnetic north is constantly moving, it is important to have a recent topographic map to help you find your way around the forest.
A similar point to remember is that compasses are calibrated for different parts of the world. So when shopping for a compass, make sure it’s designed for use in the area of your big hiking vacation.
How to find your orientation
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. Rotate the compass dial face so that compass north is parallel to map north.
3. Place the compass level in the palm of your hand and rotate until the compass needle lines up with the north mark on the front dial.
4. The large arrow on the base plate is now aligned with your heading.
Declination value and bearings: a trick to keep it
If you were to hike 10 miles through the woods in Washington state, disregarding the declination value in your heading, you would end up more than 3 miles from your target. That is too much!
To avoid confusion in the search for directions, keep in mind the central principle:
Make map heading = magnetic heading.
You’ll accomplish this in the field by adjusting your bearings to compensate for magnetic declination. There are 2 ways to take magnetic declination into account for orienteering: an adjustable compass and dialing in your non-adjustable compass.
The easiest way is with an adjustable compass. Simply turn the compass declination adjustment screw to the correct value and all readings will automatically convert to true north. If available, it is always best to have an adjustable compass on your hike.
The next best way to account for declination is to mark the declination value on your non-adjustable compass at the beginning of your trip.
That is simple to do. For example, when the map indicates a declination value of 20° west, it will mark your compass dial at 340°. If the map indicates a declination value of 10° east, it will mark your compass at 10°. Make the mark with a permanent marker, then erase with rubbing alcohol.
Now when orienting yourself in the field, place the compass level in the palm of your hand and rotate until the compass needle lines up with the declination mark on the front dial. Then the large arrow on the base plate will point towards your orientation and automatically compensate for magnetic declination.