True North & Magnetic Declination
A Trick to Make it Stick
by Will Robertson
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.
Magnetic Declination
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.
About the Author
Outdoors Adventurer, Writer and Webmaster Will Robertson lives
with his family in Little Rock, Arkansas.
Explore the best hiking vacations and gear on the planet at:
http://www.hiking-vacations-and-gear.com
Note: Link not working - if you are the author please
contact me.
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