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
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