Adjusting
Map & Compass
Bearings for Magnetic Declination
Declination is the angular difference between the geometric North Pole
(True) and the magnetic north pole
(Magnetic) measured from your location. The magnetic north pole is located in north central Canada,
not at the geometric or actual North Pole. The magnetic north pole also moves slowly over a period of years.
Always use the latest topographical map to get the declination which is shown on the map
legend. Declination can be corrected in several ways.
Map bearings (True) are always
relative to True North
Compass bearings (Magnetic) are always relative to Magnetic North - unless
corrected
Sample Declinations
White Mountains in New Hampshire: 16 degrees West
Maryland, Virginia, West Virginia: approximately 7 - 9 degrees West,
will vary slightly
Permanent Adjustment
Buy a compass that allows you to set the declination into the
compass itself. This corrects all readings whether map bearings (bearing from a map) or
field bearings (bearing to something that you see). It allows the compass orientation
arrow to be turned relative to the meridian lines on the compass. This only has to be
changed if you travel to an area where the declination is different than your present
location - typically several hundred miles.
For 7 degrees West declination, turn the screw until the dial
points to 7 degrees on the West declination side.
Temporary Map Bearing Adjustment
Use a conventional compass, which does not allow setting the
declination, and simply do the arithmetic in your head.
Take a bearing from the map
For easterly declination, decrease the bearing by the
declination (bearing - declination)
For westerly declination, increase the bearing by the
declination (bearing + declination)
Temporary Field Bearing Adjustment
Use a conventional compass, which does not allow setting the
declination, and simply do the arithmetic in your head. These corrections are just the
opposite as those for map bearings.
Take a bearing from the field
For easterly declination, increase the bearing by the declination
(bearing + declination)
For westerly declination, decrease the bearing by the declination
(bearing - declination)
Use Magnetic Bearings rather
than True Bearings
This is what most sailors use. All marine navigation charts show a dual
compass rose, one based on geometric north and a second rotated one based on magnetic
north. You can use either one. You differentiate them with a suffix of 'M' for magnetic or
'T' for true on your bearings. Backpackers can do the same thing all you need is the
corrected north for magnetic which is always shown on topographic maps. Some backpackers
add parallel meridian lines to their topo's so that they can work exclusively with magnetic
bearings. They generally draw them 1", 3", or 6" apart on the map. These
new meridian lines will be rotated with respect to the geometric meridian lines by the
declination amount. This is a lot of extra work and most land navigators do not
do this. We also do not have a chart table to work on, a long
protractor, and being inside a cabin.
Just Ignore the Declination
If you are near the line of 0 declination (roughly just East
of the Mississippi River in the US), don't need the highest
accuracy (you usually do!), or are working exclusively with the compass only and field bearings, this works
fine.
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