Many people don't realize that a compass needle does not usually point due
north, but at some angle east or west of north. This is because the
earth's geographic pole (the axis about which it rotates) is not in the
same place as its magnetic pole (the place where the magnetic lines of
force emerge from the earth). The direction to the earth's geographic pole
is called true north and the direction to the earth's magnetic pole is
called magnetic north.
by the Geological Survey of Canada in 1994 determined that the average
position of the north magnetic pole for that year was 78.3° N,
104.0° W (near Ellef Ringnes Island in the Canadian Arctic). They
also determined that the magnetic pole was moving approximately 15 km per
So, if you're using a compass somewhere on the surface of the earth, you
have to account for this difference between magnetic north (where your
compass is pointing) and true north (which you need to know). The angle
between true north and magnetic north is called the magnetic
declination and changes with your location and, at any one
location, with time.
How do we determine what the magnetic declination is?
There are some neat computer programs
which calculate the magnetic declination at a given latitude and longitude
on the earth's surface. What most geologists do, however, is simply
consult a recent topographic map for their field area available published
by the United States Geologic Survey (and available in many sporting goods
stores catering to hikers or hunters) or, if they're working outside of
the United States, the USGS also publishes inexpensive maps showing the magnetic
declination around the world.
On a typical topographic map, the magnetic declination is indicated as
The star indicates true north (toward the top of virtually all maps) and
the MN indicates the direction to the north magnetic pole from the center
of the map. In this case, the north magnetic pole is 15 deg; west of true
north. This means that your compass, unless you correct it, will point 15
deg; west of true north.
There are simple ways to correct for the magnetic declination on most
compasses. The Brunton compass, which will be discussed shortly, has an
index pin at the north end of the compass ring (the ring around the face
of the compass with the azimuths printed on it). When the compass is set
for a 0° magnetic declination, the index pin is aligned with zero.
There is a brass setscrew on the side of the compass which moves the
compass ring either clockwise or counter-clockwise. The only trick in
correcting for magnetic declination is to remember which way to turn the
compass ring for east and west declinations.
For a magnetic declination 15 deg; east of true north, you would turn the
compass ring such that the index pin was over 15 deg; (i.e. 15 deg; to the
east side of north).
For a 15 deg; west declination, you would turn the compass ring such that
the index pin was over 345 deg; (i.e. 15 deg; to the west side of north).
Let's suppose, for example, that you've set your compass for a 15°
east declination. You can check to see if the setting is correct by
orienting your compass such that the white end of the needle is at
0°. Rotate the compass 15° east and if the white end of
the needle is pointing in the same direction of the sighting arm for the
compass you're in business.