I’ve held off long enough. It’s time to reveal the terrible truth.
Grid North on the map is not the same as Magnetic North and
to confuse things further, neither are the same as True North.
In some parts of the world this could lead to huge errors in
navigation unless we allowed for this variation. So what is the
difference between the ‘three Norths’?
True North is the point around which the Earth revolves. The
problem is the Earth wobbles slightly on its axis so True North
varies, very slowly, over time.
Magnetic North, simply speaking, is what a magnetic needle
points towards. Its position on the Earth’s surface is not at
what we call the North Pole but currently lies somewhere
in north east Canada. I say ‘currently lies’ because, yes you
guessed it, it also varies, and remarkably quickly too (1/6 of a
degree each year in the UK, approximately). So as you can see
the difference between True North and Magnetic North varies
not only over time but from place to place on the Earth’s
surface. In the UK the difference is just a few degrees but in
Maine on the east coast of the United States it is close to 20
degrees.
Grid North, or north on the map, doesn’t change but the
difference between it and Magnetic North obviously does. The
magnetic variation (also called Declination) for a particular
area is normally printed on the relevant map. Remember that
an old map will be out of date in this respect. When you take
a bearing you have to add or subtract the magnetic variation
from your reading (unless you purchase a compass that allows
you to set the magnetic variation in advance by turning a small
screw).