Compasses work by detecting the magnetic north pole of the Earth. The magnetic field of the Earth extends approximately one-fourth of the way to the moon, about 59,700 miles. It is about this distance that the magnetic pull of the North Pole will no longer affect a regular magnetic compass. Even at these great heights, results of the compass will not be accurate. It is difficult enough to get an accurate reading on the surface of the Earth.
The difficulty in getting accurate readings from a compass is because the Earth has two north poles: a magnetic north pole and a true (geographic) north pole. The magnetism of the North Pole is caused by layers of molten iron deep under the surface of the planet in the outer core. From various positions in the world, a difference exists between magnetic north and true north. This difference is called declination.
In the U.S., true north and magnetic north are only the same in a narrow band running through western Wisconsin, Illinois, along the border of Arkansas and Tennessee, and through Mississippi. To make matters worse, the magnetized molten iron is constantly shifting. This means the magnetic north pole moves from year to year. At the current rate, the magnetic north pole is moving 250 miles NW every 10 years. For hundreds of years, magnetic north has been in Northern Canada, but in 100 years it will be in Northern Russia. Of course, once the movement passes true north, it will be shifting SE instead of NW.
Topographical maps usually give the declination for the map area. This helps travelers adjust their compass properly. However, even if the map lists the declination, if it is over 10 years old, it will still be inaccurate.
Posted 3691 day ago