What is North?
No, this is not a silly question, there are two types of north.
True North: (also known as Geographic North or Map North - marked as H on a topographic map) is the geographic north pole where all longitude lines meet. All maps are laid out with true north directly at the top. Unfortunately for the wilderness traveler, true north is not at the same point on the earth as the magnetic north Pole which is where your compass points.
Magnetic North: Think of the earth as a giant magnet (it is actually). The shape of the earth's magnetic field is roughly the same shape as the field of a bar magnet. However, the earth's magnetic field is inclined at about 11° from the axis of rotation of the earth, so this means that the earth's magnetic pole doesn't correspond to the Geographic North Pole and because the earth's core is molten, the magnetic field is always shifting slightly. The red end of your compass needle is magnetized and wherever you are, the earth's magnetic field causes the needle to rotate until it lies in the same direction as the earth's magnetic field. This is magnetic north (marked as MN on a topographic map). If you locate yourself at any point in the U.S., your compass will orient itself parallel to the lines of magnetic force in that area.
You can see that location makes a great deal of difference in where the compass points. The angular difference between true north and magnetic north is known as the declination and is marked in degrees on your map. Depending on where you are, the angle between true north and magnetic north is different. In the U.S., the angle of declination varies from about 20° west in Maine to about 21° east in Washington. The magnetic field lines of the earth are constantly changing, moving slowly westward (½ to 1° every five years). This is why it is important to have a recent map. An old map will show a declination that is no longer accurate, and all your calculations using that declination angle will be incorrect. As you will see, understanding this distinction becomes important when navigating with a map and a compass.
Which North to Use
So we have two types of north to contend with. When you look at your map, it is drawn in relation to true north; when you look at your compass, it points to magnetic north. To make the map and compass work together you must decide on one North as your point of reference and base all your calculations on that. As you can see the following chart, failure to take declination into account can put you way off target.
|Declination or Degrees Off Course
||Error Off Target after Walking 10 Miles
||920 feet (280 meters)
||4,600 feet (1,402 meters)
||9,170 feet (2,795 meters)
What's your Map Declination?
The first thing you need to know is where you are in relation to magnetic north. You can find this information by looking on your map legend. If you look at the map of North America in below you will see the line roughly marking 0° declination. If you are on the line where the declination is 0°, then you don't have to worry about any of this, since magnetic north and map north are equivalent. (Wouldn't it be nice if all your trips were on the 0° of declination line?) If you are to the right of that line, your compass will point toward the line (to the left) and hence the declination is to the west. If you are to the left of the line, your compass will point toward the line (to the right) and hence the declination is to the east.