The needle in the compass dial is attracted by the magnetism of the Earth and that is why the needle always points North. However, there are really two North Poles on the Earth. One is the "True North Pole" which is located geographically, while the other is the "Magnetic North Pole" which is where the magnetic lines of force come together.

Maps and directions usually are based on True North. True North is static and does not move. A compass's needle points to Magnetic North, which is located in the upper Hudson Bay region but moves slightly from year to year.

Magnetic declination is the angle between True North and Magnetic North. The amount of declination at any given point depends on the location of that point on the North American continent. When True North and Magnetic North are in the same direction, the declination is zero.

For the Invitational topographic map contest, correcting the bearing of the line on the topographic map for declination is easy. Look at the bottom of a topographic map and you will see a declination angle as pictured in Photo 1. The angle will look different depending on the location depicted on the map.

Photo 1

The line with the star above it, is the True North line. This line is parallel to the lines that form the grid work on the topographic map. The MN line with the half-arrow point is the Magnetic North line. The GN line is called the Grid North line. We are not concerned at all with GN line, so disregard it.

If the Magnetic North line points to the left (west) of the True North line, you add the degrees in the declination angle to the bearing. If Magnetic North line points to the right (east) of the True North line, you subtract the degrees in the declination angle from the bearing.

In North America, a line with zero declination runs roughly from west of Hudson Bay down along eastern Lake Michigan to the Gulf of Mexico in Alabama (see declination map above). The declination lines pictured in the map above move slightly to the west on an annual basis. So, it is important to look at a current isogonic (declination) map to know the correct declination in your area.

At any point on the west side of the zero declination line, the compass needle will point east of True North. This is called "easterly declination". At any point east of the zero declination line, the compass needle will point west of True North. This is called "westerly declination". In North America, magnetic declination varies from 30° east in Alaska to 30° west in Labrador.

The Magnetic North line (and the resulting declination angle) in Photo 1 points to the left (west) of the True North line. Therefore, any location on this specific topographic map would have westerly declination. To correct the bearing of the Point A-B line on the topographic map for declination, the declination shown in Photo 1 is added to the bearing.

There is a simple saying that makes declination determination easy to remember…east (declination) is least (subtract) and west (declination) is best (add).