Declination

Adjusting map & compass bearings for Magnetic Declination

Declination is the angular difference between the geometric North Pole and the magnetic north pole measured from your location. The magnetic north pole is located in north central Canada, not at the geometric or actual North Pole. The magnetic north pole also moves slowly over a period of years. Always use the latest topographical map to get the declination which is shown on the map legend. Declination can be corrected in several ways

Sample Declinations

White Mountains in New Hampshire: 16 degrees West

Maryland, Virginia, West Virginia: approximately 7 - 9 degrees West, will vary slightly

Permanent Adjustment

Buy a compass that allows you to set the declination into the compass itself. This corrects all readings whether map bearings (bearing from a map) or field bearings (bearing to something that you see). It allows the compass orientation arrow to be turned relative to the meridian lines on the compass. This only has to be changed if you travel to an area where the declination is different than your present location - typically several hundred miles.

For 7 degrees West declination, turn the screw until the dial points to 7 degrees on the West declination side.

Temporary Map Bearing Adjustment

Use a conventional compass, which does not allow setting the declination, and simply do the arithmetic in your head.

take bearing from the map

for easterly declination, decrease the bearing by the declination (bearing - declination)

for westerly declination, increase the bearing by the declination (bearing + declination)

Temporary Field Bearing Adjustment

Use a conventional compass, which does not allow setting the declination, and simply do the arithmetic in your head. These corrections are just the opposite as those for map bearings.

take bearing from the field

for easterly declination, increase the bearing by the declination (bearing + declination)

for westerly declination, decrease the bearing by the declination (bearing - declination)

Use Magnetic Bearings rather than True Bearings

This is what most sailors use. All marine navigation charts show a dual compass rose, one based on geometric north and a second rotated one based on magnetic north. You can use either one. You differentiate them with a suffix of 'M' for magnetic or 'T' for true on your bearings. Backpackers can do the same thing all you need is the corrected north for magnetic which is always shown on topographic maps. Some backpackers add parallel meridian lines to their topos so that they can work exclusively with magnetic bearings. They generally draw them 1", 3", or 6" apart on the map. These new meridian lines will be rotated with respect to the geometric meridian lines by the declination amount. This is a lot of extra work and most land navigators do not do this.

Just Ignore the Declination

If you are near the line of 0 declination, don't need the highest accuracy, or are working exclusively with the compass only and field bearings, this works fine.