The compass is used primarily to take bearings. A bearing is a horizontal angle measured clockwise from north (either magnetic north or true north) to some point (either a point on a map or a point in the real world). Bearings are used to accurately travel to a destination or to locate your position. If you are working from your map, it is called a map bearing and the angle you are measuring is the angle measured clockwise from true north on your map to this other point on the map. If you are taking a bearing off a real point on the landscape with a compass, you are using your compass to measure the angle clockwise from magnetic north to this point on the landscape. This is called a magnetic bearing. Remember that the bearing is measured clockwise. If you think of true north as 12 o'clock then a bearing to the right of that (1 o'clock) is greater than true north and a bearing to the left of True north (11 o'clock) is less than true north.
Map Bearings & Magnetic Bearings:
If, you think about your map as an artist's rendition of the world. It displays true north, but it doesn't include magnetic fields as the real world does, so you need to make accommodations when going from your map to the real world. The real world doesn't have a true north-it's merely a construct of the map-so you have to make accommodations when going from the real world to your map.. The basic principle is this: to correct for declination, you want the map bearing and the magnetic bearing to be equivalent. If you are lucky enough to be on the line where the declination is 0°, both are already equivalent, or if you orient your map with your compass then you have made the two equivalent. Otherwise, you will need to make your own bearing corrections by adding or subtracting the declination amount. That gives us 4 possible permutations to work with:
- West Declination - Going from a Map Bearing to a Magnetic Bearing
- West Declination - Going from a Magnetic Bearing to a Map Bearing
- East Declination - Going from a Map Bearing to a Magnetic Bearing
- East Declination - Going from a Magnetic Bearing to a Map Bearing
|If the declination is...
||Map Bearing to Magnetic Bearing
||Magnetic Bearing to Map Bearing
||Magnetic North < True North Map Bearing is < the Magnetic Bearing
||Map Bearing + Declination = Magnetic Bearing
||Magnetic Bearing - Declination = Map Bearing
||Magnetic North > True North Map Bearing is > the Magnetic Bearing
||Map Bearing - Declination = Magnetic Bearing
||Magnetic Bearing + Declination = Map Bearing