In short, failure to adjust your compass for magnetic declination could lead or keep you off course, depending on where you are in the world.
Magnetic declination is the angle between magnetic north (where the compass points) and true north (geographic north on your map). Orienteering, the process of navigating by map & compass, requires the two be aligned accurately. If they are not aligned, it can be easy think you are going north on the map when in fact you are off a bit. In mountainous areas, this could make the difference when trying to find water or navigate into the correct valley.
Decent compasses allow adjustment to the “north arrow” (red in image) to point to true north, allowing a clear understanding of direction as it relates to the map. Often a small, metal key is attached to the compass’s lanyard. This key turns a small screw to adjust the “north guide” arrow. Also, when buying a detailed trail map, look for a declination guide printed on the map. This compass-looking guide will provide the correct declination angle for the given area.
The declination angle is larger near the Earth’s poles, and because Earth’s magnetic poles move around a bit over the years, the passage of time also bears on this variance.
Important detail:
If magnetic north is east of true north, the number is positive.
If magnetic north is west of true north, the number is negative.
Orienteering remains the longest-running navigation method known to man. With an accurate map and a good compass, one can always find their way. As in all travel, the fun is in the journey as much as the destination.
Benchmark Maps – Bridger DeVille – February 2012.
