Magnetic Declination

Magnetic declination is the difference between true north (the axis around which the earth rotates) and magnetic north (the direction the needle of a compass will point). It is usually printed on the map to the left of the scale bar at the bottom of a USGS 7.5' quadrangle. After finding the declination on the map, you need to transfer the information to your compass before you ever take it into the field. If you fail to do this, any readings you get from your compass will be in error and you may wind up far from where you want to be (in other words, LOST ! ! !).

Magnetic north is determined by the earth's magnetic field and is not the same as true (or geographic) north. The location of the magnetic north pole changes slowly over time, but it is currently northwest of Hudson's Bay in northern Canada (approximately 700 km [450 mi] from the true north pole). Maps are based on the geographic north pole because it does not change over time, so north is always at the top of a quadrangle map.


However, if you were to walk a straight line following the direction your compass needle indicated as north, you would find that you didn’t go from south to north on the map. Howfar your path varied from true north would depend on where you started from. The angle between a straight north-south line and the line you walked is the magnetic declination in the area you were walking. In the example figure, if you walked 1.25 miles toward magnetic north(i.e. you followed your compass without adjusting for magnetic declination) you would end up 1/3 of a mile away from where you would be if you walked 1.25 miles toward true north.

Fortunately, magnetic declination has been measured throughout the U.S. and can be corrected for on your compass (see below). This map shows lines of equal magnetic declination throughout the U.S. and Canada.
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