IV-w6 mag north

. . . Magnetic Declination . . .

Another feature found in the legend of topographic maps is the 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)

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 walk a straight line following the direction your compass needle indicates as north, you would find that you didn’t go from south to north on the map.

How far your path varied from true north depends 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 to the right, 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.

You can get an daily reading of the magnetic declination for your area (zip code or coordinate) by clicking here:

http://www.ngdc.noaa.gov/seg/geomag/jsp/struts/calcDeclination

On the globe pictured below is a graphic example of the declination (difference) between North and Magnetic North poles and the angle formed by this difference.