Another feature found in the legend of a GTM is the north arrow and magnetic declination. The north arrow is designated by the ”N” pointing to True North (the axis around which the earth rotates). Magnetic declination is the difference between True North and Magnetic North, designated by “MN” on the map (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). GTM are based on the geographic north pole because it does not change over time, so North is always at the top of the map.
If you walk a straight line following the direction your compass needle indicates as North, you would find that you don’t go 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.
Magnetic declination has been measured throughout the U.S. and can be corrected for on your compass (see below).
The line of zero declination runs from magnetic north through Lake Superior and across the western panhandle of Florida. Along this line, true north is the same as magnetic north. If you are working west of the line of zero declination, your compass will give a reading that is east of true north. If you are working east of the line of zero declination, your compass reading will be west of true north. The exact amount that you need to adjust the declination on your compass to reconcile magnetic north to true north is given in the map legend to the left of the map scale.
The map below shows lines of equal magnetic declination throughout the U.S. and Canada.
Calculators for determining current declination can found at the National Geophysical Data Center.