Magnetic north refers to the "north pole" of the Earth's innate magnetic field. As Rick Curtis explains in "The Backpacker's Field Manual," this point migrates modestly because of the molten character of the planet's core.
By contrast, true north refers to the Earth's geographic North Pole -- that unchanging, invisible landmark where lines of longitude converge at the "top of the world." This is the "north" indicated on a map.
A compass, responding to the Earth's magnetic field, points to magnetic north, while true north is the more useful reference for a map-user. You can convert between the two references using declination, the measure of their angle of difference. An up-to-date topographic map lists the current declination. In the lower 48 states, the Mississippi River roughly marks the zero-degree declination line: If you are east of this, magnetic north exceeds true north, and you would add or subtract the declination value accordingly depending on whether you wanted a map or magnetic bearing--and vice versa for points west.
Article Written By Ethan Schowalter-Hay
Ethan Schowalter-Hay is a writer and naturalist living in Oregon. He has written for the "Observer," the Bureau of Land Management and various online publishers. He holds a Bachelor of Science in wildlife ecology and a graduate certificate in geographic information systems from the University of Wisconsin.