A compass does not always point to True North. A magnetic compass aligns itself with the planet’s magnetic field, causing the needle to point toward Magnetic North. This direction is rarely the same as the fixed geographical North Pole, which is the standard reference point for maps and global navigation systems. The difference between these two directions can be significant, sometimes measuring many degrees, depending on the traveler’s location. For accurate navigation, it is necessary to understand this distinction and account for the angular difference between Magnetic North and True North.
Geographic and Magnetic North Defined
True North, also known as Geographic North, is the fixed, northern point on the Earth’s surface where the planet’s axis of rotation meets the crust. All lines of longitude converge at this single, unchanging location. This pole serves as the ultimate reference for cartography and mapping grids, making it the direction toward which all official maps are oriented.
Magnetic North, in contrast, is the location in the Northern Hemisphere where the Earth’s magnetic field lines point vertically downward. This pole is not fixed; its position is constantly changing due to the flow of molten iron and nickel within the Earth’s outer core, a process known as the geodynamo. The magnetic pole has been steadily drifting across the Canadian Arctic toward Siberia, sometimes at an accelerated rate of up to 34 miles per year.
Because the magnetic pole is a moving target influenced by the planet’s internal dynamics, a compass needle responds to the local magnetic field, not pointing precisely at the Geographic North Pole. The distance between the two poles can vary greatly, and the difference between them is a variable that must be considered for precise direction finding. The Earth’s magnetic field is not uniform, meaning the compass aligns with the local magnetic field lines, not a straight line to the magnetic pole itself.
Understanding Magnetic Declination
The angular difference between the direction a compass points (Magnetic North) and the direction of the fixed Geographic North (True North) is called magnetic declination. Declination is measured in degrees, and its value changes based on two factors: the observer’s specific location on the globe and the current date. The Earth’s magnetic field strength and direction vary across the surface, causing the declination to be different in nearly every location.
Declination is described as either “East” (positive) or “West” (negative) to indicate which side of True North the magnetic needle is pointing. If the compass needle points to the east of True North, the declination is positive, and if it points to the west, it is negative. On maps, specialized lines called isogonic lines connect all points on the Earth’s surface that share the same magnetic declination value.
The movement of the Magnetic North Pole means that the declination value for any given location is not static. While in some areas the change may be small, the change is significant enough that older maps may have outdated declination information. A specific isogonic line, called an agonic line, marks the rare locations where the declination is zero, meaning Magnetic North and True North align perfectly.
Correcting Compass Bearings for True North
For any serious navigation, such as backcountry hiking or marine travel, accounting for magnetic declination is necessary to prevent significant navigational errors. Even a small error of just five degrees can result in a traveler being hundreds of feet off their intended course over a mile of travel. Navigators must determine the local declination value and apply a correction to their compass readings to align them with the True North used on maps.
Current declination data can be found on topographic maps, which often include a diagram showing the local declination and its annual rate of change. This information is also available through official online calculators provided by geomagnetic modeling organizations, which use the latest World Magnetic Model (WMM) to provide up-to-date values.
To correct a magnetic bearing to a true bearing used on a map, the declination value must be added or subtracted. The general rule is to add a West declination and subtract an East declination when converting from Magnetic to True North. Conversely, when converting a True North bearing from a map to a Magnetic bearing for the compass, the opposite operation is performed. Some modern compasses include an adjustable feature that allows the user to pre-set the local declination, which then automatically shifts the compass’s orientation to read True North directly.