North is not a single, absolute direction but a concept defined in three distinct ways for navigation, cartography, and surveying. These three types—True North, Magnetic North, and Grid North—each use a different reference system because the Earth is subject to dynamic physical forces. Understanding the differences between these reference points is fundamental to using a map and compass accurately.
The Fixed Reference Point: True North
True North, also called Geographic North, is the most stable directional reference point. It is defined as the fixed location where the Earth’s axis of rotation intersects the surface in the Northern Hemisphere. This point is considered permanent and does not shift over human timescales, making it the ultimate anchor for global positioning.
This geographic pole provides the basis for the global coordinate system of latitude and longitude. Lines of longitude, known as meridians, converge precisely at this fixed point. All directions measured relative to True North are known as true bearings, and this is the reference used by modern Global Positioning System (GPS) devices.
The Moving Target: Magnetic North
Magnetic North is the direction indicated by a standard compass needle, which aligns itself with the Earth’s naturally occurring magnetic field lines. This field is generated by the churning movement of molten iron and nickel in the planet’s liquid outer core, a process known as the geodynamo. Since the core fluid dynamics are constantly changing, the Magnetic North Pole is a continuously moving target that drifts across the Arctic.
For decades, the pole has been accelerating its movement away from Northern Canada and toward Siberia. This constant migration means that maps of the magnetic field, such as the World Magnetic Model, must be updated every five years to ensure navigation systems remain accurate. The physical location of the Magnetic North Pole is distinct from the fixed Geographic North Pole, which causes a predictable difference between the direction a compass points and the direction of True North.
The Mapping Standard: Grid North
Grid North is an artificial directional reference created exclusively for flat maps, specifically those that use a rectangular coordinate system like the Universal Transverse Mercator (UTM). When cartographers translate the curved surface of the Earth onto a two-dimensional plane, they must use a map projection that introduces some distortion. To simplify measurements and calculations on these flat maps, a grid of parallel lines is overlaid, and Grid North is defined as the direction of these vertical grid lines.
Unlike the meridians of longitude, which converge at True North, the grid lines representing Grid North run parallel to a central meridian assigned for that particular map zone. This system allows for easy coordinate plotting using Northing and Easting values, where all grid squares are uniform. Grid North is typically used in military operations, surveying, and large-scale land navigation because it provides a consistent, non-converging reference within a specific map area.
Navigating the Differences: Declination and Convergence
Accurate navigation requires accounting for the angular differences between these three interpretations of North. The most commonly encountered difference is Magnetic Declination, which is the angle between True North and Magnetic North. This value is not static; it changes depending on the user’s location on Earth and also changes slowly over time as the Magnetic Pole wanders.
Navigators must apply a correction factor, either adding or subtracting the declination angle to their compass reading, to convert a magnetic bearing into a true bearing. Many topographic maps include a declination diagram showing this angular relationship and the annual rate of change for the map area. Ignoring declination, which can be as high as 15 to 20 degrees in certain regions, would cause a course to be significantly off target over a long distance.
The other difference is Grid Convergence, which is the angle between True North and Grid North. Since True North meridians converge toward the pole but Grid North lines are perfectly parallel, this angle is zero only along the central meridian of a map zone. The further a navigator moves east or west away from this central line, the larger the convergence angle becomes.
Professional navigators, such as aviators and surveyors, must convert between the three systems using both declination and convergence. The combination of magnetic declination and grid convergence is sometimes calculated as a single value, known as grivation, to directly convert between magnetic headings and grid headings.