Grid North is a navigational concept that provides a consistent directional reference for coordinate systems on maps. It is defined as the direction pointing along the vertical lines of a rectangular grid overlaid on a map. This direction is fixed and does not correspond to a physical point on Earth, unlike the other definitions of north. The primary purpose of using Grid North is to create a standardized, unvarying reference for accurately plotting and referencing coordinates, such as those used in the Universal Transverse Mercator (UTM) system.
Establishing Grid North on a Map
The establishment of Grid North is a technical consequence of transforming the Earth’s curved surface into a flat map. Since lines of longitude converge at the poles on a globe, a simple north-south grid would be impossible to maintain across a large area. Mapmakers solve this problem by using a mathematical technique called a map projection, such as the widely used Universal Transverse Mercator (UTM) system.
The UTM system divides the planet into sixty longitudinal zones, each spanning six degrees of longitude. For each of these zones, a central meridian is designated as the primary north-south axis. Grid North is then established as the direction parallel to this central meridian across the entire zone.
This creates a grid of parallel lines, allowing coordinates to be expressed as simple metric distances: eastings (east-west distance) and northings (north-south distance). The vertical grid lines that define Grid North are uniform and parallel throughout the zone, making it easy to plot a location or measure a bearing on the map. The coordinates are always positive numbers, achieved by assigning the central meridian an arbitrary value, often 500,000 meters, known as a false easting.
Contrasting Grid North with True North and Magnetic North
Navigation and mapping require three distinct definitions of north: True North, Magnetic North, and Grid North. True North, also known as geographic north, is the fixed, physical direction toward the North Pole, which is the point where the Earth’s axis of rotation meets its surface. All lines of longitude converge at this single, unchanging point.
Magnetic North is the direction a compass needle points due to the Earth’s geomagnetic field. This pole is a physical location in the Arctic, but it is constantly shifting, moving several kilometers each year in response to changes in the Earth’s core. The angular difference between True North and Magnetic North at any given location is known as magnetic declination or magnetic variation.
The difference between Grid North and True North is caused by the necessary distortion introduced when projecting a curved surface onto a flat map. Only along the central meridian of a projection zone do Grid North and True North perfectly align. Away from the central meridian, the grid lines remain parallel to each other, but True North lines—the lines of longitude—are converging toward the North Pole. This means that the angle between Grid North and True North changes depending on the map user’s east-west position within the grid zone. Navigators typically use Grid North for plotting on a map, Magnetic North for directing a compass, and True North as the ultimate reference point.
Practical Application: Measuring Grid Convergence
Since Grid North and True North point in the same direction only along the central meridian, navigators must account for the difference when using a map for precise navigation. This angular difference is known as Grid Convergence. Grid Convergence is the angle formed between the grid’s north-south line and the line pointing directly to the geographic North Pole.
This convergence angle must be applied as an adjustment when converting a bearing taken from a map’s grid to a bearing that points toward True North, or vice versa. The value is not constant across a map, though the change is often minor enough to be ignored for casual use. As a navigator moves east or west away from the central meridian of the grid zone, the convergence angle increases.
Modern topographic maps typically provide the Grid Convergence value, along with the magnetic declination, printed directly on the map’s margin or legend. This information allows the user to calculate the precise bearing needed to move between a map-based grid bearing and an actual True North bearing in the field.