The Military Grid Reference System (MGRS) is the geocoordinate standard used by NATO militaries for locating points on the earth. The MGRS is derived from the UTM (Universal Transverse Mercator) grid system, but uses a different labeling convention. The MGRS is used for the areas of the earth between 84N to 80S, and 180W to 180E.

In the polar regions, MGRS is based on the Universal Polar Stereographic system. An example of an MGRS coordinate, or grid reference, would be 04QFJ12345678, which consists of three parts: An MGRS grid reference does not describe a point on the earth's surface, but rather a square area of 10 km × 10 km, 1 km × 1 km, 100 m × 100 m, 10 m × 10 m or 1 m × 1 m, depending on the precision of the coordinates provided. (In some cases, squares adjacent to a Grid Zone Junction (GZJ) are clipped, so polygon is a better descriptor of these areas.) The total number of characters must be 7, 9, 11, 13, and 15, respectively, depending on the desired precision.

All points within that square share the same MGRS coordinate. When changing precision levels, it is important to truncate rather than round the easting and northing values to ensure the more precise polygon will remain within the boundaries of the less precise polygon. Related to this is the primacy of the southwest corner of the polygon being the labeling point for the entire polygon.

In instances where the polygon is not a square and has been clipped by a grid zone junction, the polygon keeps the label of the southwest corner as if it had not been clipped. Such an MGRS coordinate, standing alone, may be converted to latitude and longitude. But you still do not know the position on the Earth, unless you also know the geodetic datum that is used.

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