The Military Grid Reference System (MGRS) is the geocoordinate standard used by NATO militaries for geo-referencing, position reporting, and situational awareness during land operations. An MGRS coordinate does not represent a single point, but rather defines a square grid area on the Earth's surface. The location of a specific point is therefore referenced by the MGRS coordinate of the area that contains it. The MGRS is derived from the Universal Transverse Mercator (UTM) and Universal Polar Stereographic (UPS) grid systems and is used as a geocode for the entire Earth. An example of an MGRS coordinate, or grid reference, is 4Q FJ 1234 6789, which consists of three parts:
4Q (grid zone designator, GZD) FJ (the 100,000-meter square identifier) 1234 6789 (numerical location; easting is 1234 and northing is 6789, in this case specifying a location with 10 m resolution) For machine-readability and database storage, all spaces may be removed. An MGRS grid reference represents a square area on the Earth's surface, rather than a single point. A grid square references a square or polygon on the Earth with a side length of 10 km, 1 km, 100 m, 10 m or 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" may be a better descriptor of such areas.)
The number of digits in the numerical location must be even: 0, 2, 4, 6, 8 or 10, depending on the desired precision. When changing precision levels, it is important to truncate rather than round the easting and northing values to ensure the more precise square will remain within the boundaries of the less precise square. Related to this is the primacy of the southwest corner of the square being the labeling point for the entire square. (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.) Google Maps recognizes MGRS grid references which have a one-meter square precision (10-digit numerical location) with spaces permitted only between the 100,000-meter square, the easting, and the northing: e.g., 4QFJ 12345 67890. The mapping application returns a dropped pin representing the centroid of the area referenced.
Grid zone designation
The first part of an MGRS coordinate is the grid-zone designation. The 6° wide UTM zones, numbered 1–60, are intersected by latitude bands that are normally 8° high, lettered C–X (omitting I and O). The northmost latitude band, X, is 12° high. The intersection of a UTM zone and a latitude band is (normally) a 6° × 8° polygon called a grid zone, whose designation in MGRS is formed by the zone number (one or two digits – the number for zones 1 to 9 is just a single digit, according to the example in DMA TM 8358.1, Section 3-2, Figure 7), followed by the latitude band letter (uppercase). This same notation is used in both UTM and MGRS, i.e. the UTM grid reference system; the article on Universal Transverse Mercator shows many maps of these grid zones, including the irregularities for Svalbard and southwest Norway. As Figure 1 illustrates, Honolulu is in grid zone 4Q.
100,000-meter square identification The second part of an MGRS coordinate is the 100,000-meter square identification. Each UTM zone is divided into 100,000 meter squares, so that their corners have UTM-coordinates that are multiples of 100,000 meters. The identification consists of a column letter (A–Z, omitting I and O) followed by a row letter (A–V, omitting I and O). Near the equator, the columns of UTM zone 1 have the letters A–H, the columns of UTM zone 2 have the letters J–R (omitting O), and the columns of UTM zone 3 have the letters S–Z. At zone 4, the column letters start over from A, and so on around the world. For the row letters, there are actually two alternative lettering schemes within MGRS:
In the AA scheme, also known as MGRS-New, which is used for WGS84 and some other modern geodetic datums, the letter for the first row – just north of the equator – is A in odd-numbered zones, and F in even-numbered zones, as shown in figure 1. Note that the westmost square in this row, in zone 1, has identification AA. In the alternative AL scheme, also known as MGRS-Old, which is used for some older geodetic datums, the row letters are shifted 10 steps in the alphabet. This means that the letter for the first row is L in odd-numbered zones and R in even-numbered zones. The westmost square in the first row, in zone 1, has identification AL. If an MGRS coordinate is complete (with both a grid zone designation and a 100,000 meter square identification), and is valid in one lettering scheme, then it is usually invalid in the other scheme, which will have no such 100,000 meter square in the grid zone. (Latitude band X is the exception to this rule.) Therefore, a position reported in a modern datum usually cannot be misunderstood as using an old datum, and vice versa – provided the datums use different MGRS lettering schemes. In the map (figure 1), which uses the AA scheme, we see that Honolulu is in grid zone 4Q, and square FJ. To give the position of Honolulu with 100 km resolution, we write 4QFJ.
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