The Military Grid Reference System (MGRS) is the geocoordinate standard used by NATO militaries for locating points on the earth. In most areas (between latitudes 80°S and 84°N), the MGRS grid is similar to the UTM (Universal Transverse Mercator) grid system, but uses a different labeling convention. In the polar regions, MGRS is based on the Universal Polar Stereographic system.
An example of an MGRS coordinate, or grid reference, would be 4QFJ12345678, which consists of three parts:
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° rectangle called a grid zone, whose designation in MGRS is formed by the zone number and the latitude band letter. The same notation is often used in UTM; the article on Universal Transverse Mercator shows many maps of these grid zones, including the irregularities for Svalbard and southwest Norway. In the map here (figure 1), you can see that Honolulu is in grid zone 4Q.
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 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.
The third part of an MGRS coordinate is the numerical location within a 100,000 meter square, given as n+n digits, where n is 1, 2, 3, 4, or 5. If 5 + 5 digits is used, the first 5 digits give the easting in meters, measured from the left edge of the square, and the last 5 digits give the northing in meters, measured from the bottom edge of the square. The resolution in this case is 1 meter, so the MGRS coordinate would represent a 1 meter square, where the easting and northing are measured to its southwest corner. If a resolution of 10 meters is enough, the final digit of the easting and northing can be dropped, so that only 4 + 4 digits are used, representing a 10 meter square. If a 100 meter resolution is enough, 3 + 3 digits suffice; if a 1 km resolution is enough, 2 + 2 digits suffice; if 10 km resolution is enough, 1 + 1 digits suffice. 10 meter resolution (4 + 4 digits) is sufficient for many purposes, and is the NATO standard for specifying coordinates.
If we zoom in on Hawaii (figure 2), we see that the position of Honolulu, with 10 km resolution, would be written 4QFJ15.
If the grid zone or 100,000-meter square are clear from context, they can be dropped, and only the numerical location is specified. For example:
One always reads map coordinates from West to East first (Easting), then from South to North (Northing). (Common mnemonics include "In the house, up the stairs," and "Left-to-right, bottom-to-top.")
In the polar regions, a different convention is used. South of 80°S, UPS South (Universal Polar Stereographic) is used instead of a UTM projection. The west half-circle forms a grid zone with designation A; the east half-circle forms one with designation B; see figure 3. North of 84°N, UPS North is used, and the west half-circle is Y, the east one is Z; see figure 4. Since the letters A, B, Y, and Z are not used for any latitude bands of UTM, their presence in an MGRS coordinate, with the omission of a zone number, indicates that the coordinates are in the UPS system.
The lettering scheme for 100,000 m squares is slightly different in the polar regions. The row letters go from A to Z, omitting I and O. The column letters use a more restricted alphabet, going from A to Z but omitting I, O, D, E, M, N, V, W; the columns are arranged so that the rightmost column in grid zone A and Y has column letter Z, and the next column in grid zone B or Z starts over with column letter A. The restricted column alphabet for UPS ensures that no UPS square will be adjacent to a UTM square with the same identification.
In the polar regions, there is only one version of the lettering scheme.
There are other geographic naming systems of this alphanumeric kind: