Military grid reference system

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The Military Grid Reference System (MGRS)^[1] 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 identical 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:

4Q (the grid zone),
FJ (the 100,000-meter square), and
12345678 (numerical location; easting is 1234 and northing is 5678, in this case specifying a location within a 10m square).

It is important to note that an MGRS grid reference does not describe a point on the earth's surface, but rather a square area of 10km x 10km, 1km x 1km, 100m x 100m, 10m x 10m or 1m x 1m, depending on the precision of the coordinates provided. The total number of digits must be 2, 4, 6, 8 or 10, respectively, depending on the desired resolution. All points within that square will have the same coordinates.

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.

1 Grid zone designation
2 100,000-meter square identification
3 Numerical location
4 Polar regions
5 References
6 External links
7 See also

[edit] 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° 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.

Figure 1. The origin of the MGRS grid, in the Pacific. You can see Honolulu in 4QFJ.

[edit] 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,^[2] also known as MGRS-New,^[3] 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,^[2] also known as MGRS-Old,^[3] 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 can not 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.

Figure 2. The MGRS grid around Hawaii. You can see Honolulu in 4QFJ15.

[edit] Numerical location

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:

If every position being located is within the same grid zone, only the 100,000-meter square and numerical location are specified.
If every position being located is within the same grid zone and 100,000-meter square, only the numerical location is specified.
However, even if every position being located is within a small area, but the area overlaps multiple 100,000-meter squares or grid zones, the entire grid reference is required.

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.")

[edit] Polar regions

Figure 3. The MGRS grid around the South Pole.

Figure 4. The MGRS grid around the North Pole.

In the polar regions, a different convention is used.^[4] 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 km 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.^[4]