Military Grid Reference System: Difference between revisions

Revision as of 16:53, 15 June 2018

Revision as of 22:43, 27 June 2018

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* FJ (the 100,000-meter square identifier)

* 12345678 (numerical ___location; [[Easting and northing|easting]] is 1234 and [[Easting and northing|northing]] is 5678, in this case specifying a ___location with 10 m resolution)

This example corresponds to [[Daniel K. Inouye International Airport|Daniel K. Inouye International Airport]] in [[Honolulu]], [[Oahu|O‘ahu]], [[Hawaii|Hawai‘i]].

16SGC3855124838 is [[Hartsfield–Jackson Atlanta International Airport|William B. Hartsfield–Maynard Jackson Atlanta International Airport]].

An MGRS grid reference is a point reference system. When the term 'grid square' is used, it can refer to a square with a side length of {{convert|10|km|mi|0|abbr=on}}, 1 km, {{convert|100|m|ft|0|abbr=on}}, 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 is a better descriptor of these 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 [[rounding|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 an 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.

* 4Q .....................GZD only, precision level 6° × 8° (in most cases)

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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,<ref name="DMA8358_1_Chapter3"/> 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.

[[Image:MGRSgridOriginSchemeAA.png|thumb|Figure 1. The origin of the MGRS grid, in the Pacific. Honolulu is in 4QFJ.]]

==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.

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* In the alternative ''AL scheme'',<ref name="MGRS_Final_Guidance"/> also known as ''MGRS-Old'',<ref name="Military_Map_Reading_201"/> which is used for some older [[datum (geodesy)|geodetic datum]]s, 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 (geodesy)|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&nbsp;km resolution, we write 4QFJ.

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==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&nbsp;km resolution is enough, 2 + 2 digits suffice; if 10&nbsp;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.

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In the polar regions, a different convention is used.<ref name="Appendix_B">[http://earth-info.nga.mil/GandG/publications/tm8358.1/tr83581f.html DMA Technical Manual 8358.1, Appendix B.] Datums, Ellipsoids, Grids, and Grid Reference Systems.</ref> 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&nbsp;m squares is slightly different in the polar regions. The column letters use a more restricted alphabet, going from A to Z but omitting D, E, I, M, N, O, 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 row letters go from A to Z, omitting I and O. 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.<ref name="Appendix_B"/>