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Military Grid Reference System
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==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 (AβZ, omitting I and O) followed by a row letter (AβV, omitting I and O). Near the equator, the columns of UTM zone 1 have the letters AβH, the columns of UTM zone 2 have the letters JβR (omitting O), and the columns of UTM zone 3 have the letters SβZ. 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'',<ref name="MGRS_Final_Guidance"> [http://earth-info.nga.mil/GandG/coordsys/MGRS_Final_Guidance.doc NGA Guidance for the Military Grid Reference System (MGRS).]</ref> also known as ''MGRS-New'',<ref name="Military_Map_Reading_201">{{cite web|url=http://earth-info.nga.mil/GandG/coordsys/mmr201.pdf|title=Military Map Reading 201|archive-url=https://web.archive.org/web/20210224185834/earth-info.nga.mil/GandG/coordsys/mmr201.pdf|archive-date=February 24, 2021}}, published by [[National Geospatial-Intelligence Agency|NGA]]</ref> which is used for [[WGS84]] and some other modern [[datum (geodesy)|geodetic datum]]s, the letter for the first row β just north of the equator β 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'',<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 cannot be misunderstood as using an old datum, and vice versa β 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. [[Image:MGRSgridHawaiiSchemeAARealigned.png|thumb|Figure 2. The MGRS grid around Hawaii. Honolulu is in the 10 km square that is called 4QFJ15.]]
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