Military Grid Reference System

The Military Grid Reference System (MGRS) utilizes the same 60 zones defined in UTM (Universal Transverse Mercator). However, it adds to that coordinate system with the addition of latitude bands and further subdivisions. Each latitude quadrilateral can be further subdivided into 100,000m by 100,000m cells, 10,000m by 10,000m cells, 1,000m by 1,000m cells, 100m by 100m cells, 10m by 10m cells, and finally 1m by 1m cells.

History of MGRS
The MGRS is a coordinate system used by the North Atlantic Treaty Organization and designed by the U.S. Army. The goal of the U.S. Army was to reduce the confusion of using long numerical coordinates as well as numerical grid zone specifications and instead replace them with letters for numerical values. The MGRS system is most commonly used by the military because of its simplicity, which allows it to be taught to young soldiers and used in the field without difficulty.

Latitude bands
Each UTM zone is segmented into 20 latitude bands. Each latitude band extends 8 degrees and is lettered starting from "C" at 80° S, increasing through the English alphabet until "X", omitting the letters "I" and "O" (because of their similarity to the numerals one and zero). The last latitude band, "X", is extended an extra 4 degrees to 84° N latitude, thus covering 12 degrees of latitude. Latitude bands "A" and "B" do exist, as do bands "Y" and Z". They cover the western and eastern sides of the Antarctic and Arctic regions respectively.  A convenient mnemonic to remember is that the letter "N" is the first letter in the northern hemisphere, so any letter coming before "N" in the alphabet is in the southern hemisphere, and any letter "N" or after is in the northern hemisphere.

Notation
The combination of a zone and a latitude band defines a grid zone. The zone number is always written first, followed by the latitude band letter. For example, a position in Toronto, Canada would find itself in zone 17 and latitude band "T", thus the full grid zone reference is "17T". The 100,000m square subdivsions are delineated further by a two letter code, such as 'CX'. The 10,000m, 1,000m, 100m, 10m, and 1m square subdivisions are delineated by a two digit code each, such as '37'. Thus, this MGRS reference to it's smallest scale would look like 17TCX37034854.

A note of caution: A method also is used that simply adds N or S following the zone number to indicate North or South hemisphere (the easting and northing coordinates along with the zone number supplying everything necessary to geolocate a position except which hemisphere). However, this method has caused some confusion since, for instance, "50S" can mean southern hemisphere but also grid zone "50S" in the northern hemisphere.

Exceptions
These grid zones are uniform over the globe, except in two areas. On the southwest coast of Norway, grid zone 32V (9° of longitude in width) is extended further west, and grid zone 31V (3° of longitude in width) is correspondingly shrunk to cover only open water. Also, in the region around Svalbard, the four grid zones 31X (9° of longitude in width), 33X (12° of longitude in width), 35X (12° of longitude in width), and 37X (9° of longitude in width) are extended to cover what would otherwise have been covered by the seven grid zones 31X to 37X. The three grid zones 32X, 34X and 36X are not used.

Picture gallery: Grid zones in various parts of the world