The AFE grid is modified from the Military Grid Reference
System (MGRS). The MGRS itself is an alphanumeric version of a
numerical UTM (Universal Transverse Mercator) or UPS (Universal
Polar Stereographic) grid coordinate. Here is a short explanation
of the UTM and MGRS.

- Projection: Transverse Mercator (Gauss-Krüger type) in zones 6° wide.
- Longitude of Origin: Central meridian (CM) of each projection zone (3°, 9°, 15°, 21°, 27°, 33°, 39°, 45°, 51°, 57°, 63°, 69°, 75°, 81°, 87°, 93°, 99°, 105°, 111°, 117°, 123°, 129°, 135°, 141°, 147°, 153°, 159°, 165°, 171°, 177°, E and W).
- Latitude of Origin: 0° (the Equator).
- Unit: Meter.
- False Northing: 0 meters at the Equator for the Northern Hemisphere; 10,000,000 meters at the Equator for the Southern Hemisphere.
- False Easting: 500,000 meters at the CM of each zone.
- Scale Factor at the Central Meridian: 0.9996.
- Latitude Limits of System: From 80°S to 84°N.
- Limits of Projection Zones: The zones are bounded by meridians, the longitudes of which are multiples of 6° east and west of the prime meridian.

Universal Transverse Mercator (UTM) coordinates define two
dimensional, horizontal, positions. The sixty UTM *zone numbers*
designate 6 degree wide longitudinal strips extending from 80
degrees South latitude to 84 degrees North latitude. UTM *zone
characters* are letters which designate 8 degree zones
extending north and south from the equator. Beginning at 80°
south and proceeding northward, twenty bands are lettered C
through X, omitting I and O. These bands are all 8° wide except
for bond X which is 12° wide (between 72-84 N).

There are special UTM zones between 0 degrees and 36 degrees longitude above 72 degrees latitude and a special zone 32 between 56 degrees and 64 degrees north latitude:

- UTM Zone 32 has been widened to 9° (at the expense of zone 31) between latitudes 56° and 64° (band V) to accommodate southwest Norway. Thus zone 32 it extends westwards to 3°E in the North Sea.
- Similarly, between 72° and 84° (band X), zones 33 and 35 have been widened to 12° to accommodate Svalbard. To compensate for these 12° wide zones, zones 31 and 37 are widened to 9° and zones 32, 34, and 36 are eliminated. Thus the W and E boundaries of zones are 31: 0 - 9 E, 33: 9 - 21 E, 35: 21 - 33 E and 37: 33 - 42 E.

The MGRS scheme (also available as an Excel table)

The Military Grid Reference System (MGRS) is an extension of the UTM system. UTM zone number and zone character are used to identify an area 6 degrees in east-west extent and 8 degrees in north-south extent. UTM zone number and designator are followed by 100 km square easting and northing identifiers. The system uses a set of alphabetic characters for the 100 km grid squares. Starting at the 180 degree meridian the characters A to Z (omitting I and O) are used for 18 degrees before starting over. From the equator north the characters A to V (omitting I and O) are used for 100 km squares, repeating every 2,000 km. Northing designators normally begin with 'A' at the equator for odd numbered UTM easting zones.

For even numbered easting zones the northing designators are offset by five characters, starting at the equator with 'F'. South of the equator, the characters continue the pattern set north of the equator. Complicating the system, ellipsoid junctions (spheroid junctions in the terminology of MGRS) require a shift of 10 characters in the northing 100 km grid square designators. Different geodetic datums using different reference ellipsoids use different starting row offset numbers to accomplish this.

If 10 numeric characters are used, a precision of 1 meter is
assumed. 2 characters imply a precision of 10 km. From 2 to 10
numeric characters the precision changes from 10 km, 1 km, 100 m
10 m, to 1 m.

- The 100,000-meter columns, including partial columns along zone, datum, and ellipsoid junctions, are lettered alphabetically, A through Z (with I and O omitted), north and south of the Equator, starting at the 180° meridian and proceeding easterly for 18°. The alphabetical sequence repeats at 18° intervals.
- To prevent ambiguity of identifications along ellipsoid junctions changes in the order of the row letters are necessary. The row alphabet (second letter) is shifted ten letters. This decreased the maximum distance in which the 100,000-meter square identification is repeated.
- The 100,000-meter row lettering is based on a 20-letter alphabetical sequence (A through V with I and O omitted). This alphabetical sequence is read from south to north, and repeated at 2,000,000-meter intervals from the Equator.
- The row letters in each odd numbered 6° grid zone are read in an A through V sequence from south to north.
- In each even-numbered 6° grid zone, the some lettering sequence is advanced five letters to F, continued sequentially through V and followed by A through V.
- The advancement or staggering of row letters for the even-numbered zones lengthens the distance between 100,000-meter squares of the same identification.
- Deviations from the preceding rules were mode in the past. These deviations were an attempt to provide unique grid references within a complicated and disparate world-wide mapping system.
- Determination of 100,000-meter grid square identification is further complicated by the use of different ellipsoids.

The MGRS coordinate for a position consists of a group of letters and numbers which include the following elements:

- The Grid Zone Designation.
- The 100,000-meter square letter identification.
- The grid coordinates (also referred to as rectangular coordinates); the numerical portion of the reference expressed to a desired refinement.
- A reference is written as an entity without spaces, parentheses, dashes, or decimal points.

Examples:

- 18S (Locating a point within the Grid Zone Designation)
- 18SUU (Locating a point within a 100,000-meter square)
- 18SUU80 (Locating a point within a 10,000-meter square)
- 18SUU8401(Locating a point within a 1,000-meter square)
- 18SUU836014(Locating a point within a 100-meter square)

To satisfy special needs, a reference can be given to a 10-meter square and a 1-meter square as:

- 18SUU83630143 (Locating a point within a 10-meter square)
- 18SUU8362601432(Locating a point within a 1-meter square)

- Dana, Peter, H.: Coordinate Systems Overview. - http://www.utexas.edu/depts/grg/gcraft/notes/coordsys/coordsys.html
- Defense Mapping Agency Technical Manual 8358.1 Datums, Ellipsoids, Grids and Grid Reference Systems. - http://164.214.2.59/geospatial/products/GandG/tm83581/toc.htm