When it comes to doing anything “there’s a RIGHT way, a WRONG way, and an ARMY way.” Every soldier has heard this saying numerous times, I’m sure.
When it comes to map reading and land navigation, the Army way has been to modify slightly some UTM grid square designations, and to adopt conventions for expressing coordinates that make them easier to work with.
The Military Grid Reference System (MGRS) uses the UTM grid, assigning two letters instead of numbers to the 100,000-meter squares. There’s a pattern for assigning the letters, but the nature of the pattern is not important as long as you know the military designation for the 100,000-meter square you’re in.
A military map provides this information in the map’s margins. Unfortunately, there’s no way to determine the letter designation for the 100,000-meter square simply by looking at a USGS topo map, making MGRS less than fully useful with these maps.
How MGRS Coordinates Are Derived from the UTM Grid
Once you understand the UTM grid, learning the ins and outs of the Military Grid Reference System is a snap.
Let’s look, for example, at a UTM coordinate defining the southwest corner of a one-meter square located at Gold Head Branch State Park in Clay County, Florida:
17 R 0408360
The MGRS coordinate for the same one-meter square would be:
17R MP 08360
The MP replaces the 04 / 33 of the 100,000 meter square, and that’s the only difference.
The coordinates above were expressed exactly that way in my GPS. Customarily, military coordinates are strung out as one long list of numbers and characters, as follows:
17R MP 0836001357
You have to know exactly where the easting / northing split is. Counting only those digits that appear after the 100,000-meter square designation (MP, in this case), you split them in your mind right down the middle, in this case, after the fifth digit.
Further, you can abbreviate military coordinates by omitting the zone and quadrant designation (17R, in this case) when there’s no question about which zone and quadrant you’re in. Some people even eliminate the two-letter 100,000-meter designation (MP, in this case) when there’s no question about which 100,000-meter square they’re in. My understanding is, however, that the Army prefers that those letters be left in place to reduce the chances of a mix-up, like say calling in an air strike on the wrong place.
Let’s look now at some specific military coordinates, and their associated abbreviations. Note that the letter designations are verbalized using the phonetic alphabet, as shown in the table following these coordinates:
2-digit coordinate — Describes the southwest corner of a 10,000-meter square.
Expressed as follows: 17R MP 00
Abbreviated as follows: MP 00
4-digit coordinate — Describes the southwest corner of a 1,000-meter square.
Expressed as follows: 17R MP 0801
Abbreviated as follows: MP 0801
6-digit coordinate — Describes the southwest corner of a 100-meter square.
Expressed as follows: 17R MP 083013
Abbreviated as follows: MP 083013
8-digit coordinate — Describes the southwest corner of a 10-meter square.
Expressed as follows: 17R MP 08360135
Abbreviated as follows: MP 08360135
10-digit coordinate — Describes the southwest corner of a one-meter square.
Expressed as follows: 17R MP 0836001357
Abbreviated as follows: MP 0836001357