For the row letters, there are actually two alternative lettering schemes within MGRS: 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 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.
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.
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.
The MGRS is derived from the UTM (Universal Transverse Mercator) grid system and the UPS (Universal Polar Stereographic) grid system, but uses a different labeling convention. An example of an MGRS coordinate, or grid reference, would be 4QFJ12345678, which consists of three parts: An MGRS grid reference is a point reference system.