These MGRS row designators are completely optional and redundant, because a UTM northing values span the entire Y dimension of a Zone.
What is not redundant is the hemisphere designator, "N" or "S".
There are 20 row designators, and the designers excluded the letters "I" and "O" to avoid confusion with the visibly-similar numbers "1" and "0". There are 26 letters in the alphabet to choose from, but they did not bother to exclude the letters "N" or "S".
It just so happens that the row which covers half of the continental United States - which is in the Northern hemisphere - is the row labeled "S".
So well-meaning GIS software and even Garmin GPS receivers often call my zone "17S", which reads as "Zone 17, Southern Hemisphere". My zone is in fact "17N", "Zone 17, Northern Hemisphere". This is especially annoying given that the row label is an artifact of MGRS and not civilian UTM.
Civilian UTM doesn’t handle things near the poles very well. MGRS explicitly uses the zone characters to change to a polar projection. Those other 4 characters not used for row identification actually identify polar areas (NE, NW, SE, SE semi-hemispheres).
(Also: Zone 17T represent!)
Since the Earth does in fact not have a circumference that's a integer multiple of 1 km, does this mean this projection just leaves out some sliver of the Earth's surface?
East to west its not too bad, just 17 metres off, but north to south that's over 800 meter of extra surface to hide!
UTM actually doesn’t cover a lot more than you think. Because at the extreme poles exaggeration would make them basically useless.I think they terminate at 84 degrees N and 80 degrees S.
It’s been a long time since I studied this stuff so I’d look this all up rather than take my word.
