A UTM projection distorts the areas above north latitude 84 and below south latitude 80 and hence another form of representation must be made. This figure will be shown in the map datum information on a map. The J represents the U.S. Military Grid Reference System zone, between 24 and 32 degrees south. Due to the greater distortion when representing areas close to the Polar Regions of Earth, above 84 degrees north and below 80 degrees South, a polar projection was developed. The MGRS, U.S. Military Grid Reference System, system divides each UTM zone horizontally into 8 sections that are assigned letters.
Most maps show the grid letter in the datum information box.
Some GPS receivers use the MGRS letter to designate position relative to the equator. Other receivers simply state above or below the equator. The complete UTM grid reference does not really include a letter as such. Some GPS units show the MGRS letter, while others only show N, North or S, South UTM Zones Eastings UTM easting coordinates are referenced to a line down the middle of the zone called the zone meridian. A zone is 6 or 666 kms wide at the equator.