function get100kID(easting, northing, zoneNumber) {
    var setParm = get100kSetForZone(zoneNumber);
    var setColumn = Math.floor(easting / 100000);
    var setRow = Math.floor(northing / 100000) % 20;
    return getLetter100kID(setColumn, setRow, setParm);
  }
Get the two letter 100k designator for a given UTM easting,
northing and zone number value.

@private
@param {number} easting
@param {number} northing
@param {number} zoneNumber
@return the two letter 100k designator for the given UTM location.