code: function get100kID(easting, northing, zoneNumber) {
    const setParm = get100kSetForZone(zoneNumber);
    const setColumn = Math.floor(easting / 100000);
    const setRow = Math.floor(northing / 100000) % 20;
    return getLetter100kID(setColumn, setRow, setParm);
  }
docs: 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 {string} the two letter 100k designator for the given UTM location.