Mgrs.fromUtm constructor

Mgrs.fromUtm(Utm utm)

Creates a MGRS grid reference from projected UTM coordinates.

May throw a FormatException if conversion fails.

Examples:

  final utmCoord = Utm(31, 'N', 448251, 5411932);
  final mgrsRef = Mgrs.fromUtm(utmCoord); // 31U DQ 48251 11932

Implementation

factory Mgrs.fromUtm(Utm utm) {
  // MGRS zone is same as UTM zone
  final lonZone = utm.zone.lonZone;

  // convert UTM to lat/long to get latitude to determine band
  final latlong = utm.toGeographic();
  // grid zones are 8° tall, 0°N is 10th band
  final band = _latBands[(latlong.lat / 8 + 10)
      .floor()
      .clamp(0, _latBands.length - 1)]; // latitude band

  // columns in zone 1 are A-H, zone 2 J-R, zone 3 S-Z, then repeating every
  // 3rd zone
  final col = (utm.easting / 100000).floor();
  // (note -1 because eastings start at 166e3 due to 500km false origin)
  final e100k = _columnLetters[(lonZone - 1) % 3][col - 1];

  // rows in even zones are A-V, in odd zones are F-E
  final row = (utm.northing / 100000).floor() % 20;
  final n100k = _rowLetters[(lonZone - 1) % 2][row];

  // truncate easting/northing to within 100km grid square & round to 1-metre
  // precision
  final easting = (utm.easting % 100000).floor();
  final northing = (utm.northing % 100000).floor();

  return Mgrs(
    lonZone,
    band,
    e100k,
    n100k,
    easting,
    northing,
    datum: utm.datum,
  );
}