distanceTo method

@override double distanceTo(Geographic destination, { double radius = 6371000.0, })

Returns the distance from the current position to destination along a rhumb line.

Parameters:

• radius: The radius of earth (defaults to mean radius in metres).

The distance between this position and the destination is measured in same units as the given radius.

Examples:

  const p1 = Geographic(lat: 51.127, lon: 1.338);
  const p2 = Geographic(lat: 50.964, lon: 1.853);
  final d = p1.rhumb.distanceTo(p2); // 40.31 km

Implementation

@override
double distanceTo(
  Geographic destination, {
  double radius = 6371000.0,
}) {
  if (position == destination) return 0.0;

  // see www.edwilliams.org/avform.htm#Rhumb

  final lat1 = position.lat.toRadians();
  final lat2 = destination.lat.toRadians();
  final dlat = lat2 - lat1;
  var dlon = (destination.lon - position.lon).toRadians();

  // if dlon over 180° take shorter rhumb line across the anti-meridian:
  if (dlon.abs() > pi) {
    dlon = dlon > 0.0 ? -(2.0 * pi - dlon) : (2.0 * pi + dlon);
  }

  // on Mercator projection, longitude distances shrink by latitude
  // q is the 'stretch factor'
  // q becomes ill-conditioned along E-W line (0/0)
  // use empirical tolerance to avoid it (note ε is too small)
  final dlatProj =
      log(tan(lat2 / 2.0 + pi / 4.0) / tan(lat1 / 2.0 + pi / 4.0));
  final q = dlatProj.abs() > 10e-12 ? dlat / dlatProj : cos(lat1);

  // distance is pythagoras on 'stretched' Mercator projection, √(Δφ² + q²·Δλ²)
  final dst =
      sqrt(dlat * dlat + q * q * dlon * dlon); // angular distance in radians
  return dst * radius;
}