computeOffsetOrigin static method

Point<double>? computeOffsetOrigin(Point<double> to, double distance, double heading)

Returns the origin when provided with a destination to, distance in meters, and original heading. Returns null when no solution exists.

Implementation

static Point<double>? computeOffsetOrigin(
    Point<double> to, double distance, double heading) {
  distance /= MathUtils.earthRadius;
  heading = toRadians(heading);
  // http://lists.maptools.org/pipermail/proj/2008-October/003939.html
  final n1 = cos(distance);
  final n2 = sin(distance) * cos(heading);
  final n3 = sin(distance) * sin(heading);
  final n4 = sin(toRadians(to.x));
  // There are two solutions for b. b = n2 * n4 +/- sqrt(), one solution
  // results in the latitude outside the [-90, 90] range. We first try one
  // solution and back off to the other if we are outside that range.
  final n12 = n1 * n1;
  final discriminant = n2 * n2 * n12 + n12 * n12 - n12 * n4 * n4;

  // No real solution which would make sense in lat-lng space.
  if (discriminant < 0) {
    return null;
  }

  double b = n2 * n4 + sqrt(discriminant);
  b /= n1 * n1 + n2 * n2;
  final a = (n4 - n2 * b) / n1;
  double fromLatRadians = atan2(a, b);
  if (fromLatRadians < -pi / 2 || fromLatRadians > pi / 2) {
    b = n2 * n4 - sqrt(discriminant);
    b /= n1 * n1 + n2 * n2;
    fromLatRadians = atan2(a, b);
  }
  // No solution which would make sense in lat-lng space.
  if (fromLatRadians < -pi / 2 || fromLatRadians > pi / 2) {
    return null;
  }

  final fromLngRadians = toRadians(to.y) -
      atan2(n3, n1 * cos(fromLatRadians) - n2 * sin(fromLatRadians));
  return Point(toDegrees(fromLatRadians), toDegrees(fromLngRadians));
}