computeOffsetOrigin static method
Returns the location of origin when provided with a LatLng destination, meters travelled and original heading. Headings are expressed in degrees clockwise from North. This function returns null when no solution is available. @param to The destination LatLng. @param distance The distance travelled, in meters. @param heading The heading in degrees clockwise from north.
Implementation
static LatLng? computeOffsetOrigin(LatLng to, num distance, num heading) {
heading = MathUtil.toRadians(heading);
distance /= earthRadius;
// 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(MathUtil.toRadians(to.latitude));
// 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;
if (discriminant < 0) {
// No real solution which would make sense in LatLng-space.
return null;
}
num b = n2 * n4 + sqrt(discriminant);
b /= n1 * n1 + n2 * n2;
final a = (n4 - n2 * b) / n1;
num 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);
}
if (fromLatRadians < -pi / 2 || fromLatRadians > pi / 2) {
// No solution which would make sense in LatLng-space.
return null;
}
final fromLngRadians = MathUtil.toRadians(to.longitude) -
atan2(n3, n1 * cos(fromLatRadians) - n2 * sin(fromLatRadians));
return LatLng(MathUtil.toDegrees(fromLatRadians).toDouble(),
MathUtil.toDegrees(fromLngRadians).toDouble());
}