haversine method
Implementation
double haversine(LatLng p1, LatLng p2) {
var lat1 = p1.latitudeInRad,
lat2 = p2.latitudeInRad;
var lon1 = p1.longitudeInRad,
lon2 = p2.longitudeInRad;
var earthRadius = 6378137.0; // WGS84 major axis
double distance = 2 *
earthRadius *
asin(sqrt(pow(sin(lat2 - lat1) / 2, 2) +
cos(lat1) * cos(lat2) * pow(sin(lon2 - lon1) / 2, 2)));
return distance;
}