a = sin²(Δlat/2) + cos(lat1)*cos(lat2)*sin²(Δlong/2)
c = 2*atan2(√a, √(1−a))
d = R*c, где R - радиус Земли

public double distanceTo(GeoPoint gp1, GeoPoint gp2) {  
        if (gp!=null){  
            int radius = 6371; // Километры
            double dLat = Math.toRadians(gp2.latitude - gp1.latitude);
            double dLong = Math.toRadians(gp2.longitude - gp1.longitude);
            double lat1 = Math.toRadians(gp1.latitude);
            double lat2 = Math.toRadians(gp2.latitude);

            double a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(lat1) * Math.cos(lat2) * Math.sin(dLong / 2) * Math.sin(dLong / 2);
            double c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
            return c * R;