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double distance(double lat1, double lon1, double lat2, double lon2) {
    double theta, dist;
    if ((lat1 == lat2) && (lon1 == lon2)) {
        return 0;
    }
    else {
        theta = lon1 - lon2;
        dist = sin(deg2rad(lat1)) * sin(deg2rad(lat2)) + cos(deg2rad(lat1)) * cos(deg2rad(lat2)) * cos(deg2rad(theta));
        dist = acos(dist); //returns the arc cosine (inverse cosine) of a number in radians.
        dist = rad2deg(dist);
        dist = dist * 60 * 1.1515;
        dist = dist * 1.609344;
    }
    return dist;
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts decimal degrees to radians             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double deg2rad(double deg) {
    return (deg * pi / 180);
}

/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
/*::  This function converts radians to decimal degrees             :*/
/*:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::*/
double rad2deg(double rad) {
    return (rad * 180 / pi);
}