public static double DistanceTo(double lat1, double lon1, double lat2, double lon2, char unit = 'K')
{
    double rlat1 = Math.PI*lat1/180;
    double rlat2 = Math.PI*lat2/180;
    double theta = lon1 - lon2;
    double rtheta = Math.PI*theta/180;
    double dist =
        Math.Sin(rlat1)*Math.Sin(rlat2) + Math.Cos(rlat1)*
        Math.Cos(rlat2)*Math.Cos(rtheta);
    dist = Math.Acos(dist);
    dist = dist*180/Math.PI;
    dist = dist*60*1.1515;

    switch (unit)
    {
        case 'K': //Kilometers -> default
            return dist*1.609344;
        case 'N': //Nautical Miles 
            return dist*0.8684;
        case 'M': //Miles
            return dist;
    }

    return dist;
}