根据经纬度计算距离（simple knowledge）

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according latitude and longtitude compute distance

1、最大圆距离

const double earth_radius=6371;
const double pi=3.1415926;
double degtorad(double deg)
{
    return ( deg *pi/ 180);
}
double GreatCircleDistance(double lat1,double lon1,double lat2, double lon2)
{
    lat1 = degtorad(lat1);
    lon1 = degtorad(lon1);
    lat2 = degtorad(lat2);
    lon2 = degtorad(lon2);
    double delta_lon = lon2 - lon1;
    double central_ang = acos ( sin(lat1) *sin(lat2) + cos(lat1) *cos(lat2) * cos(delta_lon) );
    return earth_radius * central_ang;
}

2、Haversine 距离

double HaversineDistace(double lat1,double lon1,double lat2, double lon2)
{
    lat1 = degtorad(lat1);
    lon1 = degtorad(lon1);
    lat2 = degtorad(lat2);
    lon2 = degtorad(lon2);
    double dlon=lon2-lon1;
    double dlat=lat2-lat1;
    double a=pow(sin(dlon/2),2)*cos(lat1)*cos(lat2)+pow(sin(dlat/2),2);
    double c=2*atan2(sqrt(a),sqrt(1-a));
    return earth_radius*c;
}