计算两GPS点之间的距离（比较精确）

  public static double getDistanceFromXtoY(double lat_a, double lng_a, double lat_b, double lng_b)
        {
            double pk = (double)(180 / 3.14169);
            double a1 = lat_a / pk;
            double a2 = lng_a / pk;
            double b1 = lat_b / pk;
            double b2 = lng_b / pk;
            double t1 = Math.Cos(a1) * Math.Cos(a2) * Math.Cos(b1) * Math.Cos(b2);
            double t2 = Math.Cos(a1) * Math.Sin(a2) * Math.Cos(b1) * Math.Sin(b2);
            double t3 = Math.Sin(a1) * Math.Sin(b1);
            double tt = Math.Acos(t1 + t2 + t3);
            return 6366000 * tt;
        }
#import <math.h>

-(double)getDistanceTwoGPS:(NSNumber*)lat1 lng1:(NSNumber*)lng1 lat2:(NSNumber*)lat2 lng2:(NSNumber*)lng2
{
    double pk = (double)(180 / 3.14169);
    double a1 = [lat1 doubleValue] / pk;
    double a2 = [lng1 doubleValue] / pk;
    double b1 = [lat2 doubleValue] / pk;
    double b2 = [lng2 doubleValue] / pk;
    double t1 = cos(a1) * cos(a2) * cos(b1) * cos(b2);
    double t2 = cos(a1) * sin(a2) * cos(b1) * sin(b2);
    double t3 = sin(a1) * sin(b1);
    double tt = acos(t1 + t2 + t3);
    return 6366000 * tt;
}