iphone - how to calculate two coordinates distance in objective c?

Question

as title how to? i have tried the code from google earth, but seem like the result is different with the google map calculation result. below provided the code i did

-(double)GetDistance:(double)lat1 long1:(double)lng1 la2:(double)lat2 long2:(double)lng2 {
    //NSLog(@"latitude 1:%.7f,longitude1:%.7f,latitude2:%.7f,longtitude2:%.7f",lat1,lng1,lat2,lng2);
    double radLat1 = [self rad:lat1];
    double radLat2 = [self rad:lat2];
    double a = radLat1 - radLat2;
    double b = [self rad:lng1] -[self rad:lng2];
    double s = 2 * asin(sqrt(pow(sin(a/2),2) + cos(radLat1)*cos(radLat2)*pow(sin(b/2),2)));
    s = s * EARTH_RADIUS;
    s = round(s * 10000) / 10000;
    return s;
}

-(double)rad:(double)d
{
    return d *3.14159265 / 180.0;
}

the EARTH_RADIUS value is 6378.138

by using this function by provided two coordinates the result come out is 4.5kM but when i use google map get direction between two same coordinates, it show me the distance is about 8km

can anyone help to point out the problem of my code?

Answer 1

Since this is tagged iPhone, why not use the built-in distance function rather than rolling your own? location1 and location2 are CLLocation objects.

CLLocationDistance distance = [location1 getDistanceFrom:location2];