Question

I need to find the angle between the north and a point in a map (21.4167, -39.8167).

here is my code

double MECCA_LONGITUDE = 21.4167;
double MECCA_LATITUDE = -39.8167;

-(void) getAngle :(float) phoneAngle: (float) lon:(float) lat
{

 float ad = atan (sin (lon - MECCA_LATITUDE) /  ( (cos (lat)) * (tan (MECCA_LATITUDE)) ) - ( (sin (lat)) * (cos (lon - MECCA_LONGITUDE)) ));

NSLog(@"###### %f", CC_RADIANS_TO_DEGREES(ad));
}

I’m still not getting the correct answer. Any tips would be appreciated. Thanks.

Answer 1

Your principle problem is that sin() and cos() take their arguments in radians, and you are passing degrees (you also have the longitude and latitude of Mecca interchanged).

Try:

double MECCA_LATITUDE = 21.4167;
double MECCA_LONGITUDE = 39.8167;

double r_delta_lon = CC_DEGREES_TO_RADIANS(MECCA_LONGITUDE - lon);
double r_lat2 = CC_DEGREES_TO_RADIANS(MECCA_LATITUDE);
double r_lon1 = CC_DEGREES_TO_RADIANS(lon);
double r_lat1 = CC_DEGREES_TO_RADIANS(lat);

double ad = atan2(sin(r_delta_lon) * cos(r_lat2),
    cos(r_lat1) * sin(r_lat2) - sin(r_lat1) * cos(r_lat2) * cos(r_delta_lon));