iphone - How to get angle between two POI?

Question

How do I calculate the angle in degrees between the coordinates of two POIs (points of interest) on an iPhone map application?

Answer 1

I'm guessing you try to calculate the degrees between the coordinates of two points of interest (POI).

Calculating the arc of a great circle:

+(float) greatCircleFrom:(CLLocation*)first 
                      to:(CLLocation*)second {

    int radius = 6371; // 6371km is the radius of the earth
    float dLat = second.coordinate.latitude-first.coordinate.latitude;
    float dLon = second.coordinate.longitude-first.coordinate.longitude;
    float a = pow(sin(dLat/2),2) + cos(first.coordinate.latitude)*cos(second.coordinate.latitude) * pow(sin(dLon/2),2);
    float c = 2 * atan2(sqrt(a),sqrt(1-a));
    float d = radius * c;

    return d;
}

Another option is to pretend you are on cartesian coordinates (faster but not without error on long distances):

+(float)angleFromCoordinate:(CLLocationCoordinate2D)first 
               toCoordinate:(CLLocationCoordinate2D)second {

    float deltaLongitude = second.longitude - first.longitude;
    float deltaLatitude = second.latitude - first.latitude;
    float angle = (M_PI * .5f) - atan(deltaLatitude / deltaLongitude);

    if (deltaLongitude > 0)      return angle;
    else if (deltaLongitude < 0) return angle + M_PI;
    else if (deltaLatitude < 0)  return M_PI;

    return 0.0f;
}

If you want the result in degrees instead radians, you have to apply the following conversion:

#define RADIANS_TO_DEGREES(radians) ((radians) * 180.0 / M_PI)