Point to location using compass - iphone

Question

I am trying to develop a compass for an appliation which has a set of annotations on a map. I would like to be able to choose an annotation and then have the compass point the user to the chosen location.
I have calculated the degress from the user's location to the location of the annotation and I have the magnetic heading and the true heading of the iPhone. Also I know how to rotate an image.
But I can't figure out what the next step is.
The degrees between the user's location and the annotation location is calculated like this:
// get user location
locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.distanceFilter = kCLDistanceFilterNone;
[locationManager startUpdatingLocation];
[locationManager startUpdatingHeading];
CLLocation *location = [locationManager location];
CLLocationCoordinate2D coordinate = [location coordinate];
float x1 = coordinate.latitude;
float y1 = coordinate.longitude;
float x2 = [annLatitude floatValue];
float y2 = [annLongitude floatValue];
float dx = (x2 - x1);
float dy = (y2 - y1);
if (dx == 0) {
if (dy > 0) {
result = 90;
}
else {
result = 270;
}
}
else {
result = (atan(dy/dx)) * 180 / M_PI;
}
if (dx < 0) {
result = result + 180;
}
if (result < 0) {
result = result + 360;
}
The true and the magnetic heading is retrieved like this:
- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading {
arrow.transform = CGAffineTransformMakeRotation(newHeading.magneticHeading);
// NSLog(#"magnetic heading: %f", newHeading.magneticHeading);
// NSLog(#"true heading: %f", newHeading.trueHeading);
}
Can anyone tell me what I should do now to make the arrow point to the location - even if the iPhone is rotated?

Answer 1

I had time to play with this again.
This will do it:
- (void)viewDidLoad {
[super viewDidLoad];
locationManager = [[CLLocationManager alloc] init];
locationManager.delegate = self;
locationManager.desiredAccuracy = kCLLocationAccuracyBest;
locationManager.distanceFilter = kCLDistanceFilterNone;
[locationManager startUpdatingLocation];
[locationManager startUpdatingHeading];
CLLocation *location = [locationManager location];
CLLocationCoordinate2D user = [location coordinate];
[self calculateUserAngle:user];
}
-(void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {
CLLocationCoordinate2D here = newLocation.coordinate;
[self calculateUserAngle:here];
}
- (void)locationManager:(CLLocationManager *)manager didUpdateHeading:(CLHeading *)newHeading {
compass.transform = CGAffineTransformMakeRotation(newHeading.magneticHeading * M_PI / 180);
needle.transform = CGAffineTransformMakeRotation((degrees - newHeading.magneticHeading) * M_PI / 180);
}
-(void) calculateUserAngle:(CLLocationCoordinate2D)user {
locLat = [[targetLocationDictionary objectForKey:#"latitude"] floatValue];
locLon = [[targetLocationDictionary objectForKey:#"longitude"] floatValue];
NSLog(#"%f ; %f", locLat, locLon);
float pLat;
float pLon;
if(locLat > user.latitude && locLon > user.longitude) {
// north east
pLat = user.latitude;
pLon = locLon;
degrees = 0;
}
else if(locLat > user.latitude && locLon < user.longitude) {
// south east
pLat = locLat;
pLon = user.longitude;
degrees = 45;
}
else if(locLat < user.latitude && locLon < user.longitude) {
// south west
pLat = locLat;
pLon = user.latitude;
degrees = 180;
}
else if(locLat < user.latitude && locLon > user.longitude) {
// north west
pLat = locLat;
pLon = user.longitude;
degrees = 225;
}
// Vector QP (from user to point)
float vQPlat = pLat - user.latitude;
float vQPlon = pLon - user.longitude;
// Vector QL (from user to location)
float vQLlat = locLat - user.latitude;
float vQLlon = locLon - user.longitude;
// degrees between QP and QL
float cosDegrees = (vQPlat * vQLlat + vQPlon * vQLlon) / sqrt((vQPlat*vQPlat + vQPlon*vQPlon) * (vQLlat*vQLlat + vQLlon*vQLlon));
degrees = degrees + acos(cosDegrees);
}