How can I map the iphone's compass values to the screen coordinates?

Question

Hi everyone, I'm doing an AR app that shows a UILabel in the cameraView depending of the position of a concrete place. When the compass of the iphone is leading to the place, the label is shown in the center of the screen.
My problem is that I want this label rotates to the left or the right when I turn the iphone. I guess there is some way to map the degrees of the compass to the screen coordinates so I could redraw the label in the correct position of the screen, but I cannot imagine how to do this. I don't need the accelerometer to do this, I only need transforming the compass values to the screen coordinates. Anybody knows some solution? Thank you!

Answer 1

Assuming your current position and the destination position are expressed in CLLocationCoordinate2D - use the following to get the angle (in radians) from your source, to the destination:

- (float) getHeadingForDirectionFromCoordinate:(CLLocationCoordinate2D)fromLoc toCoordinate:(CLLocationCoordinate2D)toLoc

{

    float fLat = [self angleToRadians:fromLoc.latitude];
    float fLng = [self angleToRadians:fromLoc.longitude];
    float tLat = [self angleToRadians:toLoc.latitude];
    float tLng = [self angleToRadians:toLoc.longitude];

    return atan2(sin(tLng-fLng)*cos(tLat), cos(fLat)*sin(tLat)-sin(fLat)*cos(tLat)*cos(tLng-fLng));
}  

Then just subtract your current compass heading to this heading (or vica-versa) and setRotation of your arrow/marker/view label or whatever.

Use this for angleToRadians:

-(float) angleToRadians:(float) a {
    return ((a/180)*M_PI);
}