Get the expected reading from the iPhone's compass when holding the phone in either landscape orientation.

landscape-compass-direction.m

- (void)locationManager:(CLLocationManager*)manager
       didUpdateHeading:(CLHeading*)newHeading {
    
    // If the accuracy is valid, process the event.
    if (newHeading.headingAccuracy > 0) {
        
        /**
         * The orientation affects the compass. We need to actually
         * base the orientation off of the top of the iPhone. Think
         * of the direction the phone is facing when you hold it right
         * side up in portrait mode. That is the reading the compass
         * actually gives us. So in our situation we need to actually
         * offset by 90 degrees. When the phone is facing north (0) but
         * you orientate in landscape the phone will think it is
         * actually facing east (90) or west (180) depending on which landscape
         * orientation you end up using.
         */
        if (self.interfaceOrientation == UIInterfaceOrientationLandscapeLeft) {
            float offsetDirection = newHeading.magneticHeading-90.f;
            if (offsetDirection < 0.f) {
                offsetDirection += 360.f;
            }
            self.direction = offsetDirection;
        } else if (self.interfaceOrientation == UIInterfaceOrientationLandscapeRight) {
            float offsetDirection = newHeading.magneticHeading+90.f;
            if (offsetDirection > 360.f) {
                offsetDirection -= 360.f;
            }
            self.direction = offsetDirection;
        }
    }
}