iOS-гироскоп для создания виртуального горизонта

Я решил свою проблему вот так:

- (void)updateImage: (NSNumber *)rotNum
{
    self.image.transform = CGAffineTransformMakeRotation([rotNum floatValue]);
}

// call the above function to rotate the image in reference to the horizon
[motionManager startDeviceMotionUpdatesToQueue: gyroQueue
                                   withHandler: ^(CMDeviceMotion *motion, NSError *error) {
                                                       [self performSelectorOnMainThread: @selector(updateImage:)
                                                                              withObject: [NSNumber numberWithDouble: atan2(motion.gravity.x, motion.gravity.y)-M_PI]
                                                                           waitUntilDone: NO];
                                               }];