Question

I’m trying to get the device’s declination from the magnetic North in degrees, by relying solely on the device’s magnetometer. This is the code I’ve written but I just get 0 degrees.. What am I doing wrong ?

CMMotionManager *motionManager;

motionManager = [[CMMotionManager alloc] init];

[motionManager startDeviceMotionUpdates];

CMDeviceMotion *deviceMotion;

deviceMotion = [[CMDeviceMotion alloc] init];

while(!self.stopButtonPressed)
{
    double x = motionManager.deviceMotion.magneticField.field.x;

    double y = motionManager.deviceMotion.magneticField.field.y;

    double degrees = asin(y/sqrt(pow(x, 2.0) + pow(y, 2.0))) * 180.0 / M_PI ;

    int degreesRounded = (int)degrees;

    NSLog(@"Degrees : %i", degreesRounded);
}

Answer 1

It is likely that the simulator doesn’t return normal values for these methods, so you will need to test on a real device.

The CLLocationManager‘s method, didUpdateHeading: doesn’t work on the simulator, so you are probably experiencing something similar here.

Edit:

From the docs:

“The latest sample of device-motion data. (read-only)

@property(readonly) CMDeviceMotion *deviceMotion

Discussion
If no device-motion data is available, the value of this property is nil. An application that is receiving device-motion data after calling startDeviceMotionUpdates periodically checks the value of this property and processes the device-motion data.”

Check to see if that property of your motion manager is nil. If it is, then you would get 0 for the magnetic field property.

Edit 2:

Instead of using startDeviceMotionUpdates, you should be using startMagnetometerUpdatesToQueue:. The docs say this:

“Magnetometer. Set the magnetometerUpdateInterval property to specify an update interval. Call the startMagnetometerUpdatesToQueue:withHandler: method, passing a block of type CMMagnetometerHandler. Magnetic-field data is passed into the block as CMMagnetometerData objects.”

Docs are here.