def mag_discrepancy(RAW_IMU, ATTITUDE, inclination, declination=None):
    '''give the magnitude of the discrepancy between observed and expected magnetic field'''
    if declination is None:
        import mavutil
        declination = degrees(mavutil.mavfile_global.param('COMPASS_DEC', 0))
    expected = expected_mag(RAW_IMU, ATTITUDE, inclination, declination)
    mag = Vector3(RAW_IMU.xmag, RAW_IMU.ymag, RAW_IMU.zmag)
    return degrees(expected.angle(mag))
give the magnitude of the discrepancy between observed and expected magnetic field