HMC5883L Magnetometer chip Possibilities.

[richard]

if the device is held so that the xy plane is perfectly!!!! level then the xy angle is magnetic north (sort of). if the device is tilted then the xy angle needs to compensated for that tilt . the device alone cannot really be used as a compass . the device output is a 3d vector as drawn by the black line in my previous post, as you can see the result is relative to the devices orientation , if that is unknown (and it is without an external reference ) then its virtually impossible to ascertain a direction from its output. even a small tilt creates substantial error.
furthermore magnetic field lines are not parallel with the earths surface they tilt into the earth or up to the sky depending on location. not to mention magnetic inclination/deviation etc
basically the concept of using this chip as a compass is unworkable see post 47