Paul Coulton's Forum Nokia Blog - Understanding the Magnetometer (Digital compass) Sensor

Meaning of magnetometer outputs

I am having a hard time understanding the meaning of the magnetometers outputs. I thought they were supposed to be the coordinates of the North vector expressed in local coordinates (x, y and z-axis of the cellphone) but that seems not to be the case.

For example if you measure X,Y,Z for one position of the cellphone and you rotate the cellphone 180° around z-axis, I would expect the outputs to be -X,-Y and Z. But this is not the case.

What does these outputs means if this is not the north vector in local coordinates? And also, how can you determine the orientation of the cellphone from these outputs?

Thanks for your reply.

Magnetometer data

Thank you very much for this post, this very useful.
I am currently working on an augmented reality application on a N97, and I would like to get the orientation of the phone. I was wondering if the magnetometer sensor enables to get the full orientation of the cellphone (by this I mean getting the orientation for the three axes).
How do you convert the magnetometer data to angles? My goal is to get a rotation matrix from these 3 channels (or at least the direction at which the cellphone is pointing if the whole information is not available).
Also, can you point me to some documentation about the magnetometer APIs? I did not find anything in the official documentation.

Regards,
Remi