[Solved] Compass App unreliable? How to do a compass callibration?

I have no idea how to determine the direction from the three absolute values, but these values seem plausible to me (I rotated the clock around an axis, one value remained almost constant and the other values increased or decreased, just as I would have expected when rotating around an axis). If I orientate the clock to north, I get (I thought correctly) the 0 for heading, unfortunately I get the same value for an orientation approximately to south. I do not understand the values of heading at all. Since they change reproducibly in the same way, I also assume that the calculation of heading is wrong. Maybe someone else can confirm this (?)