Finally found some time to finalize my developments for the HMC5883L sensor. At first it looked like an easy mission but I encountered some issues during development and testing. Trying to follow litterally the operational example gave me problems getting the right values. Finally after some trial and error I found out the way to go. The datasheet is not written for software developers. But that is a remark I can make for all datasheets I encountered. But anyway it gives a great feeling once you reach that breakthrough and get the right readings. You can find the most important information at HMC5883L. I found the information about calculating the heading at Calculating Heading.
When you look at the sensor you see an X and Y axis. The angle calculated is the angle between the geographic north and the X axis shown on the sensor. Well at least it is how I interprete it with the received readings. I did not go into detail on the Z axis readings.