thanks, Foersom and mehmetyldz87 for your suggestions.
I asked Lothar Freudenberg from MK and analysed some GPX log examples for myself. So I found the following conclusions, which I want to share with you...
(1) Heading = the direction, in which the kopter points. This may be the same direction the kopter moves. In this special case, heading = course. Generally, the heading and moving directions are not equal.
(2) The heading information is hidden in the parameter "compass". In older versions of the GPX logs, there are 2 compass values, in newer version these are 4. The wiki http://wiki.mikrokopter.de/GPX
explains this unfortunately very rudimentary:<Compass>349,344,356,357</Compass> --> >Yaw Gyro stabilized Compass signal ; compass signal (raw) ; Compass setpoint (corrected with declination) ; Gyro Compass corrected (declination)
An example with my interpretation:example GPX log chart
compass = original compass value
compass = averaged compass
compass = compass with applied correction of magnetic declination
compass = compass with again some averaging
I think, the best parameter for the heading is:heading = compass.
(3) Some complications appear in the special case, when the FlightControl is mounted with some angle to the front direction. The GPX files saves the "FCOrientation" in the header part. In my example the FC is moved 45 degrees ccw (seen from top) and the GPX file says:<FCOrientation>45</FCOrientation>
The compass values always refer to the compass direction, which equals the FC direction. Therefore:heading = compass + FCOrientation