As most research was a dead-end, i decided to tweak the values "until it
fits". I am happily compiling my own gpsbabel now. Only the libqt4-dev
dependency was missing from the how-to guide, but that was fixed quickly.
Right now, i am taking the altitude and convert it linearly with this line:
> double converted_alt = -alt / 3.28084 + 150;
What the 150-constant is, i have no idea. It's just the offset to bring
my first track into the -more or less- right altitude.