I uploaded the latest 3.4 firmware onto my Pixhawk and noticed that the barometric altitude does not seem to be correct. It is zero when I connect and if I increase the altitude of the Pixhawk a meter higher, the altitude actually decreases to -.01
Changing the GND_ALT_OFFSET also has very little impact.
There is definitely something wrong with the barometric altitude in the latest versions.
The recent firmware seems to work sometimes, but I needed to go back to 4/4/2016 to get the barometer working on my Pixawk connected to just the external compass. (That was just a version I knew worked in the past. I didn’t try every version since 4/4/2016.)