Now.. going back to the Altitude calculation topic (question 1). The Q500 DOES KNOW both altitude measurements and even calculates a third one, which is the one you see in the controller.
For example, the barometer sends its information based on Mean Sea Level air pressure (29.92 inHG), the GPS Module sends data in Above Ground Level, and it does so by using 4 or more satellites and calculate your distance at different angles. Then it puts that data in a 3d space and thats how it achieves your altitude above the ground.This is a really long topic in its own. Google how exactly does a GPS calculates altitude and you will be reading for 4 hours.
And then... Your ST10+ shows a different altitude than those provided by the GPS and the Baro. The Altitude you see in the Tx is a relevant information that comes from the GPS.
If you want to get technical in this, it will be something like saving baro and gps data in two different variables. Creating a third variable with 0 assigned to it. And then its all about keeping track of the changes of the GPS data and updating the third variable proportionally. So, THAT third variable is what is being shown in the ST10+. That's why sometimes it shows negative altitude when you go lower than the altitude you took off from, which I personally think its stupid to do so, but Yuneec might have its reason for this.
I would have done it differently. I would have shown the Q500 GPS data directly to the controller and problem solved... Or even better, I would have included an option for the pilot to switch from MSL or AGL if he or she wishes to do so.
So, now that we know that the Q used the GPS mainly for calculating the Q500 altitude shown in the ST10+. What is the barometer for?
Well, as long as the GPS module is ON and working properly, we can say its redundant. However, when GPS is not available, that's when the barometer comes into play. But I bet that altitude will indeed be in MSL.
The altitude lock is based on the GPS altitude. Thats why we can go to a city that is 500ft above mean sea level and we can still fly up 400 ft more. Makes sense because the FAA restriction is for 400ft above ground level.
This is my though on all of this altitude calculation topic for the Q500. I would love to see the programming lines of codes the Q uses to understand way better how it does things.