This is just my observations:
Measured (and displayed) altitude initially relates to takeoff point.
It seems to change a bit during a flight depending on what VPS sees, and what IR ground sensor and barometer are reporting.
You can trick
Tello into higher altitude by flying from a house, or hill. I live on a hill and m office is on the top level of our house so how do I know...
But keep one thing in mind: VPS does calculations based on the movement it sees. If the ground is further away then it expects based on its sensors, the relative motion of the image does not match what it's internal algorithms expect. You are simply exceeding the design parameters and VPS may go completely mad.
I had this once and
Tello did violent toilet bowls (just like if you go mad on the pitch & roll sticks). VPS position was wrong by 200m after that incident.
Even if VPS does not go mad completely it may not be able to hover by tself. So and loss of Wifi means
Tello is gone.
If you fly over a cliff VPS goes completely mad, because it expects most of what it sees in the downfacing camera to be relatively flat ground. Instead when going over the edge all it sees is a vertical wall and its algorithms are not prepared for this. If you are lucky it switches off and you can bring it back in atti mode. If not
Tello is gone.
You can fly below 0 altitude. Just keep in mind that
Tello has a nice safety "feature" that reduces rates below a certain altitude. So while
Tello is 10m high it may think its at 0m and significantly reduce rates, making it impossible to fight even lightest wind. Now how do you land
tello if it is taken away by the lightest wind...
This however does not always happen. No clue what exactly triggers this, maybe
Tello recognizes the discrepancy after a few moments and adjusts something internally (without changing the displayed altitude).
Overall: Its easily possible to trick
Tello into higher altitudes but do so only if you don't care about a fly away. Anyone complaining about a flyaway after such a trick is a fool IMO.