It's still sea-level. The transition altitude just changes the altimeter setting from one that matches the current air pressure to a standard pressure setting.
Planes report pressure altitude via their transponders. 20-30 feet up and down is very normal for an autopilot.
GPS altitude is used for vertical guidance for certain types of GPS approaches (i.e. "LPV" approaches[1]) and requires the airplane's avionics to be equipped with a WAAS[2] receiver that provides accurate altitude information.
You might be underestimating the amount of traffic Google sends to publishers through Google News. Anecdotally, I get Android notifications from CBC, Global, etc. through Google News daily and do sometimes click on them.
We had such a remediation done recently, in Alberta close to the rocky mountains.
Costs were around $2,000 CAD to install a sub-slab depressurization system (i.e. a fan that pulls air from below the house and vents it away from the house).
Radon values dropped from around 500 Bq/m3 to less than 20 Bq/m3.
Because of the question of liability: If it was lab made and accidentally released, was it due to recklessness or criminal negligence? Is someone guilty of involuntary mass-manslaughter? Or if this was state-sponsored research, could they be found liable for the damage caused?
GPS by itself isn't accurate enough to determine what lane you're in.
Even WAAS GPS [1] (used by autopilots for airplanes) is specified to be accurate only up to 25 ft, although the measured accuracy is usually around 3 ft.
https://x.com/fchollet/status/1953511631054680085