Elevation is relative to sea level, not the ground.

Yes, the elevation is based on sea level. I don't fly much and recently landed in Denver and was watching the altitude on the screen in front of me. As we were descending we landed well before I was thinking we would, about a mile in elevation above sea level.. it was "duh" obvious when it happened, but I was tired and clearly not thinking about it!

And using the ground proximity to guide a landing instead of altitude has lead to some crashes I have read.

This only holds true if you’re flying at or above the transition altitude. The transition altitude depends on where you’re flying: for example, in the USA and Canada it’s 18_000 feet MSL.

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.

I did some more reading, and it turns out I confused QNH with QFE.

The plane is measuring altitude, which is relative to a reference point, unlike elevation which is relative to sea level. And if the altitude is determined by pressure sensor, musn't it be relative to the ground directly below the plane, anyway?

(Although personally, I agree with the sibling comment that the variability is likely an artifact of the sensor resolution.)

When you climb to the top of Mt. Everest, the air pressure is about 1/3 of what it is at sea level even though you're standing on the ground.

Yeah, you make a very good point. Fortunately the blast radius of my scientific hubris is limited to whatever code I manage to deploy to the internet, and I'm not involved in designing or building aircraft.

btw: Aren't you the guy who tracks planes flying in circles? I follow you on Twitter. Such a cool project!

It's relative to sea level. After transition altitude (18k feet in most places) the pressure setting to the altimeter is changed to standard (iirc 1013 hPa) so all aircrafts are in the same reference regardless of terrain.

Most places in the US, but accurate

Planes dont measure height relative to the ground. How would that even work? Their sensor is air pressure, which is treated as a function of elevation.

They do sometimes! Via radar altimeters, when relatively close to the ground. And sometimes to keep autopilots from freaking out, we have to build radar reflectors to make the ground look level to radar even when it's really not. https://lustublog.com/2017/02/17/artificiel-mais-pas-superfi...

it's the Earth vibrating ...