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.

[1] https://en.wikipedia.org/wiki/Localizer_performance_with_ver...

[2] https://en.wikipedia.org/wiki/Wide_Area_Augmentation_System

20-30 feet change over what timeframe? The resolution of the chart data in the article is about 30 seconds. While I think the fluctuation is due to the instrumentation, 20-30 feet change over the course of a minute seems like nothing.

That's significantly slower than a typical elevator, in fact. Slow elevators run at about 200 feet per minute.

My elevator takes almost a minute to go from the first floor to the second floor. I guess I have a slow one.

Nobody without disabilities would use such an elevator.

Connect the dots…

Maybe the plane is staying level but the ground is variable terrain.

Elevation is relative to sea level, not the ground.

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.)

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...

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

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.

it's the Earth vibrating ...

When you take off, you're going up at a rate of 500 fpm to 2000 fpm. Even if you go from +1000 fpm to -1000 fpm over the course of several seconds, you aren't going to feel much.

At cruise altitude, you're moving along at 500 mph, which is 777 feet per second. So going from 30 fps up to 30 fps down in one second is just an adjustment of only about 5 degrees. You'd barely feel it, even walking down the isle.

You experience much greater changes in vertical motion on any flight you go on.

> So going from +30 feet to -30 feet in a minute is just an adjustment of only about 5 degrees. You'd barely feel it, even walking down the isle.

You would pretty obviously feel a change in pitch of 5° walking down the aisle.

You mixed feet per second and feet per minute. 60 feet of change across 777 feet of run is about 4.5° (inverse sin(60/777)), such as you'd experience if the change was in 1 second instead of in 1 minute.

Calculating 60' change in 777*60 feet, inverse sin (60/(777*60)) is 0.07°, which is why you don't feel that change in inclination of the aisle.