It reads any change in air pressure that isn’t level or horizontal in air, within a few mm or so, and the gauge moves up or down
It measures air pressure. There is no magical flat reference for it.
And do you have any evidence for your claim of a few mm or so?
So again, your argument is entirely circular.
You baselessly assert that Earth is flat so that regions of equal air pressure are flat, to baselessly assert Earth is flat.
If instead you accept that air pressure will follow the general shape of Earth, then you recognise it is useless at measuring
The most important part here, is how it actually measures for pressure. Around the entire plane, not one tiny point on the plane, in its centre or whatever.
You mean your baseless assertion, which remains vague crap which you don't even explain.
Are you suggesting that they have multiple separate altimeters which measure the altitude at separate points on the plane?
Why would they bother with that at all?
More importantly, consider just how useless that would be over the length of a plane for determining the curvature of Earth.
Your argument remains pure BS.
Level is measured over a distance or length or span, between two separate points.
No, level is measured at a point.
You can then have other things go through that point which is level.
If the Earths surface or air above it was curved, it would have to curve over the whole surface, which would be extremely difficult to measure and account for
The problem for you is that these are intrinsically tied together.
With it being so difficult to measure for over such a tiny distance, it becomes completely unnecessary to account for it.
It is only when it becomes easy to measure that it becomes a problem, at which point it is easy to account for.
Levels measure for only one thing
Perpendicular to down.
They do not measure for any particular shape.
For example, a spot level in an analytical balance, is just measuring level at that spot. It is the rest of the balance which then carries that level to be useful.
Likewise, you can level a clock, based upon the position of the 3 and 9. But that isn't making sure anything is straight.
Over a span of two feet, the curvature would be extremely small
i.e. it would be so insignificant there would be no need to account for it.
Especially given the imperfections in the tools used to measure level would be far greater.
You aren't helping yourself here. You are showing there is no problem, and then lying to everyone.
cannot EVER be flat
But can be measured as "flat" with imprecise tools which lack the precision to measure for such a tiny curve.
This has been explained to you repeatedly.
You continually ignoring this and repeatedly lying to everyone just shows you are a dishonest, lying POS with absolutely no integrity and no concern for the truth.
And again, NONE OF THAT addresses gyroscopic instruments which is what this topic was on.