The only time that is true is if you do very short "trips" (say, less than an hour) in very steady weather. In that case, it will probably mostly be right).
If you don't, and there is a low rolling in (or you rolling into it) you will suddenly have a barometer which reads as if you gain elevation. That's how it works - something you should know by now.
If it was true in general, that frigging plane mentioned earlier wouldn't need to "zero" its barometer with the airport just before landing, because they know what altitude they took off from, so its elevation gain could be read by the barometer. As the bloke who attempted to support you explained: That is simply not good enough.

No, that doesn't seem to be the case at all. The barometer doesn't need to be zeroed (for gain).

Changes in weather can sometimes be a problem. The 100 foot error for GPS in elevation is always a problem for gain.

Zeroing would do much over long trips (with significant weather changes) anyway. Unless you kept zeroing it.

By the way, the Edges can be set to "zero" the barometer to known elevations at particular locations.

What is needed for landing a plane is obviously different than estimating elevation gain for a bicycle ride.
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It's obvious that barometers need to be aligned to known elevation for accurate absolute elevation. No one said otherwise. It's also largely irrelevant to cycling.

Yes, you can. But no one is really interested in doing that because it isn't useful or necessary.

Cyclists (outside of rare exceptions) are not interested in absolute height.

No you can't check with an already known an absolute height that you are within a few hundred metres of where you are - at all times. That is ridiculous.
You are reaching so much it has become silly. I'm out. Hope to chat with you at another date about something else.

This is nonsense.

I never said "at all times". Stop making stuff up.

In any case, you don't need to do that.