Question regarding elevation

Former Member over 8 years ago

Hi,
I just came back from abroad. During the flight, just our of curiosity I looked at the elevation and while the plain was traveling at a height of ~10Km the watch showed elevation of ~2km. So I thought, obviously GPS is off, and the elevation is determined based on the pressure which perhaps is a bit lower at that height in the plane. However, when I looked at the pressure - it was almost identical to that of the ground. So this got me thinking - how on earth does the watch determined the height? sure it was wrong, but still why did it report 2km height? based on what? the barometric pressure was the same...
Obviously - this has no significance other than just my curiosity...

0 schinder over 8 years ago

The pressure it shows you is not the ambient barometric pressure, but the pressure at "mean sea level" (called "QNH"). It computes this from the ambient pressure that it's measuring and an atmospheric model. So it showed you 2 km because the aircraft cabin is pressurised, and since it thought it was at 2 km it showed you the pressure 2 km down at sea level. I don't know if there's a setting to show ambient pressure rather than QNH.

0 Former Member over 8 years ago

An aircraft cabin is not usually pressurised to sea-level pressure as. It is usually pressurised to the pressure at 6000-8000 ft or thereabouts so 2km sounds about right if the watch is using barometric pressure.

0 Former Member over 8 years ago

That's what I was thinking - but what puzzled me is that the pressure graph remained hardly unchanged during the flight - so (according to the watch) the pressure remained the same as on ground yet the elevation showed nearly 2km... this is why I could not understand how it determined the elevation... :-)