# Altimeter Question

#### PiperPilot1

##### Filing Flight Plan
OK, Here goes.
Lets say you fly to a small field without weather reporting/ATIS etc and land there, and set your altimeter to field elevation; say 100ft.

However, you made a mistake and set the altimeter to field elevation, minus 1000ft. So the big hand is on 9, the small hand is on 100. When you climb to 1000ft, the altimeter now reads 0. You climb to 1300, the altimeter reads "300".

Would the altimeter still be more or less accurate? I know totally wacky question....Im 99% sure yes it would be but just curuious.

Yes.

Thank you!

But think it through: why? The hands of the altimeter are for humans. The instrument merely responds To the difference in pressure. It doesn’t care what altitude it is at, only the pressure change.

Well, there is a greater pressure change change from 1000 -> 2000 than there is from, say 21,000 -> 22,000 right?

But think it through: why? The hands of the altimeter are for humans. The instrument merely responds To the difference in pressure. It doesn’t care what altitude it is at, only the pressure change.
Exactly. It’s simply set to a known barometric pressure and measures the change. What it’s set at doesn’t matter in how it indicates.

Well, there is a greater pressure change change from 1000 -> 2000 than there is from, say 21,000 -> 22,000 right?
Yes, but altimeters don't take that into account. That's why two planes at 21k and 22k are closer together than two at 1k and 2k.

Ah yea this is sort of what I was wondering. Well thanks for the replies! I dont know why I wondered that..I dont want to even be thinking about this stuff hahaha

Yes, but altimeters don't take that into account. That's why two planes at 21k and 22k are closer together than two at 1k and 2k.
True, but much of the variation with altitude has been erased with electronic optical sensors of the bellows device, & the signal outputted to either a motor that drives the hands or (more common now with new altimeters) a computer that paints the hands like on an electronic watch. In the past, manufacturers tried to compensate with mechanical variable ratio gearing. In those days, because if the gearing, high altitude altimeters used in airliners & bombers were way, way, more expensive than regular altimeters used in cubs.