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Altitudes Differences

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Altitudes Differences

Old 29th Sep 2022, 18:05
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Altitudes Differences

Hello,

Does anyone know why there is usually a difference of a few hundred feet between the Indicated Altitude read on the Altimeter versus the GPS Altitude? The GPS Altitude is usually a few hundred feet higher.

Thanks.
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Old 29th Sep 2022, 21:11
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Originally Posted by aviationluver View Post
Does anyone know why there is usually a difference of a few hundred feet between the Indicated Altitude read on the Altimeter versus the GPS Altitude? The GPS Altitude is usually a few hundred feet higher.
The difference will vary depending on where you are in the world and on the prevailing air pressure.

GPS altitude is relative to a reference ellipsoid which may or may not coincide with mean sea level depending on where you are in the world. What you see on the altimeter will also differ from height AMSL by an amount depending on the difference (if any) between the prevailing QNH and the subscale setting.
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Old 30th Sep 2022, 08:48
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Does the OP mean when above or below the transition altitude ?
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Old 30th Sep 2022, 11:34
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Originally Posted by Uplinker View Post
Does the OP mean when above or below the transition altitude ?
No idea.

Hence I thought it prudent to supply an answer that catered for both eventualities.
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Old 30th Sep 2022, 18:41
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Keep in mind also that even if you've set your altimeter subscale to the correct local pressure setting (QNH), the altimeter won't necessarily indicate your actual height above mean sea level if the temperature is different from ISA (as it almost always is). If it's warmer than ISA, your actual (geometric) altitude will be higher than the (barometric) altitude that your altimeter indicates. If it's colder, your actual altitude will be lower than indicated, hence the need for cold temperature corrections to altitudes on approaches.

​​​​​​Edit: aviationlover, may I hazard a guess that you fly in a fairly warm climate?

Last edited by EDHL; 30th Sep 2022 at 19:05. Reason: Added a paragraph
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Old 1st Oct 2022, 03:21
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Originally Posted by EDHL View Post
Keep in mind also that even if you've set your altimeter subscale to the correct local pressure setting (QNH), the altimeter won't necessarily indicate your actual height above mean sea level if the temperature is different from ISA (as it almost always is). If it's warmer than ISA, your actual (geometric) altitude will be higher than the (barometric) altitude that your altimeter indicates. If it's colder, your actual altitude will be lower than indicated, hence the need for cold temperature corrections to altitudes on approaches.

​​​​​​Edit: aviationlover, may I hazard a guess that you fly in a fairly warm climate?
I think he flies in a room that has central heat/air.
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Old 1st Oct 2022, 07:26
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Originally Posted by hans brinker View Post
I think he flies in a room that has central heat/air.
That's a tad unfair. The OP's posting history makes it clear that he/she has a (lapsed) FAA ATPL.
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