Altitudes Differences

[aviationluver]

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.

[DaveReidUK]

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.

[Uplinker]

Does the OP mean when above or below the transition altitude ?

[DaveReidUK]

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. :O

[EDHL]

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?

[hans brinker]

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.

[DaveReidUK]

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.