This answer claims that airspace boundaries are based on true altitudes, not any particular altimeter setting. But temperature errors can cause indicated altitudes to vary wildly from true altitudes.

For example, imagine a VFR aircraft traveling at 10500 feet indicated over the KORD class B. If the temperature at KORD is -15 degrees celsius (by no means unheard of at KORD) then the true altitude is about 9200- almost 1000 feet below the class B ceiling!

Is such an aircraft in violation of the class B airspace? If so, how should aircraft determine their true altitude in flight in order to avoid such violations?

On the flip side, if you are flying at an indicated altitude of 9500 feet (nominally inside the class B) on a hot day, then your true altitude is outside the class B. Is this allowed without a class B clearance?

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    $\begingroup$ More interesting case would be flying under a Class B... if true altitude with the correction has you under (out of) the Class B but your altimeter says you're in it... would you be in conflict with somebody whose altimeter says he's in it & is flying at the altitude as cleared by ATC? Would ATC be upset with you for being there without a Bravo clearance? $\endgroup$
    – Ralph J
    Commented Mar 11 at 1:35
  • $\begingroup$ @RalphJ Good point. I've added a similar example with a hot day with your indicated altitude being near the top of the class B. (Temperature errors are larger at higher altitudes AGL so it's easier to find a realistic example near the top than near the bottom). $\endgroup$
    – Chris
    Commented Mar 11 at 1:52
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    $\begingroup$ Not sure where one finds the answer to "prove a negative," but I would strongly doubt that Class B (or other airspace) altitudes vary from what one reads on a correctly calibrated & set altimeter due to temperatures or whatever other dynamics are out there. I think the implication drawn from the statement made in that other question is simply an incorrect conclusion. But to find the legalese to prove that or show exactly where in the thought process things break down... dunno. $\endgroup$
    – Ralph J
    Commented Mar 11 at 21:18

1 Answer 1


The referenced answer is simply incorrect.

In the USA, charted altitudes below 18,000 ft are always referenced by indicated altitude, adjusted to an altimeter setting station within 100 NM. Near class B airspace, the altimeter setting comes from either the primary airport ATIS or the destination airport ATIS/AWOS station.

The only exceptions to indicated altitude usage I'm experienced with are the class A airspace pressure levels/flight levels, and the Temperature Compensation procedure (which is not applicable to airspace boundaries anywhere that I know about).


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