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Consider two adjacent airspaces of different categories. For example, airspace G is up to 2500' and above this is an airspace D. The border is at 2500'. Which airspace are you in if flying at exactly 2500'?

I recall a definition by ICAO that said that the border between such airspaces belongs to the less restrictive one, but I can't find that document.

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  • $\begingroup$ Well, humor aside, changing weather (pressure) makes exact altitude determination difficult. They don't have fences in the sky. More importantly, where you are relative to other aircraft and directions of travel take priority. If I was close, I would abide by the rules of the MORE restrictive one to cover my assets, and follow a flight path that respects the boundary. $\endgroup$
    – Robert DiGiovanni
    Sep 4, 2019 at 19:56
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    $\begingroup$ The question makes no sense. Are you really claiming you are flying at exactly 2500'? Improbable. More likely you are above 2500.000000001 or below 2499.999999999. $\endgroup$
    – quiet flyer
    Sep 5, 2019 at 1:11
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    $\begingroup$ @quietflyer In the context, it makes perfect sense. A pilot might be instructed to "maintain 2500 feet", in which case they are considered (for the purposes of ATC instructions, clearances, etc) to be flying at 2500' (as long as instruments show them to be close to that altitude; obviously if they're down at 2200 or whatever then someone will tell them to get back to their flight level). If you've been instructed to fly at 2500', nobody is going to let you off for a violation of airspace restrictions because "my altimeter showed 2499.9999". $\endgroup$
    – anaximander
    Sep 5, 2019 at 9:23

3 Answers 3

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You recall correctly. That document is ICAO SARPs Annex 11 § 2.6:

Where the ATS airspaces adjoin vertically, i.e. one above the other, flights at a common level would comply with requirements of, and be given services applicable to, the less restrictive class of airspace. In applying these criteria, Class B airspace is therefore considered less restrictive than Class A airspace; Class C airspace less restrictive than Class B airspace, etc.

(Emphasis mine.)

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    $\begingroup$ Now i know a real life use of > and >= $\endgroup$
    – Hanky Panky
    Sep 4, 2019 at 7:30
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    $\begingroup$ @HankyPanky If you pay taxes it comes up quite a bit, e.g. tax bracket boundaries, deduction criteria, the distinction between short- and long-term gains, etc. $\endgroup$
    – Barmar
    Sep 4, 2019 at 20:54
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In the US, the VFR sectional chart often makes it explicit where the airspace begins and ends.

Aeronautical Chart User’s Guide

An example from the SF Bay area. Note that the Class C airspace for Oakland is up to the floor of the Class B for San Francisco. Also note that the Class D airspace for Hayward does not include the floor of the Class C of Oakland.

SF Bay Airspace

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According to Bold Method, the ceiling of class G airspace extends up to, but does not include, the floor of class D airspace.

At exactly 2500 feet, you are in Class D. Since radio communication is important, and transponders as well, anywhere close to the boundary may be best erred on the side of caution.

From TheBalanceCareers: "the pilot must contact the control tower PRIOR to entering Class D airspace and establish 2-way radio communication" and "you must inform Air Traffic Control of your position, altitude, transponder code, and intentions". ATC respond with your tail number or call sign to complete 2-way communication. If they don't you can NOT enter.

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