Which altimeter setting is used to define airspace boundaries?

Question

Let's say that we're flying VFR over the top of Class D airspace at 3,000 feet. The Class D extends up to 2,900 feet - we're above it, but just barely with a 100' buffer.

We're also talking to approach control for VFR flight following. Approach has given us the QNH for their nearby Class C airport, say 29.90", which we've dutifully and correctly set into our altimeter's Kollsman window.

However, it turns out there's a strong pressure gradient today. The QNH at the Class D airport underneath us is 29.80". According to that pressure setting, our altimeter would read 100 feet lower - placing us within the upper bounds of their airspace.

In a situation like this, which pressure setting actually defines the upper boundary for the Class D airspace? Would this aircraft have inadvertently busted the Class D airspace?

(One one hand, everyone within the Class D would be using the QNH reported by the local ATIS, and would expect nearby aircraft to be participating with the Class D tower. On the other, approach control would expect you using their given altimeter setting in order to comply with cruising altitudes and for vertical separation. The only rule I can find is FAR 91.121, which just requires using an altimeter setting within 100 nautical miles.)

Answer 1

Airspace boundaries are not defined by altimeter settings or Indicated Altitude. Like the field elevation, they are at True Altitude. This is defined as the actual height above the 19-year average level of the sea surface. Some boundaries are defined by the height above the surface (field elevation) which is in turn at True Altitude. The MSL vertical dimensions of the boundaries do not change based on barometric pressure or altimeter settings. They are relatively constant.

Your Indicated Altitude shown on your Altimeter may be different from the True Altitude due to barometric pressure changes, erroneous settings, or atmospheric anomalies. This does not change the boundary dimensions.

For further clarity, look at how the FAA Pilot’s Handbook of Aeronautical Knowledge defines these terms:

Types of Altitude
Altitude in itself is a relevant term only when it is specifically stated to which type of altitude a pilot is referring. Normally when the term “altitude” is used, it is referring to altitude above sea level since this is the altitude which is used to depict obstacles and airspace, as well as to separate air traffic. Altitude is vertical distance above some point or level used as a reference. There are as many kinds of altitude as there are reference levels from which altitude is measured, and each may be used for specific reasons. Pilots are mainly concerned with five types of altitudes:

1. Indicated altitude—read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
2. True altitude—the vertical distance of the aircraft above sea level—the actual altitude. It is often expressed as feet above mean sea level (MSL). Airport, terrain, and obstacle elevations on aeronautical charts are true altitudes.

Mean sea level.
The average height of the surface of the sea at a particular location for all stages of the tide over a 19-year period.

If you are on flight following, or an IFR flight plan, it is up to the controller to clear you through an airspace or hand you off to the controller with jurisdiction. If you are not on flight following, or an IFR flight plan, don’t fly so close to the boundary without giving the local tower a courtesy call to “request” transiting their airspace at your desired altitude. It is not a bad idea to do so even if you are well above their airspace. They may know about conflicting traffic of which you are not aware.

Answer 2

Airspace classes and locations are defined by regulation in 14 CFR 71, which references JO 7400.11E - Airspace Designations and Reporting Points. Here's an example of the definition of class D airspace, at KEUG (Eugene, Oregon):

ANM OR D Eugene, OR
Mahlon Sweet Field Airport, OR
(lat. 44°07'29''N., long. 123°12'43''W.)

That airspace extending upward from the surface to and including 2,900 feet MSL within a 4.6-mile radius of Mahlon Sweet Field Airport. This Class D airspace area is effective during the specific dates and times established in advance by a Notice to Airmen. The effective date and time will thereafter be continuously published in the Chart Supplement.

As you can see, the definition is given in MSL and it looks like only MSL and flight level (FL) are used in the document to indicate altitudes. MSL itself is defined in 14 CFR 1.2 as "mean sea level", which is an absolute value.

Practically speaking, in your scenario you don't need to worry about it. While on flight following, ATC is responsible for coordinating the class C/D issue. See this question and especially this answer.