What altitude system does the FAA use for classifying airspace?

The FAA classifies airspace primarily using the altimeter setting at sea level, which is essential for ensuring consistent and safe altitude measurements across different aircraft and pilots within controlled airspace. By using the altimeter setting at sea level, pilots adjust their altimeters before taking off or climbing to ensure that their altitude readings are accurately referenced to a common standard. This is crucial for maintaining proper vertical separation between aircraft, especially when flying in crowded airspaces.

Using this system allows for a standardized reference point, which is necessary for coordinating air traffic and preventing altitude conflicts. The altimeter setting provides a way for pilots to calibrate their instruments, converting the atmospheric pressure measured at their aircraft's location into an accurate altitude above sea level. This is critical for both safety and operational efficiency in aviation.

In contrast, the other options do not provide a consistent and reliable method for altitude classification within the airspace. For example, barometric pressure at flight level would vary based on weather conditions and location, while the average height of terrain is not a universally applicable standard. Likewise, the elevation of the control tower is specific to individual airports and does not serve as a common reference across different locations.