What defines the upper limit of Class G airspace?

The upper limit of Class G airspace is defined by the base of the overlying airspace. Class G airspace is considered uncontrolled airspace, and it typically extends from the surface up to a certain altitude, at which point it transitions into controlled airspace, such as Class E or Class A. Therefore, the boundary that separates Class G from the next class of airspace directly above it is crucial for determining the vertical limits of Class G.

For example, often Class G airspace may extend up to 1,200 feet agl (above ground level), but this limit can change based on the specific airspace structure in a region. The exact point where Class G airspace ends is determined by the base altitude of the overlying airspace category, making the transition points vital for pilots operating in or near these airspaces.