Steven P. McNicoll wrote:

How could it be any other way?

Just a few examples: It could be QNH everywhere (requires defined QNH
regions). It could be GPS altitude (would even simplify things a lot,
but requires all aircraft to be expensively equipped). Heck, it could
even be radar altitude everywhere, of course compensated for ground
elevation. As I said, whether it would make sense or even be practical
is a different question, but there are a lot of solutions which are
thinkable.

Stefan