I've been told I'm a naughty boy because I am "abrasive" and therefore, in the interest of no longer being "abrasive", (something it appears that no-one else in this conversation has been accused of), I will withdraw.
My assertion that AMSL cannot be derived from local pressure data is, apparently, wrong because someone else mentions ISA in a cocky and somewhat superior fashion. It seems that that automagically negates my many hundreds of hours of actual flight time in real, (Military), aircraft over several years.