Pander216
"So little knowledge about flying on a pressure altitude over high terrain?"
Please explain how my answer in post 3 is different from yours then?
NATTRACKS
That's a nice explanation of the history of grid mora's etc, but it doesn't really answer the OP question.
Once again, a far stretched question. There are not too many places where you can not get a local altimeter setting from an airport before and after crossing the high terrain, flying a four engined airplane I'm not too familiar with ETOPS, but worst case I think there should be an airport within 180 minutes at the least!!, get a couple of those and you should have an idea!!
Flying the HAMI route regularly, we get the alt from Urumqi and Hohhot and take the worst alt and lowest temp and calculate the worst correction from those and apply those to the enroute alt or mora in case of turn around BEFORE(pander) crossing the area.
Question asked and answer given!