Brendan:

> So: you must allow for :
> -different datums, ie where each country measures 0m a.s.l. to be.
> -differences in oceanic surface height
> -the natural shape of the earth
> - the different models of the shape of the earth used by each

country in its

> surveys.
> -errors in measurement over the distances from sea to Everest

But the recent GPS measurements just slice right through all that -
they're measured on top of Everest relative to the GRS80 ellipsoid,
with a geoid correction from the global geoid model laid on top.
BTW: "Local geoids" differ from the global geoid by just a few tens of
centimetres, and they're used because of differences in "sea surface
topography" - due to variations in seawater density because of
temperature and salinity. (The UK vertical datum based on Newlyn sits
about 80cm below the global geoid, for instance.) Although I can
imagine differences in SST do exist between India and China, I think
they can be ignored when the altitude of Everest is being measured -
that global geoid is the one to go for, since temperature and salinity
can only be imaginary attributes of an imaginary sea level under the
Himalayas.
So the big question is: which of the rival surveys cocked up their GPS
measurements on top of Everest? Best of three, anyone?

Grant