"Stephen!" wrote:
Jim Logajan wrote in
:

His new portable would almost certainly use WAAS when available and
would not be off by 500 ft.

It could very well be 'off' by that much quite easily. Keep in mind
that the earth is neither smooth nor a sphere. Depending on where you
are on the theoretical sphere that the GPS uses for its calculations,
its altitude calculation could be off from actual AGL by quite a bit.
I saw that quite frequently on Guam where I would be in a boat and the
GPS would swear we were a couple hundred feet below sea level.

I don't believe WAAS is yet available in Guam.