writes:

The point is that even with SA, GPS accuracy is pretty good and has nowhere
near 2 orders of magnitude altitude error compared to lateral as you stated.

You call a 500-foot error in altitude good?

Absolutely wrong.

All I have to do is look at my GPS and watch it wander up and down by tens of
metres at a time. GPS is not designed for vertical accuracy.

See the the data:

http://users.erols.com/dlwilson/gpswaas.htm

"The" data? It's just one person making experiments. I can do that, too; and
I get huge errors in altitude, so much so that I've never used GPS for
altitude measurements.

If you knew anything about real flying, you would know that you NEVER
use GPS as your altimeter under ANY circumstances for reasons entirely
unlrelated to GPS accuracy.

What are those reasons related to, if not accuracy?

If it's as accurate as you seem to believe, it should be fine for IFR. Go
ahead and use it for that, and make me a liar. Are you prepared to bet your
life on it?

But how would you know as you have never been in a real airplane with
a real altimeter and a real GPS?

I've been using GPS for many years. It's almost useless for measuring
altitude.