On Wednesday, May 30, 2012 3:52:41 PM UTC-7, Martin Gregorie wrote:
[snip]
On a slightly different topic: GPS altitude. I've always known that all
GPS altitudes are relative to the WG-84 geoid but have never known how
precisely that corresponds sea level, so I finally did some research and
it turns out that its within +/- 1 metre of AMSL.

Actually I'm not sure where you get +/- 1m difference between the WSG-84 geoid and AMSL. It's potentially larger than that (but still that does not mean GPS altitude is inherently not usable). e.g. see this article http://www..esri.com/news/arcuser/0703/geoid1of3.html

Darryl