Tom Anderson wrote:
On Wed, 13 Feb 2008, Tom Anderson wrote:

On Wed, 13 Feb 2008, Old Central wrote:

IIRC the use of GPS to determine heights is a complex topic. You need
to determine the spheroid and geoid separation in relation to the
grid used and so on. Remember that many countires use by the
different versions of these for their mapping and with different
origins.

If you want to know the height above local sea level, then yes, you
need a map of the geoid. But nobody uses that. In the UK, we use
height above the OSGB36 datum,

Hang on, no, that's rubbish. We do use the local sea level, aka Ordnance
Datum Newlyn.

Well whenever I am using GPS these days [1], I can find my altitude by
reference to my watch and a copy of Reed's Almanac. And that leads me
to the question, what sea level are you taking? Certainly most charts
I've found (Admiralty and Imray) use LAT [2] as their datum for points
below MHWS [3], and MHWS for heights on dry land.

[1] and, no, I don't rely on it, I always have several alterntive
methods of navigating at the same time, just in case.
[2] lowest astronomical tide
[3] mean high water springs

Robin