On 6 Jan 2006 05:47:35 -0800, "Tudor Hughes" wrote:

My brother's
GPS contraption was out by about 5 metres.

The GPS system wasn't designed to provide an accurate altitude.
Altitude errors can be one-and-a-half to twice the horizontal error.
The altitude that's displayed is the distance from the centre of the
earth less the distance from the centre of the earth of the local
geoid. The local geoid is stored in the receiver and may not be
particularly accurate.

E&OE

--
Alan White
Twenty-eight miles NW of Glasgow, overlooking Loch Goil and Loch Long in Argyll, Scotland.
Web cam and weather:- http://www.windycroft.gt-britain.co....her/kabcam.htm
Some walks and treks:- http://www.windycroft.gt-britain.co.uk/walks/