"
Unless the accuracy has improved markedly in the last few years, GPS is
not
particularly good for altitude. The potential error is at least 15m
depending on the sattelite geometry [=where the sattelites being
refererred
to are in relation to the receiver] and other factors (I believe the
civilian signal is deliberately "degraded" slightly), resulting in me
occasionally getting a "sea level" reading at my home! The error varies
rapidly in sign and value as the sattelite geometry changes - this can be
seen certainly on a timescale of hours if not tens of minutes.

For areas as "flat" as Cambridge, GPS is not sufficiently accurate.
Unless
you are in very hilly country, the best way of measuring your height with
GPS is to use it to get your position on a map, then read height off the
map
contours.
--
- Yokel -

That was my view until I used my Garrmin Etrex. I have used it in a number
of locations in Hertfordshire and Essex and have compared the altitude with
that read off 1:25000, OS maps and in every case the reading has agreed
within the limits of accuracy with the map contours. I was pleasnatly
surprised.

Alan