I was walking again with a hand held GPS. I started at sea level (above high tide) and I went to a point (a gate) where I had previously noted my altitude. The reading I got was 80m but the previous time, at the same spot, it was 89m, a 9m difference. At the end of the walk I noticed that at some point I was supposedly at -1.8m, though I never went for a swim 
The only thing I can think of to account for the 9m was that I was walking at a different time and so possibly triangulating with different satellites. I just looked up Garmin (it was not Garmin I was using) and they say their receivers are accurate to 15m on average. I was expecting 5-10m.
This does show the problem of trying to measure a 'true' altitude to +/- 10".
Point taken about the rope, shows how careful you have to be with requirements specifications.