My old Etrex only uses the satellites to calculate altitude and is usually pretty accurate as long as it has a good lock on 4 or more of them. I have checked against an OS map and the reading is usually within about 1%.If maintaining accuracy is important to you, do as victor suggests at the start of the ride and then check (and adjust) regularly using the spot heights on OS maps. TBH I can't see the need whist cycling, although it's a different matter when you're stuck in a whiteout up a mountain. If your GPS uses the satellites to work out altitude then forget and level of accuracy
Having read a lot of comments about how bad the altitude function was supposed to be, I was pleasantly surprised!You must have a magic one then I've used eTrex for a good few years for capturing spacial data to load into GIS at work for legal orders, so I may be looking at a higher degree of accuracy than most. Whilst the X & Y axis data has become more accurate with each version, I still don't trust the Z axis data. I use a Suunto barometric alti for that every time.