I have seen conflicting comments about the utility of "GPS altimeters" - those fixing the altitude solely by GPS geometry and not utilizing barometric correction. While some claim that this is quite inaccurate, I have found that my bare bones receiver, a Garmin Foretrex 101, is actually quite accurate.

Due to some recent street work, I know that my driveway slab is 201 feet ASL. I usually use my Garmin to track my mileage when I leave the house on a bike ride. Often the displayed altitude is within 0 to five feet of the actual elevation, if I have turned it on and given it time to settle down and figure out where it is. A lot of my riding is along the coast and I can eyeball my altitude and compare it with the GPS determination. Again, it is usually good enough for all practical purposes.

There are exceptions - I have been told I was 48 feet below sea level when I was clearly at least fifteen feet above the water, so there can be hiccups (satellite configuration?) in the system, but they generally resolve in a minute or two.

I remember an instance when I was slogging up a long grade on my bike in South Dakota. The altimeter was very useful in giving me a useful representation of how much I had gained, and how much was left. The true altitude could have been off by as much as 100 feet or so, as far as I know, but the computed altitude was still good to know.