Originally Posted by spinnaker
Not necessarily true. GPS devices can measure changes in altitude (be it not hugely accurate). It would depend on the GPS if it used this change in altitude to compute it's speed. My guess though is that most (if not all) do not use it. But sorry I really don't feel like testing my GPS.
GPS in the woods ...
On the road in open terrain (not around high rises), GPS will be pretty accurate since the error has no bias. You will be able to plot your course on a map (using a tool) and it will be pretty darn close.
Out in the woods, it's a different ball game. The foliage will obscure the satellites and you'll get TERRIBLE accuracy. GPS can measure accuracy, but only to the same precision as it can measure latitude and longitude. So if you go up and down a 10 foot hill, It may not be measured at all.