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.
Yes, most modern GPS recievers do calculate altitude, (with a minimum of 4 satellites). However, they don't take this into account in calculating speed and distance. That is exactly why the Garmin Edge 305 GPS reciever, designed specifically for use on a bicycle uses a wheel magnet to attain true speed readings.
You don't have to actually jump off a building to test it yourself. Just watch the speed that is displayed when going down a steep incline. While I usually hike at about 3.5 to 4.5 mph, when going down a steep incline, (and obviously moving faster than usual), the GPS thinks I've slowed down until I hit flat ground again.