If I strapped a GPS on my arm and went down a big hill on a skateboard would the speed be accurate?

If the hill was say 45 degrees, would the unit read 1/5th slower? pythagorus???? Or does it know when you`re going down (or up) a hill.
On a GT31.

Spocktek said..
I spose what I want to know is if the GPS uses altitude as well to give speed readings.

OK I see what you're getting at, apparently the GPS system isn't very accurate for altitude, and I think that would make the trackpoint method worse. However the doppler method may be unaffected, but I can't be sure about that, you'll have to wait and see if one of the real experts appear on this thread.

Speed = Distance/Time, so if you know the length of your downhill run you don't really need a gps

Ok I don't know much about GT 31 etc but use other GPS systems at work but a bit rusty since a while since I studied them. Basically most cheaper GPS units have one receiver and work in 2D mode as you found out. The unit is normally set to a datum (height on earths surface) which is usually WGS 84 which models the earths surface as best as possible. Ships for example enter the height of their antenna in the setup mode but this is to gain better accuracy in 2D mode. Very accurate GPS units can easily measure altitude, heading etc and work in 3D but do it using 2 receivers. The receivers which need to be a good distance apart and compare positions with each other and quickly give a very accurate heading (or course) and rate of descent or ascent. A lot of smaller commercial vessels such as tugs, fishing boats etc use these GPS compasses instead of expensive Gyro compasses as they are cheaper, more accurate, with less running costs.