do all handheld gps units give altitude data or only those with barometric altimeters?
What is available on GPS units keeps changing, but I can tell you that the GPS technology does not require barometric altimeters to tell you your altitude. GPS units use a set of satellites in extremely precise orbits to tell your position. The unit listens for several satellites which basically send out the current time down to the millisecond (or more). The unit times when the various signals arrive, and from that determines how far it is from those particular satellites. If it can hear four satellites, it can deduce your exact position on the earth in three dimensions (though more satellites will increase the precision.) Some GPS units store topo maps inside, so it can deduce your elevation from your two-dimensional position. But increasingly they can determine your elevation directly, the same way they determine your latitude and longitude. Barometric information is still useful, particularly for predicting the weather.
