About GPS altitude: This is an output from the GPS receiver. In consumer-grade GPS units this is never very accurate. This isn't because the GPS calculates altitude (the vertical plane) inaccurately (although it's not as good as in the horizontal plane), it's because the globe is not a perfect sphere - far from it. So, in order to get an accurate altitude you need an accurate globe model, and consumer grade GPS units simply don't have that. What you _could_ do, and some GPS programs do this, is to have a look-up table for your local area/areas with correction factors. For example, where I live the error is around 43 meters or so, a few hundred km south it's more like 35.