Dear Bobi

I recall reading that the vertical GPS accuracy was twice the horizontal accuracy (as stated in real time on the GPS unit) i.e. if the GPS is giving the position to within +/- 6m horizontal accuracy it should be +/- 12m on the vertical accuracy. I can't recall the source but I'll see if I can dig it up. Either way, GPS units aren't teribbly accurate for topographic surveying, even now that the selective availability signal has been turned off. You may get better results using the pressure altimeter (which I believe are accurate to +/- 2m on the Etrex Vista, although again I'll need to track down the source on that one). You'll need to use a differential GPS approach (i.e have one static and one mobile station) to counter the effect of pressure variation (which from my experience can be significant).

Good Luck

Michael

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