Kemo, One engineer in the States that the large paving (Highways) contractors use, says his specialized GPS system is accurate to within 1 inch on all three dimensions (over what distance is another question). His system costs about $40K I'm told. It appears accuracy varies depending on cost and type of system as well as all the other factors Sculp has spoken about. My aviation GPS is accurate to about 1-3 feet on approaches I'm told. JW