I was watching an old TT on discovery earlier today. They were looking for an older trench, but couldn't fnd it. That irritating git Robinson started taking the piss out of GPS and said something about tape measures never failing. Then the geophys bod happened to say that the old GPS reading was 5 years old and since then the US gov't have removed the scrambling error so the old readings could have been a long way out. It turns out that a long way is 20m or so!

So, no GPS reading taken more than 4 years ago is worth shit :-)