fixed points in survex

[Lev Bishop]

On Fri, 7 Feb 2003, John Halleck wrote:

>   Normal tape sag is an effect that affects accuracy more than the
>   accuracy that some folk *claim* that they measure to.
>   I think that a sag correction (with standard assumptions) would be
>   a lot closer to reality than what I see in cave surveys.
>   Unlike the 1" or 2" sorts of effects discussed earlier, this is
>   an effect that does make a difference.

OK, I see what you're saying. Yes tape sag is a much bigger effect than
some of what we've been discussing, and there are certainly some people
think they can read accurate to 1cm over a 20m leg without taking it into
account. Not sure about a sag correction with standard assumptions - I 
suppose it wouldn't make things worse but I'm not sure it would make 
things better - you'd have to assume values for the tension in the tape, 
the mass per unit length of the tape, and the tape's elasticity. For the 
tension in particular you'd need some kind of protocol to ensure any 
assumption be valid, and as I said, any mud or water could change the 
mass/length of the tape by a factor of 2 or so.

Lev