fixed points in survex

[John Halleck]

On Fri, 7 Feb 2003, Lev Bishop wrote:

> Date: Fri, 07 Feb 2003 12:17:14 -0500 (EST)
> From: Lev Bishop <lev.bishop@yale.edu>
> To: survex@survex.com
> Subject: Re: fixed points in survex
> 
> 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.

  True enough.  But what we are doing now already makes such assumptions.
  It just assumes a weightless tape with no stretch.

> Lev
> 
> 
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