fixed points in survex

[John Halleck]

On Fri, 7 Feb 2003, Lev Bishop wrote:

> Date: Fri, 07 Feb 2003 01:14:32 -0500 (EST)
> From: Lev Bishop <lev.bishop@yale.edu>
> To: John Halleck <john.halleck@utah.edu>
> Cc: Erin M. Lynch <elynch@cds.caltech.edu>, survex@survex.com
> Subject: Re: fixed points in survex
> 
> On Thu, 6 Feb 2003, John Halleck wrote:
> 
> > I personally think that the "clean tape" sag correction on the other
> > hand would be a lot closer to reality on a dirty tape than the 
> > totally uncorrected values being used now.
> 
> One way you could do a better sag correction is if you took 2 separate 
> tape measurements, with 2 different tensions in the tape. You'd need some 
> kind of tensioning device (I'm thinking of a spring-loaded puller thing 
> the tape end man can use - should be pretty low-tech and reliable). With 
> the two measurements you could correct for tape stretch and sag quite 
> easily and even remove "dirty tape" effects to first order. Nice in theory 
> but I can't imagine anyone doing it - cave surveying is painful enough as 
> it is without needing even more numbers to be recorded. If you really care 
> that much about accuracy in your length measurements its probably a better 
> idea to buy a laser rangefinder now that they're somewhat affordable.
> 
> Lev


  That sounds like overkill for sure.

  But, I think I'm not being clear.

  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.