fixed points in survex

[John Halleck]

On Thu, 6 Feb 2003, Lev Bishop wrote:

> Date: Thu, 06 Feb 2003 20:59:16 -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:
> 
> > On Thu, 6 Feb 2003, John Halleck wrote:
> > 
> >   And, to address some thermal effects...
> >   If the air blowing down the passage is warmer than the walls, there
> >   is a thermal (and therefore density) gradient that bends line of
> >   sight towards the walls.  And if the air blowing down the passage
> >   is cooler than the walls then there is a gradient bending line of
> >   sight towards the center of the passage.  (These conditions can, 
> >   of course, occur in any periodicly breathing cave)  This effect
> >   can EASYLY exceed 1 ft/mile of line of sight bend.
> 
> I believe your numbers and I would not be surprised if errors 10 times as
> big weren't possible in extreme cases (where different bodies of air come
> into contact and mix with each other, for example near to the entrance of
> the cave (heat-shimmer can be visible to the naked eye there), or even
> when the bodies of the surveyors set up thermal gradients in the cave (or
> they're using carbide lights)). However, the curvature is proportional to
> the square of the length of the leg, so 1ft/mile-squared, for a typical
> leg of 40' length and even taking 10 ft/mile-squared, the effect is still
> under a 100th of an inch.

  I stand corrected for my careless use of units.

  I would believe 10 times as much.  (The thermal effects overwhelm the
  normal isothermal gradient on an average day for an average land survey,
  which is why I stated that they could easily be of that magnitude. 

> Lev

  But, of course, the effects of not correcting for tape sag overwhelm
  many of the effects that have been discussed in this thread.
  How many cave surveyors do that correction?  (And of those that do,
  how many do the tape temperature correction?)