Tape Sag [Was: Re: fixed points in survex]

[John Halleck]

On Sun, 9 Feb 2003, Lev Bishop wrote:

> Date: Sun, 09 Feb 2003 15:23:58 -0500 (EST)
> From: Lev Bishop <lev.bishop@yale.edu>
> To: Matt Ryan <matt-lists-survex@mdryan.net>
> Cc: survex@survex.com
> Subject: Re: Tape Sag [Was: Re: fixed points in survex]
> 
> On Sun, 9 Feb 2003, Matt Ryan wrote:
> 
> > Perhaps I'm missing something here on tape sag, but aren't most tapes, at
> > least those intended for horizontal use, supposed to be calibrated at a
> > known tension - rather than at zero tension as you guys seem to be
> > suggesting.  The theory I thought being that you pull the tape to the
> > calibrated tension and read it at that.
> 
> I haven't looked into it in any detail but my understanding is that you
> are correct, in that the tape is calibrated for a particular tension and
> at horizontal (so it is not supporting its own weight too). However, I
> would assume that it is resting on a smooth horizontal table or whatever
> for this calibration, so that there is no sag effect and it isn't
> supporting its own weight in that way. I haven't done the calculation but
> it seems intuitively obvious to me that the effect of having it sag freely
> will change depending on the length of tape in use, so a constant
> calibration factor wouldn't be able to account for it.

  Nobody said anything about a "constant" calibration factor.

  But most survey books give the equation.
  And the manufacturers give the standard constants for their tape, that
  you can plug into the equation.

>. If no one else does
> it in the meantime I may have a go at checking what order of magnitude the
> whole tape sag effect is but I'll have to remember how to do calculus of
> variations for catenaries and such first. From personal exeperience with
> fiberglass reinforced tapes I know that by varying the tension in a say
> 20m leg one can vary the length by a few cm.
> 
> > A nice piece of work Lev - now I've had chance to read it properly.
> 
> Thanks.
> 
> > 200 GBP for a Disto-lite laser rangefinder apparently.  Tempting...
> 
> Very. For the type of surveying I've done a laser ranger is a really big 
> help.
> 
> Lev
> 
> 
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