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

[John Halleck]

On Sun, 9 Feb 2003, Matt Ryan wrote:

> Date: Sun, 09 Feb 2003 12:25:37 +0000 (Asia/Chongqing)
> From: Matt Ryan <matt-lists-survex@mdryan.net>
> To: survex@survex.com
> Subject: Tape Sag [Was: Re: fixed points in survex]
> 
> 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

  No, they read direct distance.
  (If they were just to be read directly at a specific tension
  the feet would be different lengths, and they aren't.)

  The "standard" tension is to use a standard correction factior.

> suggesting.  The theory I thought being that you pull the tape to the
> calibrated tension and read it at that.  Obviously using anything other
> than a brand new clean tape is going to have a huge effect, as are
> anything other than horizontal legs.  And then there's also the fact that
> at very best people just guess the correct tension.

  True.

  But enough theory.   Go out and try it.
  Measure a 40 foot distance with the tape on the ground, and with
  it off the ground.  A bigger difference than some folk (with a
  straight face) tell me that they measure to.
  [ The easy test is to measure the corners of a building, both
  by laying in on the sidewalk from corner to corner, and by  doing 
  the same measurement at normal eye level.
 
> A nice piece of work Lev - now I've had chance to read it properly.

  I agree..  Lev's paper was one of the best reads I've seen on
  the list in a long time.

> 
> 200 GBP for a Disto-lite laser rangefinder apparently.  Tempting...
> 
> -Matt
> 
> -- 
> Matthew Ryan
> matt@mdryan.net    <-- Please note my new e-mail address
> www.mdryan.net
> 
> 
> 
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