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

[Lev Bishop]

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. 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