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

[Wookey]

On Sun 09 Feb, Lev Bishop wrote:
> On Sun, 9 Feb 2003, Matt Ryan wrote:

> 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.

I recall a paper by (IIRC) Irwin and Stenner from the late 1960s which I
believe covers this in some detail, including a load of sums. I think this
gives a good idea of the actual likely error values. I'll try and dig it
out. 

> 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.

I'd agree with that.

> > 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.

We used a current generation disto on the mulu trip I got back from
yesterday <big grin>, and I have to agree that it is bloody marvelous. I'm
going shopping for one ASAP. It's particularly good for surveying in the
jungle as you don't have to keep rethreading the tape the other side of
trees. Also perfect for passage dimensions, especially in big passage.
Andit's much harder to break the end off :-)

Wookey
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