[Therion] standard deviation and survey grade

[Olly Betts]

On Wed, Oct 29, 2008 at 05:43:01PM +0100, Xavier Pennec wrote:
> The value retained by survex and therion correspond to 3*sigma (i.e. 
> 99.7%) although it is said to be 95% in the comments.

At least for Survex, that was fixed over 6 years ago:

http://trac.survex.com/changeset/1931

So either you're looking at a rather old release, or I missed some
places with that fix (if so, please point out where!)

> They do correspond 
> to the BCRA definition of these grades 
> (http://bcra.org.uk/surveying/index.html) except for the angular 
> measures in grade 3 which is 10 degrees instead of 2.5 degrees:

That was also corrected by the change above.

> Thus it seem that Therion's grade 3 definition should be updated (and 
> 95% changed to 99.7% in the comments). In practice, however, I found 
> that using this definition of grade 3 for unreliable surveys is quite 
> good as the tape reading is usually very easy (it is resonnable to 
> assume that it is within +/- 0.5m) while compass and clino measurements 
> can be easliy wrong by 5 to 10 degrees for untrained people. Thus, I use 
> in practice:

The BCRA grades generally seem harder of compass readers than tape
readers, but I think it's confusing to keep a "bcra3" grade which
doesn't actually match BCRA grade 3.

That said, it would be interesting to come up with some sets of expected
SDs for "real world" measurements for various instrument types.

As you say, reading the tape to BCRA accuracy is much easier than the
angular requirements.  The required station position accuracy is fairly
easy to achieve too, at least if you choose stations with it in mind
(i.e. make sure you can get your reading eye next to them, or offset
from both stations to compensate).

Clino and compass being the same is a little dubious too since magnetic
effects (from equipment, minerals, and fluctuations in the Earth's
field) affect only the compass, and also the force on the card is much
weaker than gravity, so the reading settles less decisively on the
compass than the clino.

Cheers,
    Olly