Mangled 3d file

[Olly Betts]

On Tue, Mar 03, 2015 at 04:53:44PM -0600, Bill Gee wrote:
> Wow, that is interesting!  I set the SD because otherwise cavern issues a 
> warning on every set of readings that do not match exactly.

The default isn't to require an exact match - the tolerance of the check
is based on the SDs set for the fore and back instruments, and the
default SDs are 0.5 degrees, so the test is based on a combined SD of
0.707 degrees (based on the assumption that the fore- and backsight
errors are independent).

The check currently seems to be if we're outside of 2 SDs, which if the
SDs are correct you'd expect to be true for 5% of non-blundered
readings:

http://en.wikipedia.org/wiki/68%E2%80%9395%E2%80%9399.7_rule

So that translates to a default threshold of 1.414 degrees for the
difference.  Unless you read the instruments to better than 0.5 degrees,
that means the threshold is effectively 1 degree.

> I don't want to see 200 warnings for readings that are within the
> tolerance we chose.

Wookey also commented a few days ago that these warnings were very
noisy.  I've not dealt with much data involving backsights, so it would
be useful to get feedback if these warnings are too readily triggered.

On the 107 legs in AllieSpringCave.svx, the default SDs give 35
warnings, though some of those look like genuine issues (e.g. a 7 degree
difference).  It seems to be noisier than expected, probably partly due
to the effective threshold being lower.  Perhaps also 0.5 isn't entirely
realistic for the SD of a sighting compass reading.

I also suspect a 2 SD check probably isn't the right criterion anyway,
as even 5% false positives is unhelpful.  A 3 SD check would be expected
to give a false positive about once every 370 readings (or 185 legs),
and for the default SDs would mean a threshold of 2.121 (effectively 2)
degrees.

When surveying with sighting instruments using backsights, what
threshold is generally regarded as acceptable?

> I set it for only the forward readings because I assumed cavern would apply it 
> to the backward reading as well.  It is, after all, looking at BOTH readings 
> to determine whether a warning should be issued.

In general the two sights may be made with different instruments, so
Survex allows you to specify the details of those instruments
separately.

And it is because both readings are involved that the SDs of both
readings matter.

Cheers,
    Olly