Mangled 3d file

[Bill Gee]

Hi Olly -

Most surveys in the United States have adopted a standard tolerance of one 
degree for both compass and clino.  If the forward and backward shots are 
within one degree of each other, it is an acceptable reading.  

For some individual shots greater tolerance is needed.  Getting a good clino 
reading when shooting uphill can often be a challenge.  In those situations 
the survey team will often circle one of the readings as preferred.

I am probably not using the *sd directive as it is intended.  I don't really 
care about the mathematical standard deviation.  Indeed, I cannot imagine what 
use it would be in this situation.

My REAL goal is to reduce the number of warnings to some manageable and 
informative level.  I don't want to see a warning just because the forward and 
backward shots are 1/2 degree different.  That is well within our tolerance 
range.  I want to see the shots where they differ by more than the tolerance.

For the Allie Spring Cave data, I found about a half-dozen legs where I had 
entered the data incorrectly.  Most often I forgot to put in the minus sign on 
a clino shot.  Two positive clino shots is a sure sign of a problem!  These 
bad survey legs are already corrected in the data file you have.

The survey leg you mentioned with a difference of 7 degrees needs to be 
checked.  I will be in the cave this weekend and will try to verify it.  The 
remaining survey legs are probably OK.

Perhaps we should handle this by adding an item to the Survex wish list.  It 
would be handy to have an directive like *tolerance <reading> <number> <unit>.  
For example:

*tolerance clino backclino 1.0 degrees 
*tolerance compass backcompass 1.25 mils

Then any survey leg which exceeds the tolerance is flagged with a warning.  

This would be useful in Therion, too.

Thanks - Bill Gee


On Wednesday, March 04, 2015 02:16:21 Olly Betts wrote:
> 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