Mangled 3d file

[Olly Betts]

On Wed, Mar 04, 2015 at 01:22:54PM +0000, Wookey wrote:
> +++ Olly Betts [2015-03-04 02:16 +0000]:
> > 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.
> 
> The tolerance used on Erin's China trips is 2 degrees, so most
> readings are within 2 degrees.
> 
> As you show, this doesn't exactly match the *SD-related warning,
> although it's similar.  I wonder if one should change the SD to fit
> the 'expected' tolerance, or add an explict 'tolerance' warning
> threshold? The two numbers are related but not actually identical.

I think the current 2 sigma test isn't actually the right test, since
with a correctly specified SD it's actually expected to have a fairly
high false positive rate, and that isn't helpful.  I've changed that
to 3 sigma in commit 4dc9899, which should reduce the noise.

A way to specify the tolerance required directly seems clearer than
forcing jiggling of SD values, and at least provides a direct way
to tell cavern about a project policy.  But if we're just thinking
about it as a way to stem the flood of warnings, if the SD values are
set appropriately, then the warnings ought to be appropriate, so the
change to 3 sigma should cut down the noise to a more useful level.

As point out below, an appropriate tolerance may vary with other
readings on the leg.  Perhaps the tolerance should be specifiable as an
angle or in "sigmas", with the default being "3 sigmas".  That would
allow for the current approach or for automating a project policy on
the tolerance.

> A large project inevitably collects a lot of fore/back sights that are
> out of tolerance over time, and thus compiling a big system gives a
> lot of warnings, which obscures genuine errors in the new survey you
> just added. Perhaps just having an option (like gcc) to turn off
> specific warnings is one way to deal with this?

Perhaps an optional flag per leg indicating how to resolve fore vs back
with options like average, quiet average, use only fore, use only back
- this provides the ability to "ring the preferred reading".  I guess
it would need to be specifiable for clino and compass separately.

> > When surveying with sighting instruments using backsights, what
> > threshold is generally regarded as acceptable?
> 
> Ask on an American survey list. But I'm familiar with 2 degrees, Bill
> says 1 degree on his project. 
> 
> For actual SDs I'd say that (well-calibrated) distoXs have better SD
> than sighting compasses, especially at high angles, but if you end up
> with a pair that are not well-calibrated it can be a struggle getting
> to 2 degrees.

Perhaps not so different to two non-calibrated sighting compasses.

> (Sighting compasses should strictly have an SD function
> that takes into account the clino angle (as opposed to a constant)- so
> there is a step at 15 degrees where it starts to stick, then it rises
> to several degrees at +89 clino).

There's actually a formula in use which effectively increases the
expected compass error for steeper legs (in "normal" style), but it
currently does its job when we calculate the covariance matrix, so the
SDs used for the tolerance check are currently exactly those specified
with *SD (or the defaults).

The formula doesn't give a sudden step, but I think a step is probably
not really desirable, as it's building in an assumption about the type
of instrument in use (I've not used one, but I suspect the brunton style
of compass behaves differently), and also in how it is being used.
With the suunto/silva style of sighting compass you can get a decent
reading on really quite steep legs by holding the tape (or a string) on
the stations (or as a plumb from the upper station if it's a free hang
below) and sighting to that.

Note that by the time you get to +89 degrees clino, errors in the
compass reading have become much less relevant.  In the worst case
(180 degree compass error), it's the same effect as a 2 degree clino
error.  This presumably means there's a worst possible angle to read
your compass at somewhere in between.

Cheers,
    Olly