Backsights and default accuracy estimates

[Martin Green]

----- Original Message -----
From: "John Halleck" <John.Halleck@utah.edu>
To: "Olly Betts" <olly@survex.com>
Cc: "M.J. Green" <mjg54@hermes.cam.ac.uk>; <survex@survex.com>
Sent: Tuesday, May 28, 2002 11:06 PM
Subject: Re: Backsights and default accuracy estimates

>
> Forgive my slow reply, I've been out of the country for about 5 weeks...
>
> > On Thu, May 09, 2002 at 07:05:04PM +0100, M.J. Green wrote:
> > > Strictly with the grade 5 BCRA limits, errors should be called for
greater
> > > than 2 degrees, for fore sight to back sight discrepancies.
>
>   Fore and back site discrepancies of much larger than that can be
>   *CORRECT* for places with large magnetic problems.  Such data can
>   be processed properly to distinguish errors from magnetic
>   anomolies.  (As I've written up before.)
>

Survex does not deal with magnetically annomalty problems, therefore it
should call up an error, if such data is encountered.
In these senarios the error along a chain of reading increasing to the power
1.5 with no. of stations, due to the errors in each direction measurement
adding to all the previous errors(to the power 0.5 as independant) and the
errors of where the leg started( to the power one as dependant), rather than
to the power 0.5 in the case of each leg being measured with respect to
magnetic north.  If anyone impliments code for magnetic annomily caves, they
need to be careful because of this.  Places where fore sights and back
sights are taken should not be treated in the special magnetic annomily way,
unless there is evidence to suggest the magnetic field is doing funny
things, otherwise the errors will unescesarily explode.  Sorting out the
errors may be tricky in terms of loop closurers.
How bad can magnetic variations get?  How quickly do they vary?  Do many
caves suffer from this?  Are entire caves affected or only part of caves?
It may be that if the magnetic fields are only devitated slightly, then the
errors assosiated with the fore sight backsight method would become
comparable to assuming that north does not change, over the size of a
moderate cave system.  Particularly if any pitches are involved, where the
magnetic readings become rather poor in my experience.
Perhaps you have already mentioned some of this in the article you wrote, I
read it a while ago so can not remember.

> [...]
>
> > > Perhaps this could be defined so that 2 degrees is when it shouts.
>
>   If you are going to shout, then a figure based on the SD's that you
>   expect seems cleaner.    For extreme example, if the instrument I'm
using
>   is my naked eyeball without an instrument, and I have (for example)
>   10 Degree SD, then a 2 degree discrepancy is not remarkable.
>   If I'm using a Total station then a few minutes of arc descrepancy
>   might possibly be appropriate.
>
> > [...]
>
>

This remark was made with regard to when survex should complain about
discrepancies when surveying to grade 5 and there are no magnetic
annomilies.  The problem being the definition of angle measuremnts to grade
5 standard, are that they are not greater than 1 degree out.  It does not
mention anything about gaussian distributions, so I was using a cyclic
argument about how to determine how many standard deviations to complain at,
given I am uncertain about how big one standard deviation should be if the
only info is the survey said it was done to grade 5.  This would make the
surveying software consistent with the grade 5 standard, in the fore sight
back sight no magnetic anomily senario.
Further E-mails that I wrote, mention other ways of deciding how to tell
what 1sd in a grade 5 survey might be.

Martin