XML: was Bedding planes, rose diagrams and bug fix (fwd)

[Lev Bishop]

On Wed, 3 May 2000, John Halleck wrote:

> 1) Covariance of the instrument observations.  A symmetric 3x3 matrix, so one
>    only needs to store 6 numbers.
>    This is generally the same for an entire cave, but can change from shot to
>    shot in truely bizarre surveys.
>    99.9% of the time it is the same for an entire survey... so I'd consider it
>    a candidate for
>    <sometag D="..." A="..." I="..." DA="..." DI="..." AI="..."> survey </sometag>>
>    style marking.
>    [I'm assuming Distance Asimuth Inclination as was is measured.]
>    (D, A, and I are manditory, DA, DI, AI are optional)

Under what circumstances will the off-diagonal elements ever be nonzero?
If you are measuring with seperate compass, clino, tape then you have 3
independent measurements. Perhaps if you have some kind of instrument
which relies on a platform that needs levelling, or you are electronically
gimballing your compass with you electronic clino, the AI element could be
nonzero, but I can't think how DA or DI can be nonzero.

However, I can see a few occasions under which the covariance matrix will
be different for each shot. For example, if the instruments are
electronic, they might output an estimated accuracy with every shot (the
leica disto will do this). Also, you might want to take account of the
fact that off-horizontal compass shots are likely to be less accurate,
perhaps with a 1/cos(I) weighting, or somesuch.

> 2) Covariance of the adjuted points.  Roughly the same as the above in form,
>    but with
>    <sometag X="...", Y="...", Z="...", XY="...", XZ="...", YZ="...">

Except, that in this case the covariance matrix will certainly be
different for every point.

L


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| Lev Bishop            The Queen's College, Oxford, OX1 4AW     |
| Physics student                                                |
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