Defaults in STC

[Guy Rixon]

"Unknown" is, technically,  a null value.  XML has a specific way of
expressing nulls: the nilable attribute in the schema and the nil (Sp?)
attribute in the instances.  I suggest that we use this rather
than just writing a value of "Unknown".

On Thu, 7 Oct 2004, Anita Richards wrote:

> > There are two kinds of uncertainties, especially for historical data.
> >
> > One is "I don't know exactly when this plate was taken - it was in
> > 1925 or 1926".
> > In principle, the observing time range, coupled with the observatory
> > location, should give one a huge uncertainty in Doppler velocities, so
> > no special provisions are needed.  And inaccuracies in, say, positions
> > can be handled through the error elements.
> >
> > The other is "unknown": "I don't know whether this velocity is radio
> > or optical definition, etc".
> > I think the only reasonable way to handle this, if we must, is to
> > allow "Unknown", "Not available", or something like it, as an allowed
> > value.  It does put the bruden on the user to figure out (a) whether
> > the data are still useful, and (b) if needed, what would be a
> > reasonable guess for a default.
> >
> > So, somewhat reluctantly, I am leaning toward accepting the concept of
> > "Unknown" as allowed value for a number of coordinate system elements.
> > If I understand your previous message correctly, you would agree with
> > that solution.  Correct?
>
> Thanks, yes, that is all exactly what I was trying to say, with the added
> proviso that (as you mention in the original document), e.g. velocity
> convention = "unknown"  would cause problems for software using the model
> to deduce redshift from velocity _unless_ the application software
> intrpreted "unknown" as e.g. optical convention and added the appropriate
> error; knowing a redshift to a low accuracy might still be very useful.
>
> That would be outside the province of the STC model but is something whcih
> people writing applications could take care of.  Does that sound sensible?
> (i.e. caveat emptor, and your model stays pure :)
>
> cheers
>
> a
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> Dr. Anita M. S. Richards, AVO Astronomer
> MERLIN/VLBI National Facility, University of Manchester,
> Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, U.K.
> tel +44 (0)1477 572683 (direct); 571321 (switchboard); 571618 (fax).
>

Guy Rixon 				        gtr at ast.cam.ac.uk
Institute of Astronomy   	                Tel: +44-1223-337542
Madingley Road, Cambridge, UK, CB3 0HA		Fax: +44-1223-337523