Defaults in STC

[Arnold Rots]

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?

Cheers,

  - Arnold

Anita Richards wrote:
> On Tue, 28 Sep 2004, Arnold Rots wrote:
> 
> > Defaults are a nasty issue.  I have some sympathy for your argument,
> > but don't agree.
> >
> > Most HI data repository keepers DO know what conventions were used.
> 
> If only! that was just an example, there are others.
> 
> > If we allow data providers to specify "default", they will use it,
> > because the conventions are so obvious to them, in their own context.
> 
> We should make it encouraged to be specific where ever possible, and for
> new data or where data providers do knnow 'obviously'' then it is just as
> easy to type LSR as N/A, so I think that people will be accurate if it is
> little effort.  However where the rlevant information is lost in the mists
> of time I think that we will just not get the data and that would be a
> shame, unless we allow defaults.
> 
> > In many cases it might not matter, but people will have forgotten what
> > they were doing and not realize that there are datasets where they do
> > need to be more specific.
> That's just it - in many cases it is entirely forgotten!
> 
> > For instance, there is the infamous example of the HI absorption line
> > that had disappeared in a WSRT observation in the early 70s - until it
> > was realized that the velocity that had been specified was optical and
> > the online frequency calculation assumed radio: as a result, the line
> > was completely out of the band.
> >
> Been there, done that (got a lovely radiocontinuum image instead of a
> megamaser).
> >
> > What I am trying to say is that it is usually trivial for the data
> > providers to be explicit and that allowing defaults in cases where it
> > does not matter will lead to their being used in situations where it
> > does matter.  Local defaults are usually known - why not insert them
> > explicitly in the STC elements?
> Absolutely, where things are known.  But I am aware from my own attempts
> to track down systemic velocities with enough accuracy to observe
> megamasers in a 16-MHz b/w (needing accuracy of a few parts in
>  10^6) that in many cases it requires extensivedetective work, looking at
> other observations with the same system/papers by the same author which
> can be cross-referneced to objects with well-defined velocities etc. etc.
> I am suggesting that in such cases the data are probably very useful e.g.
> for comparisons, variability etc. even with a poorly-known reference frame
> and if there wa some sort of warning (like the scoring of metadata
> accuracy) then if the user did need exact absolute velocities they
> would
> have to take the risk.
> 
> Heinz Andernach (e.g. 1999irpa.conf....1A) has identified a large number
> of candidates for
> electronic publishing among radio and other catalogues going
> back decades and I am sure many of them will not have full metadata, and
> that the problem for historic optical data is probably even worse.
> 
> cheers
> Anita
> 
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> 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).
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head.cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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