Defaults in STC

[Anita Richards]

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).