Referencing STC in VOTable: IDREF problem

[Francois Ochsenbein]

Hi Mark,

Thank you a lot for your remarks (a carefull reading by new eyes
is always useful). Of course I fully agree with the "good practices"
asessment -- I did use a simple validator (XSV) but apparently this
one does not really care about the ID/IDREF rule...

I see essentially 2 (easy?) solutions which could work:

1. Use Arnold's proposed way with 
   xlink:href=ivo://STClib/CoordSys#UTC-ICRS-TOPO" and 
   xlink:href=ivo://STClib/CoordSys#UTC-ICRS-TOPO",
   as it is used in VOEvent; it requires an additional
   namespace (and is not always understood by validators)

2. Specify the frame in a parameter with 
   utype="stc:AstroCoords.coord_sys_id"

The alternatives would look like:

1. xlink usage:
    <GROUP ID="Coo1" utype="stc:AstroCoords"
      xlink:href="ivo://STClib/CoordSys#UTC-ICRS-TOPO"
      xlink:type="simple">
      <FIELDref ref="RAJ2000"/>
      <FIELDref ref="DEJ2000"/> 
      <!-- etc -->
    </GROUP>
    in the first example, 

2. Specification via the "coord_sys_id" of the STC schema:
   <GROUP ID="Coo1" utype="stc:AstroCoords">
      <PARAM name="cooframe" datatype="char" arraysize="*"
         utype="stc:AstroCoords.coord_sys_id"
         value="UTC-ICRS-TOPO"/>
      <FIELDref ref="RAJ2000"/>
      <FIELDref ref="DEJ2000"/> 
      <!-- etc -->
   </GROUP>

Although I would prefer the 2nd alternative (full compatibility with
the VOTable schema without additional namespaces), I'm open to 
other opinions...

Merci, Francois

>
>On Tue, 16 Sep 2008, Francois Ochsenbein wrote:
>
>> 1. http://www.ivoa.net/internal/IVOA/IvoaVOTable/note_stc_v0.8.pdf
>> 	proposed note on "Referencing STC in VOTable"
>
>Francois,
>
>there is a problem with the scheme outlined in this Note.
>
>Section 4 says that the ref attribute of the second required GROUP
>element may reference an STC 'library' coordsys by using an
>identifier along the lines "ivo://STClib/CoordSys".
>
>This practice is exemplified in the Example in section 5.1:
>
>    <GROUP ID="Coo1" utype="stc:AstroCoords"
>    ref="ivo://STClib/CoordSys#UTC-ICRS-TOPO">
>
>However, since the GROUP element's ref attribute is defined in the
>VOTable 1.2 schema with a type of "xs:IDREF" it is syntactically
>restricted in what characters it can contain: in particular non-XML-name
>characters such as "/" are illegal here.  Hence this example cannot
>be valid against the VOTable 1.2 schema.
>Even apart from the syntactical issue, I don't *think* that the ID/IDREF
>mechanism can perform crossreferencing between different documents,
>here the VOTable instance and the STC library, which is what appears
>to be intended here, though it may be that my understanding
>of that issue is wrong (can anyone else confirm/deny?)
>
>The exact same problem afflicts the example in section 3.1 of the
>VOTable 1.2 document draft.
>
>A rethink is required.  One possibility would be to keep the same
>general idea of referencing STC library documents, but to make it
>informal rather than enforced by the ID/IDREF mechanism.
>I recall however that Arnold was not keen on this approach when a
>similar solution was suggested to the problems arising from
>concatenating STC documents a year or two ago.
>
>I have two more general comments:
>
>    1. I would consider it good practice when writing a document such
>       as this to validate any examples quoted against the schema prior
>       to publication.  As well as the issue above, there are a number
>       of other errors in the first example of the Note which validation,
>       and in some cases even XML parsing, would have picked up
>       (stray </FIELD> close tag, missing IDs for the 'second referencing'
>       refs).  I haven't checked the other examples.
>
>    2. The approach outlined by this Note is quite complicated, though
>       perhaps that is unavoidable given the job it is trying to do.
>       Even assuming the problem I have identified above will be fixed,
>       I am not therefore very hopeful about seeing applications capable
>       of making sense of VOTables containing coordinate data in this form.
>       I would of course be quite happy to be proved wrong about that.
>
>Mark
>
>--
>Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
>m.b.taylor at bris.ac.uk +44-117-928-8776 http://www.star.bris.ac.uk/~mbt/
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