content, format, ctype, or xtype ?

[Patrick Dowler]

On Monday 04 May 2009 07:49:17 Rob Seaman wrote:
> Surely we don't need another inscrutable meta-meta-concept?!?

This is not intended to be a metadata concept at all, just an indication of 
the format a value is expressed in. 

As far as I am aware, a FIELD has one utype attribute and a service with some 
data model will need to use that to describe it's content. That model could 
have multiple dates in it, or multiple positions: just specifying the STC 
utype is not sufficient to describe this. Now, if there is a way to connect some 
data model with STC so that the utype has both and the STC part can be 
recognised, then maybe that is OK. 

As it stands, I don't think it is good for the TAP spec to specify a usage for 
the utype attributes or somehow require a specific usage to express low level 
metadata... but this has already been asked and I think not answered:

Rick Wagner wrote:
>Pardon any ignorance on my part, but I don't see how we can use  
>utypes to define both the format of a FIELD, and its relationship to  
>a data model. For example, how can I specify that a FIELD represents  
>the execution time of a simulation using  
>utype='simdb:Simulation.ExecutionTime', and that is in ISO format,  
>utype='stc:AstroCoords.Time.TimeInstant.ISOTime'?

Rob Seaman wrote:
>...and I'll second Rick's question.  Can a single quantity have more  
>than one utype?  What would this mean?

Sure, we could jam both of those in there, but it would be ugly:

utype='simdb:Simulation.ExecutionTime at stc:AstroCoords.Time.TimeInstant.ISOTime'

where @ is some arbitrary separator chosen to cause the least pain and  
suffering. Ugh.

There is a fundamental question here": how does one "use" a data model inside 
another data model? We don't know the answer to that, but I am also pretty 
sure TAP is not the place to answer it. However, since we don't know the 
answer, we don't even know if it is safe to defer to some other standard. 

Even then, suppose there is some way to combine all the above metadata. It 
*still* does not specify the format of the content. It could still be STC-S or 
STC-X (elements from the stc namespace) or some other format. So I still think 
we need a format/content attribute for FIELD (and PARAM) so applications can 
grok the content (or happily not do so if they are not capable).

-- 

Patrick Dowler
Tel/Tél: (250) 363-0044
Canadian Astronomy Data Centre
National Research Council Canada
5071 West Saanich Road
Victoria, BC V9E 2M7

Centre canadien de donnees astronomiques
Conseil national de recherches Canada
5071, chemin West Saanich
Victoria (C.-B.) V9E 2M7
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ivoa.net/pipermail/dal/attachments/20090504/8f46d151/attachment-0003.html>