[TAP] data type for column metadata

[Patrick Dowler]

On 2009-03-31 14:49:16 you wrote:
> Is the intention now for the TAP engine to do spatial transformations?
> For example: "Select points from the X catalog (in B1950 as seen from
> Jupiter) which are in the Y region (in heliocentric supergalactic
> coordinates)".

This is absolutely not the intention -- in fact it is the opposite: if we 
wanted to require services to do transformations we would expose nothing and 
the query language would have to be able to carry all that metadata instead. 
It is the presence of coordinate system in ADQL (REGION et al) that implies 
transformations are possible (and lack of ability to do so an error), not 
anything in this discussion.

The intention is simply to say (to the service) "if you know these things and 
want to tell users about them, do it in this way" and (to the user) "if you 
care about these details, you can look for them here".

Anyway, this is not a proposal; it is simply a description of what needs to be 
included to more or less correctly describe the values *in some columns*. The 
alternative you are implying (just derive meaning from the column name, maybe 
have some documentation somewhere) is also a viable choice, and it would work 
well enough for direct use of a single TAP service. However, it would then be 
very hard to write applications that help people use TAP services more 
effectively, specifically to help them write queries. 

PS-Of course, most of this metadata applies to ADQL query writing; in PQL the 
param specification imposes limitations on values and systems and that does 
require that services implement some transformations (if their content is not 
in the standard system). 

-- 

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