Comments on SSAP V0.95, UCDs

[Doug Tody]

Another point - as I mentioned to Randy earlier, we may want to consider
treating the observation position as a special case, and giving these
dedicated fields in the query response rather than digging them out
of characterization.  Perhaps this is more what you were asking.
One would like to avoid redundancy, but we already did something
similar to this in SIA 1.0, where RA/DEC were specified separately from
the WCS, which includes somewhat redundant positional information and
in any case is optional.  In either case I think everyone agrees we
need to ensure that the most fundamental metadata is easily available
in a standardized fashion.

The real issue with the observation position is coordinate systems.
It is not clear that we can afford to continue to limit ourselves to a
fixed equatorial coordinate system for queries.  In this last interop
we had solar observers starting to use IVOA data access interfaces,
and there has always been interest in supporting galactic coordinates
as well as equatorial.  We should consider making more use of the
reference frame when specifying coordinates; this may be a case of
making things too simple.   - Doug


On Tue, 27 Jun 2006, Doug Tody wrote:

> Hi Gretchen -
>
> This is NOT a complex hierarchical interface.  The idea with the query
> response is that it is a flat table; each field has a UTYPE and other
> tags (name, ucd, etc.).  UTYPE is just a simple fixed string, so to
> access a field one can just do something like  getField(<utype>).
> Hence you still have a simple direct reference, although there may
> also be some logical structure behind what is presented.
>
> Another way to look at it is that we have component data models with
> some limited internal structure, however using the UTYPE mechanism
> we can map everything to a flat name space, be it a VOTable, a hash
> table, a parameter file, or whatever.  This makes things referentially
> very simple but still provides some logical structure with reusable
> components.
>
> 	- Doug
>
>
> On Tue, 27 Jun 2006, Gretchen Greene wrote:
>
>> Doug,
>> 
>> Let me get this clear,  to specify ra, dec you are seriously considering
>> something to level 4 hierarchy?
>> 
>> -Gretchen
>> 
>> -----Original Message-----
>> From: owner-dal at eso.org [mailto:owner-dal at eso.org] On Behalf Of Doug
>> Tody
>> Sent: Tuesday, June 27, 2006 2:01 PM
>> To: Randall Thompson
>> Cc: dal at ivoa.net; Inga Kamp; Myron Smith
>> Subject: Re: Comments on SSAP V0.95, UCDs
>> 
>> 
>> On Tue, 27 Jun 2006, Randall Thompson wrote:
>>
>>>   What I was hoping for was something equivalent to the
>>> UCDs POS_EQ_RA_MAIN and POS_EQ_DEC_MAIN defining
>>> the "primary" coordinates for a given observation, that would be
>>> included in the query results returned for any protocol. Will there be
>> 
>>> a single equivalent UTYPE I can use instead?
>>>   Randy
>> 
>> Yes - Assuming we still try to use characterization for this, probably
>> it will be provided by something like
>> SpatialAxis.Coverage.Location.whatever.
>> If that proves too complex for something this fundamental (we still
>> don't know how to specify a coordinate at this level), we will define a
>> special field for this purpose with a fixed coordinate system reference
>> frame, as in the past.  Whatever is decided, it will be a well defined
>> interface element with a fixed UTYPE so that you can just look up the
>> position by its UTYPE string.
>>
>> 	- Doug
>> 
>> 
>> 
>