Expressing position in RDF

[Rob Seaman]

ICRS = International Coral Reef Symposium
ICRS = International Christian Retail Show
ICRS = International Cartilage Repair Society
ICRS = International Cannabinoid Research Society
ICRS = International Celestial Reference System

"International" is a quaint adjective to attach to celestial  
coordinates.  (I much prefer the chutzpah of "Universal" Time :-)   
Presumably it was a placeholder for the International in IAU, this new  
standard's momma organization.  ICRS is fixed with respect to the  
ICRF, a grid of 608 VLBI (way distant) sources stationary in space and  
orientation by construction.  See http://aa.usno.navy.mil/faq/docs/ICRS_doc.php 
  or Pat Wallace's paper from ADASS XVII.

Precession and nutation don't go away - the Earth still wobbles like a  
top - but these effects don't appear in ICRS coordinates.  The Vernal  
Equinox has been defined out of existence because ICRS is no longer an  
equatorial coordinate system, unlike FK5 and all those J's, B's, etc.

The implications for the Art of Astronomy are still unclear.  Very  
smart people, but too few engineers perhaps, were involved in defining  
ICRS.  The net effect for the purposes of this virtual observatory  
discussion appears to be to avoid the need to specify just one scalar  
- the equinox.  All the other STC-like and non-STC issues persist.   
Real observatories face issues like all their equatorial mount  
telescopes turning into tilted alta-az telescopes :-)

Rob
---

On Oct 15, 2008, at 6:06 AM, Ed Shaya wrote:

> Matthew,
>
> Precession must have been explained before somewhere in VOEVENT  
> discussions, but here is another try at emphasizing it.
>
> Precession rotates the equatorial system at the rate of about 50.3  
> arcsec per year (approximately 360 degrees divided by 25,700 yr), or  
> 1 degree every 71.6 years.  Thus, one year of precession is almost  
> as large as the distance from the center of the HST/WFPC2 field to  
> the edge.  We avoid vast confusion by stating the positions in a  
> given frame fixed at a particular epoch (hence J2000, B1950 etc).
> In two years GAIA will fly and there will be a vastly superior  
> reference frame, if all goes well (knock wood).  Logically, we will  
> all flock to this new reference frame which will probably be set at  
> a current epoch like 2015.  So, in not too many years your VOEVENT  
> database will be a mixed bag of J2000 and G2015.  With about 12.5  
> arcminute disagreements for the positions of anything with low  
> declination,  10 times larger than the JWST field of view from  
> center to edge.
> Just the facts.  You can do what you want with them.
> You could, in 2015, replace all positions with G2015 positions, for  
> instance.  But the harder issue is how do you inform all  
> applications (including someone's one line IDL procedure) that query  
> your database that this change has been made?
>
> Ed
>
> PS - And hopefully there will be an S2020 from SIM.
>
>
> Matthew Graham wrote:
>> Hi,
>> RDF is just like XML so questions of production, presentation and  
>> storage are really quite secondary. It's the representation of  
>> information that is the primary concern and the issue here is does  
>> the IVOA have a succinct way of representing celestial positions in  
>> RDF. As Ed, I think, said, RDF and ontologies are supposed to  
>> remove the need for context to provide meaning to the information  
>> and so how the RDF statement is used is also quite secondary. The  
>> statement means what it says whatever.
>> SPARQL is the query language for RDF and is very SQL-like so an  
>> inequality is just a constraint on the search predicate. The exact  
>> syntax is dependent on how verbose the RA expression is.
>> I would not use a SPARQL query to do intersections, though - we  
>> have STC and Footprint Services for that. There might an RDF  
>> statement that a data object has a footprint associated with it and  
>> but then the object will the URI for the footprint itself.
>>    Cheers,
>>    Matthew
>> On Oct 14, 2008, at 7:35 PM, Roy Williams wrote:
>>> Matthew
>>>
>>> I like Doug's question, asking how the RDF will be used: produced,  
>>> presented, stored etc. Another question in the same genre asks  
>>> what *queries* will run against this knowledge base. I suspect  
>>> that "RA" will be used in an arithmetical inequality -- "Give me  
>>> all sources with RA>240.0", also as part of a cone search or  
>>> polygon. Is it Sparql, the query language for RDF? How would an  
>>> inequality be framed as part of a Sparql query?
>>>
>>> More abstract, we could utilize RDF at a higher level -- not  
>>> details of coordinate systems, as in this naked quantity "RA". How  
>>> about the RDF handles "Regions" (of spacetime). With Regions, the  
>>> questions are no longer arithmetical, but rather boolean choices,  
>>> about intersection: "Does the Region in which this event lies  
>>> intersect with the spacetime coverage Region of the Catalina  
>>> Survey?"
>>>
>>> Roy
>>>
>>>
>>> -- 
>>>
>>> California Institute of Technology
>>> 626 395 3670
>>>