[QUAR] Re: Expressing position in RDF

[Matthew Graham]

Hi,

On Oct 14, 2008, at 1:17 PM, Rob Seaman wrote:

> Matthew Graham wrote:
>
>> I'm happy for the philosophical discussion but am also trying to  
>> figure out how to actually do something empirical.
>
> Empiricism is a branch of philosophy, of course :-)
>
>> I can see exactly the same arguments that we had about using STC in  
>> VOEvent applying to representing positions in RDF. The 90:10 rule  
>> should apply and whilst it is wonderful that I can describe any  
>> position in any coordinate system using ontology X, why can't we  
>> have ontology Y that is small and simple (that word again):
>>
>> :myStar stc:UTC-TOPO-FK5#RA 134.56
>
> This is an opportunity, not a bump in the road.  First, start on p.  
> 179 of the ADASS XVII proceedings with Pat Wallace's contribution.   
> Use ICRS, not FK5.

The identifying string from the STC in the VOEvent packet is UTC-FK5- 
TOPO.

> Second, what is your use case?  It appears to be the expression of  
> coordinates of an unresolved object fixed to the celestial sphere.   
> You are seem to be implicitly in a regime in which proper motion can  
> be ignored.  Perhaps this should be made explicit in some fashion.   
> Note that "unresolved", "fixed", and "no proper motion" are  
> orthogonal to the STC issues.
>
> One STC issue you must address (implicitly or explicitly) is  
> parallax.  Is the "star" far enough away that this is negligible for  
> your purpose(s)?  Another STC issue is whether you are you  
> describing spectral observations - i.e., is a doppler correction  
> (for the motion of the observer) needed?  Depending on the science  
> to be pursued, other coordinate issues will arise.
>
> Third, what is the goal of your application?  Are you describing  
> coordinates in support of follow-up observations?  Or are the  
> coordinates themselves somehow pertinent to scientific inferences?   
> In the former case, these are targeting coordinates as with  
> VOEvent.  In the latter case, perhaps you don't want equatorial  
> coordinates at all, but rather some galactic coordinate system.
>
> In short, the celestial coordinates are not inherent to the stars  
> themselves.  There are other such quantities, some pertaining to the  
> point of view, some to the intervening environment.  My  
> recommendation would be the same for representing these:
>
> 	X is a star
> 	X has environment Y
> 	Y has reddening Z
>
> Not:
>
> 	X has reddening Z

My goal is to represent the RA, Dec information contained in the  
VOEvent packet in RDFa.

I think the answer to my original question is that there is nothing  
currently existing in the VO to allow me to do this succinctly.

	Cheers,

	Matthew