Expressing position in RDF

[Brian Thomas]

Mathew, all,

On Tuesday 14 October 2008 10:16:09 am Ed Shaya wrote:
> "Let a hundred flowers blossom"  -- Mao
> 
> In a certain context I might actually say, "HDNNNN has RA 13.53" and 
> totally leave out epoch, equinox, system, units, and error and it would 
> be useful.  In a peer reviewed journal, I would not think of saying this 
> unless  all  those  metadata are  clearly tied down. 
> [snip}
> The VOEVENT software might prefer  "MDNNNN  
> a:hasRA  13.53^^float",  and that is fine. But, a  more elaborate 
> analysis package  might simply not accept such statements.  With some  
> intense coding, it may  understand that this statement sets an RA in 
> some equatorial system at some epoch, and it will either set these as 
> unknowns or put out a request for additional information. 

I agree with Ed here. One should basically pick the semantic expression which
is suitable to the application. Its really hard to get away from having some 
information being built into the context in which the RDF is being used. Building
RDF documents from 'first principles' will result in very many triples to express
even the simplest concepts, and is quite unwieldy for both storage and query.

Perhaps you needed a middle ground not expressed prior by Ed or others? 
What about choosing rich associations rather than building up from simpler 'isa' or 
'hasa' associations to express complex semantics succinctly? For example:

a:star b:hasJ2000RAInDegrees 13.53^^xsd:float;

This triple may then be exploded into many simpler triples with the appropriate mapping
from the 'b' namespace association 'hasJ2000RAInDegrees' to get at the units, coordsystem,
and other 'builtin' metadata/triples, but provides a one-line expression (and simple query) 
of the power you seem to want (similar to what Ed sent earlier, below).

Cheers,

=brian


> >>>> Here is "MyStar has a J2000 RA of 34.5343 degrees, it would be 
> >>>> something like this in N3:
> >>>>
> >>>> :MyStar a a:star;
> >>>>    ivoa:hasPosition :MyCoordinates.
> >>>>
> >>>> :MyCoordinates a 
> >>>> a:J2000Coordinates;                                                       
> >>>> geo:hasCoordinate :myRA.
> >>>>
> >>>> :MyRA a a:rightAscension;
> >>>>   q:hasFloat 34.5343^^xsd:float;
> >>>>   q:hasUnits ui:degree.
> >
> >
>