RDF Proposal (Was: Re: SKOS concepts in VOTable

[Norman Gray]

Greetings, all.

On 2012 Jun 5, at 23:30, Norman Gray wrote:

> For those who aren't familiar with it, the idea of RDFa is that it's a smallish extension to the HTML DTD which allows one to embed a broad range of RDF statements into an HTML document.  There's a good example in the Wikipedia article <http://en.wikipedia.org/wiki/Rdfa>, but
> 
>    <p xmlns:dc="http://purl.org/dc/elements/1.1/">This page was
>    written by <span property='dc:creator'>Norman</span>.</p>
> 
> ...illustrates how it can intersperse normal HTML and RDF triples like "<> dc:creator 'Norman'."
> 
> Now, RDFa is defined with respect to HTML, but there's no reason why one couldn't define an RDFa-like thing for VOTable, and the registry, and any other XML used in the IVOA.  It would mean defining a couple of extra attributes in each of the relevant schemas, and mandating that they're ignored by existing applications.  I think the result of that thought-experiment would look very similar to what you're proposing.

Actually, I've just tried this, and it works rather well.

Below is a cropped version of one of the sample VOTables in the VOTable spec:

<?xml version="1.0"?>
<VOTABLE version="1.2"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xmlns="http://www.ivoa.net/xml/VOTable/v1.2" 
         xmlns:stc="http://www.ivoa.net/xml/STC/v1.30"
         xmlns:dc="http://purl.org/dc/elements/1.1/"
         xmlns:skos="http://www.w3.org/2004/02/skos/core#"
         xmlns:phys="http://purl.org/astronomy/vocab/PhysicalQuantities/"
         xmlns:ivoa='http://purl.org/astronomy/vocab/XXXivoa-relations/'>
  <RESOURCE name="myFavouriteGalaxies">
    <TABLE name="results">
      <DESCRIPTION property='dc:title'>Velocities and Distance estimations</DESCRIPTION>
      <PARAM name="Telescope" datatype="float" ucd="phys.size;instr.tel" 
             unit="m" value="3.6"/>
      <FIELD name="R" ID="col6" ucd="pos.distance;pos.heliocentric" 
             datatype="float" width="4" precision="1" unit="Mpc"
             typeof="ivoa:DatabaseColumn" about='#col6'>
        <DESCRIPTION property='dc:title'
                     rel='ivoa:hasSkosConcept' href='phys:Distance'
                     >Distance of Galaxy, assuming H=75km/s/Mpc</DESCRIPTION>
      </FIELD>
      <DATA>
        <TABLEDATA>
        <TR>
          <TD>010.68</TD><TD>+41.27</TD><TD>N  224</TD><TD>-297</TD><TD>5</TD><TD>0.7</TD>
        </TR>
        </TABLEDATA>
      </DATA>
    </TABLE>
  </RESOURCE>
</VOTABLE>

Notice the addition of the extra namespaces, and the addition of various property, typeof, about, rel and href attributes.  When I send this through a program which extracts RDF from RDFa (namely rapper from librdf.org, but there should be plenty others), I get:

% rapper -irdfa -oturtle sample-votable-cropped.xml file:///Users/norman/Desktop/vo-rdfa/sample-votable-cropped.xml
@base <file:///Users/norman/Desktop/vo-rdfa/sample-votable-cropped.xml> .
@prefix rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#> .
@prefix xsi: <http://www.w3.org/2001/XMLSchema-instance> .
@prefix : <http://www.ivoa.net/xml/VOTable/v1.2> .
@prefix stc: <http://www.ivoa.net/xml/STC/v1.30> .
@prefix dc: <http://purl.org/dc/elements/1.1/> .
@prefix phys: <http://purl.org/astronomy/vocab/PhysicalQuantities/> .
@prefix ivoa: <http://purl.org/astronomy/vocab/XXXivoa-relations/> .

<>
    dc:title "Velocities and Distance estimations" .

<#col6>
    ivoa:hasSkosConcept <phys:Distance> ;
    dc:title "Distance of Galaxy, assuming H=75km/s/Mpc" ;
    a ivoa:DatabaseColumn .


The "ivoa:" namespace is a fake one, but the others are real.

Immediate problems are that RDFa [1,2] tends to be good about adding these annotations to existing information that's in the form of elements, but adding properties to existing attributes looks slightly more intricate.  A theoretical problem is that RDFa is defined for XHTML, but I can see no reason why it wouldn't work in general for any other XML.

The point of RDFa is that there are no _changes_ to the content of (in this case) the VOTable, but the extra attributes allow precise semantics to be overlaid on the information already present in the file.  It's "principled screenscraping".

Obviously, this is designed to be processed by being parsed into RDF, but there's no _requirement_ that RDF be the end-point, and an application could process these annotations in any way it wanted that was consistent with the RDFa semantics.

So: do we have concrete examples of what it'd be good to do here?  Can we find things we'd want to express, which can't be done this way?  Does this seem to match what you were suggesting, Brian?

All the best,

Norman

[1] RDFa Primer: http://www.w3.org/TR/xhtml-rdfa-primer/
[2] RDFa Syntax: http://www.w3.org/TR/rdfa-syntax/


-- 
Norman Gray  :  http://nxg.me.uk
SUPA School of Physics and Astronomy, University of Glasgow, UK