VOTable-1.3 Erratum 1 - COOSYS un-deprecation

Author: Pierre Fernique - Application Working Group Chair
Date: 31 March 2017
Accepted by TCG: 14 May 2017

The following set of Errata amends the VOTable-1.3 (Ochsenbein et al. 2013) recommendation.

Rationale

The 1.3 VOTable document has introduced several references to the IVOA note "Referencing STC in VOTable" version 1.1 (Ochsenbein et al. 2009) and version 2.0 (Demleitner et al. 2010) for replacing progressively the usage of the basic COOSYS element used since VOTable 1.0. and dedicated to describe coordinate systems.

Due to their status, these notes have not been endorsed by the IVOA. And as any IVOA note, they are just expressing suggestions from and opinions of the authors, and are not a recommendation. Additionally, these two versions of the same note describe different and not compatible methods. This fact introduces inconsistencies: the example provided in the VOTable 1.3 document has been retrieved from the version 2.0 of this note. The bibliographic reference at the end of the document points to the note version 1.1.

Consequently, most of generated VOTables continue to use COOSYS, or worst, just remove the COOSYS element but without replacing it by the solution described in the note 1.1 or alternatively note 2.0. In absence of coordinate system description, VO clients may assume that the default coordinate system is FK5 or ICRS, epoch J2000, and it is generally the case. The first release of the Gaia mission - in 2016 - based on ICRS coordinates in epoch J2015 breaks this assumption.


This problem has been exposed and demonstrated at the IVOA Interop meeting at Trieste in 2016 (T.Boch - VO access to Gaia). A consensus was reached for writing this errata document, in order to remove the references to these IVOA notes, and to undeprecate and encourage the usage of COOSYS until the next version of the VOTable standard is released.

Change of standards text

  • page 10 - The example is replaced by this example:
          <?xml version="1.0"?>
          <VOTABLE version="1.3" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xmlns="http://www.ivoa.net/xml/VOTable/v1.3"
           xmlns:stc="http://www.ivoa.net/xml/STC/v1.30" >
            <RESOURCE name="myFavouriteGalaxies">
              <COOSYS ID="sys" equinox="J2000" epoch="J2000" system="eq_FK5"/>
              <TABLE name="results">
                <DESCRIPTION>Velocities and Distance estimations</DESCRIPTION>
                <PARAM name="Telescope" datatype="float" ucd="phys.size;instr.tel"
                       unit="m" value="3.6"/>
                <FIELD name="RA"   ID="col1" ucd="pos.eq.ra;meta.main"
                       datatype="float" width="6" precision="2" unit="deg" ref="sys"/>
                <FIELD name="Dec"  ID="col2" ucd="pos.eq.dec;meta.main"
                       datatype="float" width="6" precision="2" unit="deg" ref="sys"/>
                <FIELD name="Name" ID="col3" ucd="meta.id;meta.main"
                       datatype="char" arraysize="8*"/>
                <FIELD name="RVel" ID="col4" ucd="spect.dopplerVeloc" datatype="int"
                       width="5" unit="km/s"/>
                <FIELD name="e_RVel" ID="col5" ucd="stat.error;spect.dopplerVeloc"
                       datatype="int" width="3" unit="km/s"/>
                <FIELD name="R" ID="col6" ucd="pos.distance;pos.heliocentric"
                       datatype="float" width="4" precision="1" unit="Mpc">
                  <DESCRIPTION>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>
                  <TR>
                    <TD>287.43</TD><TD>-63.85</TD><TD>N 6744</TD><TD>839</TD><TD>6</TD><TD>10.4</TD>
                  </TR>
                  <TR>
                    <TD>023.48</TD><TD>+30.66</TD><TD>N 598</TD><TD>-182</TD><TD>3</TD><TD>0.7</TD>
                  </TR>
                  </TABLEDATA>
                </DATA>
              </TABLE>
            </RESOURCE>
          </VOTABLE>
    
  • page 10 - The last paragraph "This example also contains....their STC metadata in this way if possible" is removed;
  • page 11 - The paragraph "An essential difference with the version...express the coordinate components" is replaced by the definition of COOSYS issued from the VOTable 1.1 document page 9;
"Space coordinates: The COOSYS element
This element defines a celestial coordinate system, to which the components of a position on the celestial sphere refer. It has an ID attribute -- required if the COOSYS element has to be referred to via the ref attribute of the position components, which is generally the case -- a system attribute which specifies the coordinate system among "ICRS", "eq_FK5", "eq_FK4", "ecl_FK4", "ecl_FK5", "galactic", "supergalactic". equinox is the parameter required to fix the equatorial or ecliptic systems (as e.g. "J2000" as the default "eq_FK5" or "B1950" as the default "eq_FK4"), and epoch specifies the epoch of the positions if necessary.
Note that the COOSYS may be deprecated in the future in favor of a more generic way of describing the conventions used to define the positions of the objects studied in the enclosed tables. "

  • page 15 - The two last paragraphs of section 4.6 "The utype attribute is especially useful to specify the spatial and temporal coordiantes present.... in the dedicated note "Referencing SRC in VOTable"" are removed;
  • page 25 - The tag "RIP" (deprecated) on COOSYS element are removed in the schema;
  • page 27 - section 9.1 - the first item "COOSYS is deprecated, in favor of a reference to ... Referencing STC in VOTable" is removed;
  • page 28 - References - the reference [7] "Referencing STC in VOTable" is removed.

Impact Assesment

The changes described by this erratum should impact only the 2 services which are using note 1.1 description (IMCCE) and note 2.0 description (GAVO) (see P.Fernique presentation "Coordinate issue"). Both agreed to re-introduce COOSYS definitions. The IVOA services which had totally removed coordinate system description are encouraged to re-introduce it based on COOSYS.

References


IVOA note - Referencing STC in VOTable - version 1.1
F.Ochsenbein et al. 2009
http://www.ivoa.net/documents/latest/VOTableSTC.html

IVOA note - Referencing STC in VOTable - version 2.0
M.Demleitner et al. 2010
http://www.ivoa.net/documents/Notes/VOTableSTC/

Interop presentation - "VO access to Gaia DR1"
T. Boch - 2016 - Trieste
http://wiki.ivoa.net/internal/IVOA/InteropOct2016Apps/Apps2-TBoch-VO-access-GaiaDR1.pdf

Interop presentation - "Coordinate issue"
P. Fernique - 2014 - Banff
http://wiki.ivoa.net/internal/IVOA/InterOpOct2014Applications/AppsWG-VOTable_discusion-Coordinate-issue.pdf

Topic revision: r5 - 2017-06-01 - PierreFernique
 
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