Erratum 1 VOTable 1.3 Announcement

[Pierre Fernique]

Dear Apps members,

This is an announcement of *the **Erratum 1 concerning VOTable 1.3*, 
which has been accepted by the IVOA TCG during the last Interop Meeting 
at Shanghai.

Consequently of this erratum, we encourage any VOTable providers to 
re-introduce as soon as possible the coordinate system description 
thanks to the COOSYS mecanism for avoiding bad VO tool assumptions for 
coordinates provided in other epoch than J2000 (notably the case for 
Gaia DR1: J2015 epoch) - see details below.

Best regards
Pierre Fernique

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    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 <http://wiki.ivoa.net/twiki/bin/view/IVOA/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.


      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.

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The whole Errata document is available at this address: 
http://wiki.ivoa.net/twiki/bin/view/IVOA/VOTable-1_3-Err-1



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