Coordinates in VOTable after Banff

[Pierre Fernique]

Dear VO App members,

During the last Interop meeting at Banff, we discussed about the very 
bad situation that *most of VO members (providers & clients) are not 
able to use correctly the coordinate descriptions recommended in 
VOTable* *standard*. For people not present at this meeting, have a look 
on this PDF : 
http://wiki.ivoa.net/internal/IVOA/InterOpOct2014Applications/AppsWG-VOTable_discusion-Coordinate-issue.pdf

In one word, since we deprecated COOSYS (2009), *only one VO provider* 
(IMCCE) has been achieved to describe correctly their coordinates and 
*only one VO client* (Aladin) is parsing the current recommended method 
("/Referencing STC in VOTable/" note). After 5 years, the worst 
consequence is certainly the fact that some providers prefer to remove 
totally the coordinate description rather than use the deprecated 
COOSYS, or try to implement the STC VOTable note.
Concretely, *VO providers and clients assume that coordinates are 
probably provided in ICRS/ep2000*. Unfortunately, this assumption will 
be no longer a solution as some new major missions will be not provided 
in epoch J2000 - for instance GAIA epoch J2010. *So the situation is 
critical*.



In order to come back to a clean situation, the participants to the 
Banff meeting mainly agree to take these two actions:
*"Un-deprecate" COOSYS to clean up the situation immediately.**
**Move to the future VODML STC2 description of coordinates when it will 
be ready**(deprecate the STC note)
*
The 2 methods (COOSYS, VODML) do not clash, allowing to manage a smooth 
transition.

I contacted directly the unique good student of the class-room (IMCEE - 
not present at Banff) and he kindly agreed to adapt his VOTables 
according to this solution. In the other side, Aladin will continue to 
support the STC note even it will be deprecated.

But, may be, you were not present at the Banff meeting, and you have a 
divergente opinion.
So please, raise your hand and bring your argument (have a look on your 
own VOTables before contesting too hardly ;-)

In the other hand, if you are generating VOTables without any coordinate 
description, please insert as soon as possible the minimal COOSYS 
according to your space reference : for instance: <COOSYS system="ICRS" 
epoch="J2000" />, see VOTable REC 1.1 - section 3.3


        /"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//", 
"//barycentric//", "//geo_app//" and a user-defined "//xy//" value. 
//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. //"//
/



Best regards
Pierre Fernique

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