s_region problem

Thomas Boch thomas.boch at astro.unistra.fr
Mon Nov 27 18:00:39 CET 2017

Hi all,

I would like to add my 2 cents from my perspective, as a client 
application developer.

1. One thing I quite like about STC-S description is that it is 
self-contained, the string by itself is sufficient for a reasonably 
smart client to make something useful with it, for instance display it 
on the sky.

2. I'm a bit worried that having different ways of expressing roughly 
the same thing (the region on the sky covered by an observation) goes 
against interoperability and makes client implementation more difficult.

Please take my comments with a grain of salt as I might not have in mind 
the big picture of how DAL protocols are interlinked.



Le 27/11/2017 à 13:12, François Bonnarel a écrit :
> Dear all,
> My understanding of this context as a DAL chair involved in several of 
> the specifications as author and (supposed to be) aware of the others.
> Most of this stuff is extracted from a discussion we had in a more 
> limited group.
> 1 ) Obscore
>     The discussion here is actually invloving at least 3 DAL standards 
> + Obscore (which is DM oficially but actually "DM-DAL") : TAP, ADQL, 
> DALI and Obscore. The DAL dillemma is the following : as it 
> encompasses a vast variety of situations and use cases and various 
> interface shapes, DAL technology cannot be defined by a single 
> "SPEC_OF_EVERYTHING". DAL has defined a great deal of different 
> protocols, which of them evolving its own rythm. We have to be 
> carefull all this keep consistent and to be carefull also we do not 
> break everything
>      Each of the 4 standards involved here have two versions : TAP 
> 1.0, 1.1, ADQL 2.0, 2.1, DALI 1.0, 1.1. Obscore 1.1 and DALI 1.1 are 
> IVOA recommendations since 2017 May the 17th. TAP1.1 and ADQL 1.1 are 
> still in discussion.
>      1 ) For s_region what does Obscore spec says ? ObsCore is a model 
> (or a view of a larger model) each class/attribute is perfectly 
> defined. To map such a model in a TAP service one has to express it in 
> a TAP schema. The OBscore spec defines precisely this TAP schema. + 
> xtypes for each TAP schema column. In OBscore 1.0 the xtype for 
> s_region is "adql:REGION". Obscore 1.1 maintains the same definitions 
> for s_region (and other ObsCore attributes) than 1.0. But it adds that 
> the definition given in the spec is consistent with TAP 1.0 (the only 
> valid one at the moement it was defined) but that the situation may 
> evolve after the release of TAP 1.1.
>      2 ) So what does TAP says about xtypes ? 1.0 was defining a 
> couple of them, all bound to adql functions, including adql:Region. 
> adql:REGION WAS AT THE TIME OF WRITING OF 1.0 considered as STC-S. TAP 
> 1.1 gets rid of this and claims that as far as possible xtype should 
> map to the definitions given in DALI. It says "SHOULD" and not MUST of 
> course. You are still allowed to define or use other xtypes if really 
> necessary.
>      3 ) DALI 1.0 didn't say anything about xtypes. DALI 1.1 defines a 
> couple of them. xtypes CIRCLE and POLYGON are defined as arrays of 
> doubles. This has also been discussed to be xtypes of some of SODA 
> input parameters (together witn interval). These xtypes are those 
> refered by TAP 1.1  as those   that SHOULD be used if no other xtype 
> is required by the use case.
>      4 ) ADQL 2.0 Defined a couple of functions which TAP was mapping 
> to xtypes: Among them REGION. ADQL 2.1 is considering dropping 
> string-typed REGION and keep more double array oriented CIRCLE and 
> POLYGON. This has consequences for querying. This has no consequence 
> on adql:REGION xtype which can remain defined as in ADQL 2.0 if necessary.
>      Discussion :
>                       With the main goal of keeping the defintions and 
> formats stable for a while in the vO in a given context ( the 
> ObsCore/OBstap one here) I think nothing prevent us to keep 
> adql:REGION (equivalent to STC-S string) as an xtype for the s_region 
> attribute in ObsCore.
>                        I don't know how DACHS is implemented but I 
> assume it generates xtypes in the VOTABLE response of the services 
> from the TAP schemata. So if the the ObsCore TAP schema keeps 
> adql:REGION as the xtype of s_region instead of xtype=polygon, clients 
> like Aladin could still recognize and parse it immediatly.
>                        Actually it makes more sense than it may 
> appear. The data/metadata (coord system and whatever) distinction is 
> not an issue for Obscore. Everything is ICRS in there. So the STC-S 
> string is unambiguously a polygon, a circle, or any of other stc 
> flavors of space area perfectly understandable. s_region is not only a 
> space area. It's the support of the observation (as defibed by ObsCore 
> and characterisation) expressed as a space area. So it's not that 
> strange to have a spoecific xtype for it, and to keep the historical 
> adql:REGION/STC-S then.
>                         For POlygon or CIRCLES in general, which have 
> plenty of usages different from "dataset support" (exemply gratia 
> "region of interest", "survey or catalog coverages" , "cross match 
> search regions" ,etc... and which may require different coordinate 
> systems than ICRS going to the POLYGON/CIRCLE xtypes with arrays is 
> the most standard option for the future...
> Lets' go to Pierre questions in detail
> Le 24/11/2017 à 12:26, Pierre Fernique a écrit :
>> Dear DAL and Apps members,
>> It seems that we have a problem concerning the *s**_region* usage.
>> Recently, we discover that some data providers have changed their 
>> syntax, moving from the regular STC-S string to a new one which seems 
>> to be an array of doubles, alternating long and lat. Both Aladin V10 
>> and Aladin Lite fail to manage a such new syntax, and we had 
>> complaints from final users which do no longer see the FoV of their 
>> observations.
>> After investigating about this unexpected evolution, it appears that 
>> this new syntax is generated by some versions of DaCHS testing TAP1.1 
>> future possible recommendation.
>> So, I have several questions for TAP 1.1 authors and DAL members :
>>  1. Is is really true that STC-S s_region should be removed from
>>     VOTable results ? Personally, I did not see this consequence in
>>     the TAP 1.1 document. or ?
> No. Nothing prevent us to have string field in VOTable
>> 1.
>>  2. And if yes : is it only in TAP ? or also in SIA2 or SSA ?
> TAP itself doens't prevent to use string-typed FIELDS in the response. 
> IT only recommands usage of fIELDS with wtype.
> In the case of s_region the basic thing is not TAP or SIAV2, the 
> discovery protocol, but ObsCOre. ObsCore has to be stabilized.
> Common discovery metadata in the case of ADQL queries and the case of 
> Parameter query language has been a major progress recently...
> We should not break it/
> For SSA si next answer.
>>  1. How the client can know the logic (lon/lat or lat/lon,
>>     counter-clock or anti-counterclock, ...), the frame and other
>>     coordinate parameters for such array of double ?
> I think xtype=POLYGON assumes it's lon/lat. At the moment coordsystem 
> can aonly be given by the de-deprecated COOSYS. So reference to a 
> COOSSYS element seems to be the only solution.
>>  1. How providers who already generated complex s_region will do now
>>     ? (for instance ESO)
> I think they should not change that. ObsCore has been designed and 
> discussed durin a long time to be a stable basis.
>> 1.
>>  2. How we will manage the transition period ?
> In my opinion no transition period is needed for the specific case of 
> s_region which is STCS ICRS.
> DAL "build up" is always a balance between stability and backward 
> compatibility and controlled progress in the definitions.
> Cheers
> François
>> Best regards
>> Pierre Fernique

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.ivoa.net/pipermail/dal/attachments/20171127/d6fdcce2/attachment-0001.html>

More information about the dal mailing list