dal Digest, Vol 147, Issue 5

[gilles landais]

Hello all!

VizieR provides tables and  preserve (whenever possible) the format 
chose by authors - sexagesimal is commonly used in literature (253 
VizieR catalogues published in 2021 have sexagesimal).

This observation explains sexagesimal column in CDS (despite the VizieR 
option to get computed column in decimal - option -oc.form=dec).

For a long time now, VizieR provides Sexagesimal information in unit 
('d:m:s', 'h:m:s') - it was a CDS convention which was not a problem in 
the past, but since VOTAble 1.4, vounits are required.
We are agree, that unit are not really adapted for sexagesimal which is 
more a format or a composition of units. May be a semantic like 
unit='composit:axis' should be better.

To provide compliant output is -of course- the CDS wish ! But more 
important are the clients points of view  - it they need unit or xtype 
(and if xtype fixes the multitude of formats used in sexagesimal) ?


Gilles Landais (CDS)

Le 11/03/2022 à 12:00, dal-request at ivoa.net a écrit :
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>     1. Re: Sexagesimal metadata (Markus Demleitner)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 11 Mar 2022 09:41:39 +0100
> From: Markus Demleitner <msdemlei at ari.uni-heidelberg.de>
> To: dal at ivoa.net, semantics at ivoa.net
> Subject: Re: Sexagesimal metadata
> Message-ID: <20220311084139.2kiltxzvipunae6h at victor>
> Content-Type: text/plain; charset=utf-8
>
> Hi,
>
> On Thu, Mar 10, 2022 at 05:17:23PM +0000, Mark Taylor wrote:
>> But one issue that remains is how to flag up sexagesimal quantities,
>> which are currently marked up like this:
>>
>>     <FIELD name="RAJ2000" ucd="pos.eq.ra;meta.main" ref="J2000"
>>            datatype="char" arraysize="12" unit="'h:m:s'">
>>     <FIELD name="DEJ2000" ucd="pos.eq.dec;meta.main" ref="J2000"
>>            datatype="char" arraysize="13" unit="'d:m:s'">
>>
>> The unit, by long VizieR tradition, is quoted as 'h:m:s' or 'd:m:s'
>> (including the single quotes), which is probably recognised as an
>> ad hoc indication by quite a bit of client code out there.
> I'll need to start with a disclaimer:  my personal opinion has always
> been that sexagesimal positions should be considered part of
> provenance and hence be squeezed out of operational use as much as we
> can.  I'll spare you some inappropriate joke about Babylon (where
> these things came from).
>
> But well, they're there, far too many people love them, and it's
> clear we have to do something with them.
>
> *If* there's substantial client code evaluating unit strings as
> above, then I think we have little choice but to introduce an extra
> section into VOUnits ("Units on string values") where we document
> (and perhaps deprecate?) the practice.  Since VOUnits is currently in
> WD, this would actually be a good time for that.  Do we have
> indications for who actually looks at these strings?  Is the practice
> widespread enough to justify this uglification of the spec?
>
> If, on the other hand, we're free to invent something less ugly, I'd
> say Pat (in a sibling mail) is right, and this is partly a job for
> xtypes; I'd say "hms" and "dms" might work to zeroeth order, but as
> Anita (in another sibling mail) has rightly pointed out, that is
> regrettably not enough, as we may still encounter anything from
> 12.12.12.12 to +12:12:12.12 to 12 12 12.12 to 12h12m12.12s in the
> wild, not to mention 12 12.12 (which was likely intended to mean
> 12.202 deg rather than 12.2033333, i.e., decimal minutes).
>
> We certainly do not want xtypes for all these variations, and we
> probably don't want to invent unit strings to tell them apart,
> either.  Hence, this problem would persist even if we were to
> sanction units on sexagesimal strings in VOUnits.
>
> I suppose we could define a canonical grammar that would cover most
> of what's out there, but that wouldn't be pretty because, if we want
> it to be context-free (and that we certainly do), we'd essentially
> have to enumerate the various full specifications to ensure the
> separator characters match.
>
> Or we could designate a single canoncial format and require everyone
> to comply.  But then many data providers would have to touch their
> data, and then wouldn't it be more prudent to just tell people to go
> all the way to decimal angles in VOTables (except for provenance) and
> leave the sexagesimal formatting to clients?
>
>              -- Markus
>
>
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