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<div class="moz-cite-prefix">Hi all,</div>
<div class="moz-cite-prefix"><br>
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<div class="moz-cite-prefix">Just to illustrate the context, I did a
short survey of the time column definitions found in VizieR. Here
the result of my quick view for an half hundred of random tables
(selected by UCD time.epoch). As you can see, we get a lot of
various timeoffsets, generally based on JD origin, and MJD
(<=> timeOffset:2400000.5) is just one case among others. We
also find decimal years, with or without offset. And very few
calendar date (ISO or others)</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Cheers<br>
Pierre<br>
</div>
<div class="moz-cite-prefix">
<ol>
<li><b>JD </b>(19x) [CDS/I/337/rrlyrae, CDS/VI/44/catalog,
CDS/J/A+A/459/321/table4, CDS/J/AcA/54/207/table1,
CDS/J/AcA/49/223/lmc_sc, CDS/J/AcA/49/437/smc_sc,
CDS/J/AcA/48/563/smc_sc, CDS/J/AJ/133/1470/tsvsc1,
CDS/J/ApJ/781/22/table1, CDS/J/ApJS/225/9/table2,
CDS/J/ApJS/217/18/table1, CDS/J/ApJS/208/16/koi,
CDS/J/MNRAS/447/3342/table3a, CDS/J/MNRAS/447/3342/table3b,
CDS/J/other/IBVS/5721/table3, CDS/J/other/JAVSO/37.169/nsvs,
CDS/J/PASP/123/412/table1, CDS/II/213/pvar]<br>
</li>
<li><b>MJD </b>(11x) [CDS/B/psr/psr, CDS/J/AJ/152/158/table3,
CDS/J/AJ/146/21/table1, CDS/J/AJ/144/93/table,
CDS/J/AJ/142/160/table3, CDS/J/AJ/142/160/v3,
CDS/J/ApJS/217/31/KOIs, CDS/J/MNRAS/441/1230/catalog,
CDS/J/other/IBVS/5969/nl80, CDS/II/340/summary,
CDS/J/A+A/619/A50/catalog]</li>
<li><b>JD timeOffset: 2450000.0</b> (9x) [CDS/J/AcA/63/323/ecl,
CDS/J/AcA/60/1/catalog, CDS/J/AcA/60/17/catalog,
CDS/J/AcA/58/163/catalog, CDS/J/AcA/54/1/table2,
CDS/J/AcA/53/1/eb, CDS/J/AcA/53/93/catalog,
CDS/J/AJ/151/68/catalog, CDS/J/MNRAS/443/432/HEBs]</li>
<li><b>YEARS timeOffset 1900 </b>(5x) [CDS/J/A+A/509/A3/eoc4,
CDS/I/149A/catalog, CDS/I/180/gcpa, CDS/I/128/data,
CDS/I/206/ppmb]</li>
<li><b>YEARS</b> [CDS/J/AJ/153/95/table3,
CDS/J/AJ/150/107/table1, CDS/J/ApJ/807/170/table2,
CDS/J/ApJ/756/185/table1, CDS/I/188/pmc88, CDS/I/121/cpm,
CDS/I/78/catalog, CDS/J/A+A/618/A44/tablea7]</li>
<li><b>JD timeOffset: 2454900.0</b>
(3x)[CDS/J/AJ/151/110/table1, CDS/J/ApJS/210/19/table1,
CDS/J/ApJS/197/2/table2]</li>
<li><b><b>DATE "Y:M:D"</b> </b>(3x) [CDS/J/ApJS/235/5/table2,
CDS/J/ApJS/235/5/table3, CDS/VI/25/nposs]</li>
<li><b>JD timeOffset: 2400000.0</b> (2x)
[CDS/J/AJ/141/83/table1, CDS/J/ApJ/663/249/table1]</li>
<li><b>JD timeOffset: 2454800.0</b> (1x)
[CDS/J/ApJ/773/54/table1]</li>
<li><b>JD timeOffset: 2454833</b> (1x)
[CDS/J/ApJS/199/24/table1]</li>
<li><b>JD timeOffset: </b><b>2454000</b><b> </b>(1x)<b> </b>[CDS/J/MNRAS/429/2001/table2]</li>
<li><b>YEARS timeOffset 1800 </b> (1x) [CDS/I/310/agk1]</li>
<li><b>DATE 'Y-M-D"</b> (1x) [CDS/II/283/sncat]</li>
<li><b>DATE "MMMYY"</b> (1x) [CDS/II/218/sn]</li>
<li><b>ISOTIME </b> (1x) [CDS/J/A+A/619/L3/list]</li>
<li><b>Dedicated code</b> (1x) (R0 =RA, DEC in the J2000
coordinates, R5 =RA, DEC in the 1950 coordinates )
[CDS/V/150/variabls, ]</li>
</ol>
</div>
<div class="moz-cite-prefix"><br>
</div>
<div class="moz-cite-prefix">Le 06/12/2018 à 12:03, Mark Taylor a
écrit :<br>
</div>
<blockquote type="cite"
cite="mid:alpine.LRH.2.21.1812061033370.21669@andromeda.star.bris.ac.uk">
<pre class="moz-quote-pre" wrap="">On Wed, 5 Dec 2018, Markus Demleitner wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap="">On Tue, Dec 04, 2018 at 03:29:31PM +0000, Tom Donaldson wrote:
</pre>
<blockquote type="cite">
<pre class="moz-quote-pre" wrap=""> ii. If the attribute is needed, I’d still need to be
convinced that the convenience values JD-origin and MJD-origin are
worth the added complexity.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
Since I'd rather not have them either, I'd say those who want them
should again make their points. The one thing almost making me like
them is that it's easy to say 240000.5 instead of 2400000.5, whereas
NJD-origin is easily caught.
</pre>
</blockquote>
<pre class="moz-quote-pre" wrap="">
Really just human-friendliness: readability, writability,
comprehensibility. While I don't think that VOTable is or should
be mostly a human-readable format, there will inevitably be
human eyes on this from time to time, including when writing
[the code to write] these elements. If you see:
<TIMESYS ID="_tframe" refposition="BARYCENTER" timescale="TCB"
timeorigin="JD-origin"/>
...
<FIELD ID="obs_time" datatype="double" name="obs_time" ref="_tframe"/>
it's really obvious what's going on, even if you've never heard of
the TIMESYS element. If instead you see timeorigin="0" you're
definitely going to have to locate and read the document describing
this element to work out if the reference is to JD, MJD, Unix epoch etc.
(I admit that if you see timeorigin="2400000.5" you've probably got
a good chance of guessing right). And also the typo thing that
Markus notes.
To me the cost, namely additional complication in the XSD and in
parsing code, is not very high. As somebody implementing code
that may have to understand this, it certainly doesn't worry me,
in terms of additional code complexity or likely bug opportunities,
to put a couple of special cases in the parser for these
numerical aliases.
I initially thought that having aliases for JD and MJD would cover
a large fraction of the tables that need to use TIMESYS, however Ada,
who I don't doubt has a much better idea about this, said not
(<a class="moz-txt-link-freetext" href="http://mail.ivoa.net/pipermail/apps/2018-November/001316.html">http://mail.ivoa.net/pipermail/apps/2018-November/001316.html</a>).
If usage of these named time origins is really not very common,
that would weaken the case for it. And if most people don't like it
in any case, so be it. But I still think it's a good idea.
--
Mark Taylor Astronomical Programmer Physics, Bristol University, UK
<a class="moz-txt-link-abbreviated" href="mailto:m.b.taylor@bris.ac.uk">m.b.taylor@bris.ac.uk</a> +44-117-9288776 <a class="moz-txt-link-freetext" href="http://www.star.bris.ac.uk/~mbt/">http://www.star.bris.ac.uk/~mbt/</a></pre>
</blockquote>
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