Coordinates model - Working draft.

[Markus Demleitner]

Hi,

I know I had vowed to keep mum for a while, but this one thing hasnt'
been make explicit so far, so:

On Fri, Jan 18, 2019 at 11:59:42AM +0100, François Bonnarel wrote:
> Le 15/01/2019 à 13:42, Markus Demleitner a écrit :
> > The information that something is, say, a floating point number in
> > Julian years is already in the container format,
> I don't think we have something in VOTAble or DALI for Julian years.

Well, sure: <FIELD datatype="float" unit="yr"/>  (or unit="Byr" for
Besselian years).

It's certainly an important use case for me to properly annotate such
fields within STC objects.  To get an idea of what kinds of data are
being represented in this way, try

  select * from tap_schema.columns where unit='yr'

on the TAP service http://dc.g-vo.org/tap.

Right now, that's 112 columns; you can of course try the same on
VizieR, where you get back 1584 such columns. 

You'll find things like epu in ucac5.main, which is the epoch of
observation and as such a central piece of data for the basic use
case "bring two catalogs onto the same epoch".  There's the equinoxes
of coordinates for historical plates (which probably don't need too
much annotation).  There's coverages, there's dates when two objects
are closest, and of course there's lots of mean epochs, which would
become important if we wanted to cover the extended use case "bring
two catalogs onto the same epoch and give sensible error estimates".


Last round of broken record, really: There's absolutely no reason to
exclude this (or any other) time representation.  JSON or YAML or
whatever else either have their own ones already (and probably in
ways incompatible with the current choice of JD, MJD, or ISO), or
we'll have to define them separately anyway when we say how these
things are supposed to be used in the VO.  There's simply no way a
data model could magically bring them to par with VOTable or FITS --
that's always going to have to be a separate, format-specific
document.

       -- Markus