STC-S Velocities

[Markus Demleitner]

Dear DM group,

Over on the DAL list I've posted in the context of datalink an
example for noting an STC description in STC-S, viz,

    Time TT "Date"
    Position ICRS SPHER3 Epoch J2010 "alpha" "delta" "distance"
    Velocity "mualpha" "mudelta" "radialvelocity"
    Redshift OPTICAL "z"

[Fixed the errorneous CART3 in the original to SPHER3]

Arnold commented:

On Thu, Jun 20, 2013 at 05:40:36PM -0400, Arnold Rots wrote:
> One more aside: I noticed that Markus had an example which listed
> three velocity components (mu_ra, mu_dec, and radial_velocity) and
> a redshift. However, if that radial velocity is actually a Doppler
> velocity (and I may be wrong on this), then it is in the wrong
> place and should be on the Redshift coordinate axis (which then
> should be called Doppler velocity, to be more helpful).

Since this is unrelated to the original discussion but sounds like
something that might be relevant to others, too, I've taken this
here, although the question probably is mainly for Arnold -- why
do you want to move radialvelocity?  Given that this thing has
distances, it seems fairly natural to me to have the derivative of 
that distance in Velocity.  Would clients be expected to do things
differently if the radial velocity came in the redshift phrase rather
than as the third component of Velocity?

Also, since Position and Velocity share a common system, I have a
third Velocity component anyway -- so, what would I write there?
If I understand correctly, you'd like the above to roughly look like
this:

    Position ICRS SPHER3 Epoch J2010 "alpha" "delta" "distance"
    Velocity "mualpha" "mudelta" ???
    Redshift VELOCITY OPTICAL "radialvelocity"

-- to me,  it would seem odd to me to put some artificial NULL value
to where there's ??? now.  Or am I missing something fundamental
here?

Remark 1: The situation for this dataset is even worse -- it is a
simulation of the Gaia result set and thus we are dealing with a
radial velocity that
would-be-a-doppler-velocity-if-it-were-a-measurement-in-the-first-place.
*And* they have redshifts, too, mainly for the convenience of the
quasars that Gaia will observe, too, and for which the radial
velocities look a bit daunting.   But that's probably beside the
point, and I might even be convinced that that Redshift belongs to
another STC instance.

Remark 2: If you're worried  about all the quotes in these
expressions because you've never seen those mentioned in STC-S:
Don't.  These are DaCHS-local extensions to STC-S that (essentially)
allow column references whereever STC-S has actual literals and that
I don't propose for standardization since I don't think anything of
this kind should ever leave a data center.  It's how we give assign
STC roles and metadata to our tables -- for the question here, you
can just as well write 0.1 whereever there's something quoted.

Cheers,

      Markus