Space-Time Coordinate metadata schemata

[Arnold Rots]

David,

Thank you very much for your thoughtful comments; they are helpful.
I will need some time to work through them, but here are some quick
comments; if you are going to be in Cambridge, next week, it might be
good to spend some time on this together.

In general, what I tried to do was design a framework that implements
our current needs but is sufficinetly general that it can accommodate
extensions of the kind that you mention, either by extending the lists
of enumerations (or extending elements in general) or by the
CoordRefPosition - which could actually point to a time-dependent table.

I definitely intend to produce some UML-like figures this week.

The Doppler velocities are tricky.  I thought they should be included
since they are so often used in conjunction with spatial coordinates.
I actually started wondering whether the best thing would not be to
include the spectral coordinate with space-time in order to
encapsulate all dependencies.

Cheers,

  - Arnold

David Berry wrote:
> Arnold,
> 
> Some initial comments on the "Space-Time Coordinate Specification for VO
> Metadata" (based on the HTML explanation document):
> 
> - Inclusion of Doppler velocities and redshifts: Since these are
> basically re-scaled forms of wavelength or frequency, should they not be
> included with the wavelength/frequency/energy schema instead of the
> space-time schema? Including them in the space-time schema may produce
> confusion between these "formal" velocities and "real" velocities.
> 
> - Regarding the DopplerReference item, is "LSR" the kinematical or
> dynamical LSR (presumably the kinematical, but should support be included
> for the dynamical LSR?) See:
> 
> http://www.starlink.rl.ac.uk/star/docs/sun67.htx/node227.html
> 
> - Planetary coordinate systems are included, but there doesn't seem to be
> any mention of solar or STP coordinate systems.
> 
> - The planetary coordinate systems seem to be restricted to spherical
> longitude/latitude. Is it possible to describe ellipsoidal surfaces?
> (e.g. geodetic longitude/latitude instead of geocentric).
> 
> - Spherical coordinates seem to be restricted to longitude/latitude
> systems. Is it possible to use co-latitude in place of latitude? Some
> solar coordinate systems require this.
> 
> - Positions within a single physical Domain (such as "the sky", or "the
> solar system", or "a spectrum", etc) can in general be described using
> many different coordinate systems. It is common for the Mapping between
> these coordinate systems to vary with time.  To enable conversion between
> such coordinate systems to be possible, it is therefore necessary to
> specify a moment in time which defines each coordinate system. I couldn't
> find such a time in the CoordsSystem element (although it's quite likely
> that I missed it). TimeRefPos seems to be for a different purpose.
> 
> - It would be interesting to see a UML diagram (or equivalent) showing a
> class hierarchy which could be used to represent the elements of the
> schema. Presumably you would need two basic classes to represent the
> CoordSystem element, a Domain (representing a physical domain, "SKY",
> "TIME", "SOLAR-SYSTEM", "SPECTRUM", "SUN", "JUPITER", etc), and a
> CoordinateSytem class which indicates the type of coordinate system
> (Cartesian, polar, lon/lat, long/co-lat, etc). Each Domain would
> encapsulate a set of named CoordinateSystems each corresponding to a
> different coordinate system which can be used to describe positions
> within the Domain. For instance, the SkyDomain class (a sub-class of
> Domain) would have several LonLatCoordSystems corresponding to FK4, FK5,
> ICRF, FK4-NO-E, GAPPT, etc.
> 
> 
> 
> Apologies if any of these comments reveal the fact that I am new to this
> discussion, and havn't fully grasped the background!
> 
> The schema seems to have been specifically tailored to the current needs
> of the bulk of astronomical data. How easy would it be to extend the
> schema in future to cover other coordinate systems not currently in
> demand? For instance, could a cylindrical coordinate system describing
> the solar system (or a galaxy) be easily incorporated?
> 
> 
> David
> 
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Arnold H. Rots                                Chandra X-ray Science Center
Smithsonian Astrophysical Observatory                tel:  +1 617 496 7701
60 Garden Street, MS 67                              fax:  +1 617 495 7356
Cambridge, MA 02138                             arots at head-cfa.harvard.edu
USA                                     http://hea-www.harvard.edu/~arots/
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