[TAP] data type for column metadata - units
Anita M. S. Richards
a.m.s.richards at manchester.ac.uk
Tue Mar 31 15:21:50 PDT 2009
> The simple way is NOT to implement astronomical time in TAP!
> These automated coordinate transformations are scary enough with Time. Is the
> intention now for the TAP engine to do spatial transformations? For example:
> "Select points from the X catalog (in B1950 as seen from Jupiter) which are
> in the Y region (in heliocentric supergalactic coordinates)".
> Will it be "allowed" by the IVOA to have a TAP engine that does not have full
> support for coordinate transformations?
We need to distinguish between the need to specify units, and to convert
them. A lot of problems arose in the early VO implementations because we
decided that we would mandate that e.g. search radii were in decimal
degrees and omit the unit, and then people published data in decimal
There is a Units model under development in the Data Model working group
and also a EuroVO AIDA workpackage to provide (modest) effort for this.
This will bring a proposal for units to be adopted internally by the IVOA.
The scope of this package is purely linear or self-contained conversions
(e.g. m to km, arcsec to deg in the same system not spatial, temporal
coordinate frames). This would allow a query to ask for JD from a data
set in MJD. However, if no unit was specified or the data set does not
have a valid unit, you get back the original data unit (so some warning
should also be present if the unit is ambiguous or not what was
So Roy's simplest case could be supported. On the other hand, a frequent
user complaint is problems querying catalogues (or formulating queries) in
Galactic coordinates, or wanting data in a certain time range. This is
outside the scope of the Units model but there are excellent libraries
(used bY SIMBAD, TopCat etc.) which do temporal and spatial coordinates to
high accuracy and which have been successfully wrapped for VO
applications. There is undoubtedly a demand for this, the question is
whether we want to write generic VO services which the user would hook
in, or ideally the service provider. I think that this would be a great
deal better use of effort than many things, and if Pat's model enables
this (without disallowing services which can't or for whom it is
irrelevant), so much the better.
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Dr. A.M.S. Richards, UK ARC Node/AstroGrid,
Jodrell Bank Centre for Astrophysics, Alan Turing Building,
University of Manchester, M13 9PL
+44 (0)161 275 4124
MERLIN/VLBI National Facility, Jodrell Bank Observatory,
Cheshire SK11 9DL, U.K. +44 (0)1477 571321 (tel) 571618 (fax)
"Socialism or barbarism?" Rosa Luxemburg (1915)
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