1 MetaData and UCDs
The catalogues and tabular data present in VizieR
are all described in a uniform way. This data description,
what is also called metadata, includes among other descriptors,
the following basic elements:
- the UCD (Unified Content Descriptors)
is a categorization of any parameter present in a column
of a table or catalogue in a predefined scheme.
- the units in which a parameter is expressed
(e.g. ct/s, erg/cm2/s, mJy,
10-15W/m2/keV, etc); in some cases,
the parameters may also be expressed as decimal logarithms.
The knowledge of these 2 elements are a help for automatized conversions
of the parameters, needed for cross-correlation and data-mining operations.
2 The UCD's
Attaching a meaning to a parameter reported in a table
or catalogue may be frustrating: on one hand, it is expected
that the description is detailed enough to permit accurate
comparisons between similar parameters reported independently
in different catalogues; and on the other hand, a classification
scheme has to remain simple in its usage for data-mining purposes.
The Unified Column Descriptor scheme has been
developed in the frame of the ESO-CDS data-mining project,
and first presented in a
at ADASS VIII (1998, in ASP Conf. Ser., Vol. 172,
ADASS VIII, eds. D.M. Mehringer, R.L. Plante, & D.A. Roberts, 379).
2.1 The UCD vocabulary
The UCD vocabulary is currenly a 4-level
hierarchical tree, made of approximately 450 words
representing the variety of parameters present in the
4000 catalogues present in VizieR.
The current list of valid words may be viewed here
2.2 Availability from VizieR
Almost all of the 105
columns of the catalogues contained in
VizieR have associated UCDs. The UCDs of the VizieR columns
are currently specified in the XML output of VizieR queries –
as for instance in the
of the 1RXS catalogue
3 Conversions between parameters
The knowledge of the units in VizieR already permits the
simplest unit transformations (computation and application of a
scaling factor, e.g. convert from ct/s
Conversions to/from logarithmic scales is not yet operational,
but could be added rather easily.
Other transformations, like a conversion of e.g. ct/ks
into erg/cm2/s, are much more questionable: they require
a knowledge of the instrument (area and efficiency of the
detector as a function of the photon energy); the conversion also
depends on the spectral energy distribution of the source
detected. The only way we can see presently to make such transfomation
is on a case-by-case basis.
Version 0.6, 23 Aug 2004