Unified Content Descriptors (UCDs)

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 paper 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 XML output 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 to ct/ks). 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