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Derriere, S., Ochsenbein, F., Boch, T., & Rixon, G. T. 2003, in ASP Conf. Ser., Vol. 295 Astronomical Data Analysis Software and Systems XII, eds. H. E. Payne, R. I. Jedrzejewski, & R. N.
Hook (San Francisco: ASP), 69
Metadata for the VO: The Case of UCDs
Sébastien Derriere, François Ochsenbein, Thomas Boch
CDS, Observatoire Astronomique de Strasbourg,
11 rue de l'Université,
F-67000 Strasbourg, France
Guy T. Rixon
Institute of Astronomy,
University of Cambridge,
Madingley Road,
Cambridge. CB3 0HA,
U.K.
Abstract:
The UCDs (Unified Content Descriptors) were first developed
in the ESO/CDS data mining project, to describe precisely the contents
of the individual fields (columns) of tables available from a data
center. They have been used to describe the content of the
columns available in the different VizieR tables.
Owing to the wide diversity and high heterogeneity of table contents,
UCDs constitute an excellent starting point for a hierarchical description
of astronomy, for general data mining purposes.
We present different applications of UCDs: selection of
catalogues, based on their content; identification of catalogues
having similar fields; automated data conversion allowing direct comparison
of data in cross-identifications. The compatibility of UCDs with
semantic descriptions developed in other contexts (data models for
space-time coordinates or image datasets) will also be addressed.
Astronomical tables can come from many different sources, and the
original descriptions are therefore very heterogeneous. Automated processing
of the contents of these datasets, which is one of the Virtual Observatory (VO)
applications, requires a uniform description for the catalogues (with
standardized metadata).
The UCDs (Unified Content Descriptors), first developed in the
ESO/CDS data mining project (Ortiz et al. 1999), are metadata describing
precisely the contents of the individual fields (columns)
of tables available from a data center. They have been applied to
describe the content of the columns available in the different
VizieR tables (Ochsenbein, Bauer & Marcout 2000).
Some tools using UCDs have been developed and are available online:
http://vizier.u-strasbg.fr/UCD/.
The UCDs consist of a 4-level hierarchical structure, with approximately
1500 elements. Different branches of the tree correspond to different
domains of the semantic classification (e.g., time, position, instrument).
A tool has been developed to visualize and explore the tree (Figure 1).
Figure 1:
The UCD browser, on the left, is used to locate relevant UCDs in the
hierarchical structure. For each UCD, the list of VizieR catalogues containing
this UCD in at least one field can be displayed and queried.
 |
A javascript and an applet version of the browser are available. The presentation
of the tree is similar to a file system browsing engine, with folders being nodes
of the UCD tree and documents being the UCD leaves, actually describing
the catalogue columns.
Clicking on a leaf gives access to:
- a definition of the corresponding UCD;
- statistics on column labels and units associated to this UCD
(Figure 2);
- usage statistics for this UCD in VizieR (catalogues and tables where it occurs).
The wide heterogeneity of the original description of astronomical data is
clearly visible when making statistics on the column names and units used to
represent a single physical quantity (Figure 2).
Figure 2:
Example of statistics on the different column names and units used in all
VizieR tables for one UCD.
 |
These statistics help pointing out possible errors in the catalogue description,
or in the UCD assignation, and are thus useful for data validation.
One of the most important use of UCDs is that
they allow to select catalogues which exactly contain a given measurement.
Instead of searching all the ``infrared'' catalogues for a K-band magnitude,
all catalogues with a Johnson K magnitude can be retrieved instantly.
This selection can be done with the browser (see Figure 1). It is
also possible to translate plain text into relevant UCDs.
One provides one or several terms to describe in natural language the desired
quantity (e.g., `proper motion'). The answer is a list of corresponding
UCDs, tentatively ordered by relevance. These can be used to select the
relevant catalogues.
If two fields in two tables are described by the same UCD, these fields can be compared
because they contain the same quantity. Automated data conversion can then be applied
if these fields are expressed in different units (Figure 3).
Figure 3:
Example of automated conversion for columns with the same UCD.
 |
Because UCDs precisely describe the contents of catalogues, they can be used to
find similar catalogues.
Given a reference catalogue, the list of UCDs which are present in this catalogue
is used as criteria to perform a search among all other catalogues: similar
catalogues are those that will have many UCDs in common with the reference one.
Suggestions have been made to improve the current structure of UCDs. The evolution
towards an ``atomic'' rather than hierarchical structure is studied. UCDs could
be built by assembling atomic elements (principal nouns, adjectives, complementary nouns)
selected among a predefined set of standard atoms. This scheme allows more flexibility
in defining new UCDs, avoids dispersion of related quantities in different branches
of the tree, and describes the data more completely.
Examples of combinations of atoms (compared to current UCDs):
- angle/declination (current UCD is POS_EQ_DEC);
- length/wavelength/johnson-V (central wavelength of the band, no UCD);
- length/wavelength/extent/johnson-V (bandwidth of the band);
- energy-flux-density/uncertainty/johnson-V (current UCD is ERROR).
UCDs are currently used in VizieR to describe the semantics of astronomical
content. They offer new ways of selecting relevant datasets, and enable
cross catalogue/archive interoperability. Owing to the wide
diversity of table contents, UCDs constitute an excellent starting
point for a hierarchical description of astronomy, for general data
mining purposes.
An improved structure relying, for example on atomic keywords, could
provide building blocks for the development of astronomical ontologies.
References
Ortiz, P. et al. 1999, in ASP Conf. Ser., Vol. 172, Astronomical Data Analysis
Software and Systems VIII, ed.
David M. Mehringer, Raymond L. Plante, &
Douglas A. Roberts (San Francisco: ASP), 379
Ochsenbein, F., Bauer, P. & Marcout, J. 2000, A&AS, 143, 23
© Copyright 2003 Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, California 94112, USA
Next: Russian and fSU Resources to be Integrated in the IVO
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