%APN3_PROCEEDINGS_FORM%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%
% TEMPLATE.TEX -- APN3 (2003) ASP Conference Proceedings template.
%
% Derived from ADASS VIII (98) ASP Conference Proceedings template
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\documentclass[11pt,twoside]{article}  % Leave intact
\usepackage{adassconf}

% If you have the old LaTeX 2.09, and not the current LaTeX2e, comment
% out the \documentclass and \usepackage lines above and uncomment
% the following:

%\documentstyle[11pt,twoside,adassconf]{article}

\begin{document}   % Leave intact

%-----------------------------------------------------------------------
%			    Paper ID Code
%-----------------------------------------------------------------------
% Enter the proper paper identification code.  The ID code for your
% paper is the session number associated with your presentation as
% published in the official conference proceedings.  You can           
% find this number locating your abstract in the printed proceedings
% that you received at the meeting or on-line at the conference web
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% of your presentation. 
%
% This will not appear in your paper; however, it allows different
% papers in the proceedings to cross-reference each other.  Note that
% you should only have one \paperID, and it should not include a
% trailing period.
%
% EXAMPLE: \paperID{O4-1}
% EXAMPLE: \paperID{P7-7}
%

\paperID{P3-17}
%%%% ID=P3-17

%-----------------------------------------------------------------------
%		            Paper Title 
%-----------------------------------------------------------------------
% Enter the title of the paper.
%
% EXAMPLE: \title{A Breakthrough in Astronomical Software Development}
% 
% If your title is so long as to fill the page header when you print it,
% then please supply a short form as a \titlemark.
%
% EXAMPLE: 
%  \title{Rapid Development for Distributed Computing, with Implications
%         for the Virtual Observatory}
%  \titlemark{Rapid Development for Distributed Computing}
%

\title{UCD in the IVOA context}
%\titlemark{ }

%-----------------------------------------------------------------------
%		          Authors of Paper
%-----------------------------------------------------------------------
% Enter the authors followed by their affiliations.  The \author and
% \affil commands may appear multiple times as necessary (see example
% below).  List each author by giving the first name or initials first
% followed by the last name.  Authors with the same affiliations
% should grouped together. 
%
% EXAMPLE: \author{Raymond Plante, Doug Roberts, 
%                  R.\ M.\ Crutcher\altaffilmark{1}}
%          \affil{National Center for Supercomputing Applications, 
%                 University of Illinois Urbana-Champaign, Urbana, IL
%                 61801}
%          \author{Tom Troland}
%          \affil{University of Kentucky}
%
%          \altaffiltext{1}{Astronomy Department, UIUC}
%
% In this example, the first three authors, "Plante", "Roberts", and
% "Crutcher" are affiliated with "NCSA".  "Crutcher" has an alternate 
% affiliation with the "Astronomy Department".  The fourth author,
% "Troland", is affiliated with "University of Kentucky"

\author{S\'ebastien Derriere\altaffilmark{1}, Norman Gray\altaffilmark{2}, Jonathan McDowell\altaffilmark{3}, Robert Mann\altaffilmark{4}, Fran\c{c}ois Ochsenbein\altaffilmark{1}, Pedro Osuna\altaffilmark{5}, Andrea Preite Martinez\altaffilmark{1,6}, Guy Rixon\altaffilmark{7}, Roy Williams\altaffilmark{8}}
%\affil{CDS, Observatoire de Strasbourg, 11 rue de l'Universit\'e, 67000 Strasbourg, France}
%\author{Norman Gray}
%\affil{University of Glasgow}
%\author{Jonathan McDowell}
%\affil{CfA Harvard}
%\author{Robert Mann}
%\affil{ROE}
%\author{Pedro Osuna}
%\affil{ESA}
%\author{Guy Rixon}
%\affil{Institute of Astronomy, Cambridge}
%\author{Roy Williams}
%\affil{CACR, Caltech}

\iffalse%%% MOD FO
\altaffiltext{1}{CDS}
\altaffiltext{2}{University of Glasgow}
\altaffiltext{3}{CfA Harvard}
\altaffiltext{4}{Royal Observatory Edinburgh}
\altaffiltext{5}{ESA}
\altaffiltext{6}{IASF}
\altaffiltext{7}{Institute of Astronomy, Cambridge}
\altaffiltext{8}{CACR, Caltech}
\else
\vspace*{2ex}   % Corrected style FO
\altaffilmark{1}\affil{CDS, Observatoire de Strasbourg, France}
\altaffilmark{2}\affil{University of Glasgow, UK}
\altaffilmark{3}\affil{CfA Harvard, USA}
\altaffilmark{4}\affil{Royal Observatory Edinburgh, UK}
\altaffilmark{5}\affil{European Space Agency}
\altaffilmark{6}\affil{IASF, Italy}
\altaffilmark{7}\affil{Institute of Astronomy, Cambridge, UK}
\altaffilmark{8}\affil{CACR, Caltech, Pasadena, USA}
\fi

%-----------------------------------------------------------------------
%			 Contact Information
%-----------------------------------------------------------------------
% This information will not appear in the paper but will be used by
% the editors in case you need to be contacted concerning your
% submission.  Enter your name as the contact along with your email
% address.
% 
% EXAMPLE:  \contact{Dennis Crabtree}
%           \email{crabtree@cfht.hawaii.edu}
%

\contact{S. Derriere}
\email{derriere@astro.u-strasbg.fr}

%-----------------------------------------------------------------------
%		      Author Index Specification
%-----------------------------------------------------------------------
% Specify how each author name should appear in the author index.  The 
% \paindex{ } should be used to indicate the primary author, and the
% \aindex for all other co-authors.  You MUST use the following
% syntax: 
%
% SYNTAX:  \aindex{Lastname, F. M.}
% 
% where F is the first initial and M is the second initial (if
% used).  This guarantees that authors that appear in multiple papers
% will appear only once in the author index.  
%
% EXAMPLE: \paindex{Crabtree, D.}
%          \aindex{Manset, N.}        
%          \aindex{Veillet, C.}        
%
% NOTE: this information is also used to build the author list that
% appears in the table of contents.  Authors will be listed in the order
% of the \paindex and \aindex commmands.
%

\paindex{Derriere, S.}
\aindex{Gray, N.} 
\aindex{McDowell, J. C.} 
\aindex{Mann, R.} 
\aindex{Ochsenbein, F.} 
\aindex{Osuna, P.} 
\aindex{Preite Martinez, A.} 
\aindex{Rixon, G.} 
\aindex{Williams, R.} 

%-----------------------------------------------------------------------
%		      Author list for page header	
%-----------------------------------------------------------------------
% Please supply a list of author last names for the page header. in
% one of these formats:
%
% EXAMPLES:
% \authormark{Lastname}
% \authormark{Lastname1 \& Lastname2}
% \authormark{Lastname1, Lastname2, ... \& LastnameN}
% \authormark{Lastname et al.}
%
% Use the "et al." form in the case of seven or more authors, or if
% the preferred form is too long to fit in the header.

\authormark{Derriere et al.}

%-----------------------------------------------------------------------
%			Subject Index keywords
%-----------------------------------------------------------------------
% Enter a comma separated list of up to 6 keywords describing your
% paper.  These will NOT be printed as part of your paper; however,
% they will be used to generate the subject index for the proceedings.
% There is no standard list; however, you can consult the indices
% for past proceedings (http://adass.org/adass/proceedings/).
%
% EXAMPLE:  \keywords{visualization, astronomy: radio, parallel
%                     computing, AIPS++, Galactic Center}
%
% In this example, the author noticed that "radio astronomy" appeared
% in the ADASS VII Index as "astronomy" being the major keyword and
% "radio" as the minor keyword.  The colon is used to introduce another
% level into the index.

\keywords{metadata, Virtual Observatory, standards, semantics, UCD}

%-----------------------------------------------------------------------
%			       Abstract
%-----------------------------------------------------------------------
% Type abstract in the space below.  Consult the User Guide and Latex
% Information file for a list of supported macros (e.g. for typesetting 
% special symbols). Do not leave a blank line between \begin{abstract} 
% and the start of your text.

\begin{abstract}          % Leave intact
UCDs (Unified Content Descriptors) are metadata that have been used in the VizieR 
catalogue service since they were first developed in the ESO/CDS data mining project. 
Because the catalogues currently described by UCDs cover many different domains, 
UCDs can be used to describe a large fraction of astronomical concepts. The 
different Virtual Observatory (VO) projects, collaborating in the IVOA (International 
Virtual Observatory Alliance) have therefore decided to support UCDs, and to 
collaborate to define a new set of standard metadata. This new set of UCD will 
be more coherent and complete than the old one. It uses a new syntax, and a list 
of atomic keywords from which UCDs are built. The sharing of a core set of metadata 
is very important to ensure interoperability between the various VO elements. We 
present how this new list of UCDs has been defined, and how it will be expanded 
in the future. We also present the mechanisms to assign UCDs to (possibly large) 
datasets, check for validity of UCDs, and make suggestions for including new 
concepts in the standard core of UCDs. We also discuss several fields of the 
VO where UCDs will be used, what kind of applications of UCDs are foreseen, how 
they can be used and how software using UCDs can be designed.
\end{abstract}

%-----------------------------------------------------------------------
%			      Main Body
%-----------------------------------------------------------------------
% Place the text for the main body of the paper here.  You should use
% the \section command to label the various sections; use of
% \subsection is optional.  Significant words in section titles should
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% automatically. 
%
% EXAMPLE:  \section{Introduction}
%           ...
%           \subsection{Our View of the World}
%           ...
%           \section{A New Approach}
%
% It is recommended that you look at the sample papers, sample1.tex
% and sample2.tex, for examples for formatting references, footnotes,
% figures, equations, html links, lists, and other special features.  

\section{From UCD1 to UCD2}


The UCD (Unified Content Descriptors) have been initially developed at CDS, in order to describe 
homogeneously the contents of the VizieR tables (Ortiz et al., 1999).
This first set of terms (hereafter UCD1) seemed to be a very good starting point for a 
standard description of astronomy, but had some inconveniences (lack of flexibility, 
missing concepts) which hampered broader use in the emerging VO projects.

Some tools have already been built to demonstrate the possible applications of UCDs in the
VO context (Derriere et al., 2003).
In order to generalize the usage of UCDs in the different VO components,
it has been decided to build a new set of rules and standard terms, named UCD2.
This new set will be validated by a committee at the IVOA level. The list of UCD2 terms
will become the reference for use by any VO application.


\section{A Controlled Vocabulary}

Defining a controlled vocabulary, and getting common agreement is a nearly impossible task, mainly 
because different people have different views of the same things (e.g. the FITS keyword NAXIS1 can be described
as the ``number of pixels along the $x$ image axis'', or as the ``size of the detector'').

The main role of UCDs is to describe quantities that are used in practice (and represented as numbers, 
character strings, ...) at some level 
between the fuzziness of natural language and the accuracy of attributes of data models. The objective is to ensure 
interoperability between services in the VO, by the use of a controlled vocabulary that can be interpreted 
by machines and still readable (and writeable) by humans.

We have tried to reconcile a bottom-up approach (describe what is found in existing datasets) with 
the ontology-related vision (cf. section~\ref{P3-17-onto}). The resulting vocabulary is a compromise, 
intended to be flexible enough, but less ambiguous than natural language. 
Of course, the resulting list of words, as any standardization process,
is somehow subjective (for example, only one word is kept
when there are synonyms of some concept). But words are created to describe the quantities 
used in practice, taking into account the UCD1 (i.e. the contents of astronomical tables), FITS keywords, etc.,
so they try to cover the semantic field.

\section{Syntax of UCD2}

The syntax of UCD1 reflected the underlying hierarchical organization, with different
levels separated by underscore characters (e.g. POS\_EQ\_RA\_MAIN).
In UCD2, this would be written \verb|ivoa:pos.eq.ra;meta.main|.

There are three reserved characters in UCD2:
\begin{itemize}
  \item the colon \verb|:| is used to separate the optional namespace from words;
  \item the semicolon \verb|;| is used to separate composed words;
  \item the period \verb|.| is used to concatenate atoms to build composed words.
\end{itemize}

The semicolon is reserved for possible future usage of an optional namespace.
  The use of a leading namespace should be avoided as far as possible. Standard UCD2 
(defined by the IVOA board) can be written with the \verb|ivoa:| namespace, but it is 
recommended not to write it (e.g. \verb|pos.eq.ra;meta.main|).



A UCD2 is composed of several {\em composed words}, the most important (carrying
most of the meaning) being the first one, that is called {\em primary word}.
The primary word describes a property; it is a first-order description of the quantity.
Following words, if present, give additional precision: they can add precision to
either the concept 
to which the property refers, or the context in which it was measured.

The composed words are made of {\em atoms} separated by periods. They are arranged, only for convenience, 
in a hierarchical tree. This structure does not imply the existence of an underlying model. 

\section{UCD and Ontologies\label{P3-17-onto}}

Currently, UCD are not an ontology.
In the future, a project called UCD3 will apply knowledge representation methods developed 
outside astronomy to the controlled vocabulary.

However, to ease the transition between UCD2 and 3, we have tried in the definition of UCD2 
to take into account the ideas of concepts, properties, classes and instances.

\begin{itemize}
 \item The concepts are the top-level elements of an ontology: they are analogous to 
 classes in OO-programming.
 \item A concept has properties (or slots, or parameters).
 \item The concept has instances (which are still something abstract to the computer).
 \item Instances of properties are the real data, that computers deal with.
\end{itemize}


The thumb rule is that {\em primary words} refer to {\em properties}. For example:
\begin{itemize}
 \item  phys.temp;instr.telescope represents the temperature of a telescope (the property being here phys.temp); 
 \item  stat.error;pos.eq.ra  represents the error on the right ascension.
\end{itemize}

\section{List of Standard Terms}

The recommendations for the use of UCD2, and the list of standard terms will be made available
on the \htmladdnormallinkfoot{IVOA web site}{http://www.ivoa.net/}.

The list of proposed roots for the tree of standard composed words
currently contains: {\em meta} (for metadata-related quantites), {\em instr} (instrument), 
{\em obs} (observation), {\em phot} (photometry), {\em src} (source), {\em stat} (statistical), 
{\em em} (electromagnetic)...

The {\em em} branch is a special case: it has been created mainly to indicate in which part of the electromagnetic spectrum
a measurement is made.
Specific words have been created to identify the frequently used bandpasses and filters. The 
electromagnetic spectrum has been divided into 8 domains (radio, sub-mm, IR, optical, UV, EUV, X-ray, gamma),
in agreement with other IVOA representations. 
Further divisions are made to define large bands classically used in the different domains. 
For photometric quantities, the property that is measured is phot.mag, or phot.flux or phot.count,
and possible valid UCD2 could be \verb|phot.mag;em.opt.V| or \verb|phot.count;em.x-ray.hard|.

There will be a committee in charge of studying the suggestions for additions 
of new terms in the list of standard UCDs.

\section{Tools and Services}


A set of tools for UCD2 will be progressively made available (similarly as for UCD1):
\begin{itemize}
 \item  Tools to list and browse the existing list of UCD2, with their definitions. These could
 be available as Web Services;
 \item  A validation/resolver service, that will check for the validity of a UCD, and provide the 
 corresponding definition;
 \item  A search engine/assignment tool, designed to suggest the appropriate UCD 
 corresponding to a description in natural language;
 \item  An interface to send feedback, and suggest new UCDs to the board in charge of 
 maintaining the UCD2 standard list.
\end{itemize}

\begin{figure}[htbp]
%  \centering
  \plotone{P3-17_1.eps}
  \caption{Services use UCDs to exchange information. A translation layer is used
 to interpret the internal description in terms of UCDs.}
  \label{P3-17fig:translation-layer}
\end{figure}

It is important to note that data providers do not need to change the internal description 
of their existing databases to use UCDs. 
Interoperability only requires a translation layer able to associate UCDs to parameters 
used internally. This layer is used to make the conversion
from (resp. to) UCDs to (resp. from) the internal description, as shown on 
Fig.~\ref{P3-17fig:translation-layer}, in the case of two services exchanging data.

  This translation layer only needs to be built once, and the assignment tool
  can be used to facilitate this step.
  
%-----------------------------------------------------------------------
%			      References
%-----------------------------------------------------------------------
% List your references below within the reference environment
% (i.e. between the \begin{references} and \end{references} tags).
% Each new reference should begin with a \reference command which sets
% up the proper indentation.  Observe the following order when listing
% bibliographical information for each reference:  author name(s),
% publication year, journal name, volume, and page number for
% articles.  Note that many journal names are available as macros; see
% the User Guide listing "macro-ized" journals.   
%
% EXAMPLE:  \reference Hagiwara, K., \& Zeppenfeld, D.\  1986, 
%                Nucl.Phys., 274, 1
%           \reference H\'enon, M.\  1961, Ann.d'Ap., 24, 369
%           \reference King, I.\ R.\  1966, \aj, 71, 276
%           \reference King, I.\ R.\  1975, in Dynamics of Stellar 
%                Systems, ed.\ A.\ Hayli (Dordrecht: Reidel), 99
%           \reference Tody, D.\  1998, \adassvii, 146
%           \reference Zacharias, N.\ \& Zacharias, M.\ 2003,
%                \adassxii, \paperref{P7.6}
% 
% Note the following tricks used in the example above:
%
%   o  \& is used to format an ampersand symbol (&).
%   o  \'e puts an accent agu over the letter e.  See the User Guide
%      and the sample files for details on formatting special
%      characters.  
%   o  "\ " after a period prevents LaTeX from interpreting the period 
%      as an end of a sentence.
%   o  \aj is a macro that expands to "Astron. J."  See the User Guide
%      for a full list of journal macros
%   o  \adassvii is a macro that expands to the full title, editor,
%      and publishing information for the ADASS VII conference
%      proceedings.  Such macros are defined for ADASS conferences I
%      through XI.
%   o  When referencing a paper in the current volume, use the
%      \adassxii and \paperref macros.  The argument to \paperref is
%      the paper ID code for the paper you are referencing.  See the 
%      note in the "Paper ID Code" section above for details on how to 
%      determine the paper ID code for the paper you reference.  
%
\begin{references}
\reference Derriere, S. et al.\ 2003, \adassxii, 69
\reference Ortiz, P.\ et al.\ 1999, \adassviii, 379
%\reference Ochsenbein, F., Bauer, P.\ \& Marcout, J.\ 2000, \aaps, 143, 23



\end{references}

% Do not place any material after the references section

\end{document}  % Leave intact
