%%%%%%%%%%%%%%%%%%%%%%%%%% To allow same version tex / htx \def\ifhtx{\iffalse} % Lines used only for the HTML version \ifhtx % Declarations for HTX (HTML) \begin{head} \tag{META HTTP-EQUIV="CONTENT-TYPE" CONTENT="text/html; charset=iso-8859-1"} \begin{title}VOTable \Version\ Proposal\end{title} \tag{META NAME="robots" content="index"} \tag{META NAME="description" content="VOTable Proposal"} \tag{META REV="made" HREF="mailto:VOTable@ivoa.net"} \tag{META NAME="GENERATOR" CONTENT="StarOffice/5.2 (Linux)"} \tag{META NAME="AUTHOR" CONTENT="Francois Ochsenbein, Roy Williams"} \tag{META NAME="CREATED" CONTENT="20020315;23390000"} \tag{META NAME="CHANGEDBY" CONTENT="Francois Ochsenbein"} \tag{META NAME="CHANGED" CONTENT="20020315"} \end{head} \Beg{body}{\bg{white}} \def\Arefx#1#2{\Aref{#1}{#2}} \else % Declarations for LaTeX \documentclass[10pt,notitlepage,onecolumn]{ivoa} %\documentclass[12pt,notitlepage,onecolumn]{ivoa} %\documentclass{article} %\usepackage[pdftex]{graphicx} % Accept Images 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Format Definition} % Give date and version number \date{2004-01-30} % Choose one document type from below %\ivoatype{IVOA Note} \ivoatype{IVOA Working Draft} %\ivoatype{IVOA Proposed Recommendation} %\ivoatype{IVOA Recommendation} \version{1.091} % Give author list: separate different authors with \\ % You can add email addresses with links \url{mailto:yourname@ivoa.net} \def\name#1{\makebox[10em][l]{#1}} \author{ \normalsize \name{Fran\c cois {\bf Ochsenbein}} \quad{\em Observatoire Astronomique de Strasbourg, France} \\ \normalsize \name{Roy {\bf Williams}} \quad{\em California Institute of Technology, USA} \\ \normalsize \hspace*{-0.75em}\name{{\em with contributions from:}}\\ \normalsize \name{Clive {\bf Davenhall}} \quad{\em University of Edinburgh, UK} \\ \normalsize \name{Daniel {\bf Durand}} \quad{\em Canadian Astronomy Data Centre, Canada} \\ \normalsize \name{Pierre {\bf Fernique}} \quad{\em Observatoire Astronomique de Strasbourg, France} \\ \normalsize \name{David {\bf Giaretta}} \quad{\em Rutherford Appleton Laboratory, UK} \\ \normalsize \name{Robert {\bf Hanisch}} \quad{\em Space Telescope Science Institute, USA} \\ \normalsize \name{Tom {\bf McGlynn}} \quad{\em NASA Goddard Space Flight Center, USA} \\ \normalsize \name{Alex {\bf Szalay}} \quad{\em Johns Hopkins University, USA} \\ \normalsize \name{Mark B. {\bf Taylor}} \quad{\em Physics, Bristol University, UK} \\ \normalsize \name{Andreas {\bf Wicenec}} \quad{\em European Southern Observatory, Germany} \\ } \urlthisversion{\url{http://cdsweb.u-strasbg.fr/doc/VOTable/votable-1-1.htx}} \urllastversion{\url{http://cdsweb.u-strasbg.fr/doc/VOTable/votable-1-0.htx}} \previousversion{1.0 (2002-04-15)} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %\begin{document} \maketitle % print header in standard form \section*{Abstract} This document describes the standards adopted for the version 1.1 of the VOTable format, and supersedes the previous version 1.0 of 15 April 2002. The differences between versions 1.0 and 1.1 are summarized in \Aref{diff}{the last section}. \noindent The main part of this document describes the adopted part of the VOTable standard; it is followed by appendices presenting extensions which have been proposed and/or discussed, but which are not part of the standard. \section*{Status of this document} This is an IVOA Proposed Recommendation for review by IVOA members and other interested parties. It is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use IVOA Working Drafts as reference materials or to cite them as other than ``work in progress''. A list of current IVOA Recommendations and other technical documents can be found at http://www.ivoa.net/Documents/. \section*{Acknowledgments} This document is based on the W3C documentation standards, but has been adapted for the IVOA. \tableofcontents %\clearpage \section{Introduction} The VOTable format is a %proposed XML standard for representing a set of tables. In this context, a table is an unordered set of rows, each of a uniform format, as specified in the table {\em metadata}. Each row in a table is a sequence of table cells, and each of these contains either a primitive data type, or an array of such primitives. VOTable is derived from the Astrores format [1], itself modeled on the FITS Table format [2]; VOTable was designed to be closer to the FITS Binary Table format. \subsection{Why VOTable?} Astronomers have always been at the forefront of developments in information technology, and funding agencies across the world have recognized this by supporting the Virtual Observatory movement, in the hopes that other sciences and business can follow their lead in making online data both {\it interoperable} and {\it scalable}. VOTable is designed as a flexible storage and exchange format for tabular data, with particular emphasis on astronomical tables. Interoperability is encouraged through the use of standards (XML). %because physical quantities are tagged not only with units, but also %through a Uniform Content Descriptor (UCD) that expresses the nature %of the quantity (eg. Gunn J magnitude, declination). The XML fabric The XML fabric allows applications to easily validate an input document, as well as facilitating transformations through XSLT (eXtensible Style Language Transformation) engines. \subsubsection*{Grid Computing} VOTable has built-in features for big-data and Grid computing. It allows metadata and data to be stored separately, with the remote data linked. % according to the Xlink model. Processes can then use metadata to `get ready' for their input data, or to organize third-party or parallel transfers of the data. Remote data allow the metadata to be sent in email and referenced in documents without pulling the whole dataset with it: just as we are used to the idea of sending a pointer to a document (URL) in place of the document, so we can now send metadata-rich pointers to data tables in place of the tables themselves. The remote data is referenced with the URL syntax {{\sf protocol://location}}, meaning that arbitrarily complex protocols are allowed. When we are working with very large tables in a distributed-computing environment (``the Grid"), the data stream between processors, with flows being filtered, joined, and cached in different geographic locations. It would be very difficult if the number of rows of the table were required in the header -- we would need to stream in the whole table into a cache, compute the number of rows, then stream it again for the computation. In the Grid-data environment, the component in short supply is not the computers, but rather these very large caches! Furthermore, these remote data streams may be created dynamically by another process or cached in temporary storage: for this reason VOTable can express that remote data may not be available after a certain time (\attr{expires}). Data on the net may require authentication for access, so VOTable allows expression of password or other identity information (the `{\attr{rights}}' attribute). \subsubsection*{Data Storage: Flexible and Efficient} The data part in a VOTable may be represented using one of three different formats: TABLEDATA, FITS and BINARY. TABLEDATA is a pure XML format so that small tables can be easily handled in their entirety by XML tools. The FITS binary table format is well-known to astronomers, and VOTable can be used either to encapsulate such a file, or to re-encode the metadata; unfortunately it is difficult to stream FITS, since the dataset size is required in the header (NAXIS2 keyword), and FITS requires a specification up front of the maximum size of its variable-length arrays. The BINARY format is supported for efficiency and ease of programming: no FITS library is required, and the streaming paradigm is supported. We hope that VOTable can be used in different ways, as a data storage and transport format, and also as a way to store metadata alone (table structure only). In the latter case, we can imagine a VOTable structure being sent to a server, which can then open a high-bandwidth connection to receive the actual data, using the previously-digested structure as a way to interpret the stream of bytes from the data socket. Alternatively, the metadata can be sent alone as an implicit query to a server, which will respond with the data part of the table filled in. VOTable can be used for small numbers of small records (pure XML tables), or for large numbers of simple records (streaming data), or it can be used for small numbers of larger objects. In the latter case, there will be software to spread large data blocks among multiple processors on the Grid. Currently the most complex structure that can be in a VOTable Cell is a multidimensional array. \iffalse \subsubsection*{Future} In future versions of the VOTable format, we expect to benefit from both experience and tool-building. Such tools include presentation and transformations of the metadata (and data too, when it is in XML), using XML transformation language and software: XSL and XSLT. We would like to migrate to the more powerful document validation provided by XSchema [8] rather than DTD -- a draft version of the VOTable format in XSchema is included as \Aref{xsd}{Appendix}. We also expect XSchema to allow better modularization of the document schema, so that, for example, users might put whatever serialized objects they wish into the table cells. In this way, we expect to use VOTable for handling the flow of large data objects through Grid resources, objects such as FITS files or XDF [7] documents. Also, it would also mean, for example, that the description of a table could contain arbitrary HTML instead of the current version -- plain text with paragraph markers; or that an XML definition of non-standard astronomical coordinate systems could be seamlessly integrated. VOTable is derived from {\em Astrores}, which is specified not only as a way to write a data table, but also as a way to specify how to address a {\it request} to data tables. We expect to sharpen and formalize this dichotomy with the benefit of experience, building into VOTable the ways of making sophisticated querying mechanisms and protocols. We expect to add features for efficiency in the future also: to specify that the data stream has a particular sort order, to specify that a column in one table is a key in to another table; to specify that one table is an index into another. The binary format will be extended to facilitate large-scale streaming. \fi \subsection{XML Conventions} VOTable is constructed with \A{http://www.w3.org/XML/}{XML} (extensible Markup Language), a powerful standard for structured data throughout the Internet industries. It derives %through simplification from SGML, %which has been a standard used in the publishing industry and for technical documentation for many years. XML consists of {\it elements} and payload, where an element consists of a {\it start tag} (the part in angle brackets), the payload, and an {\it end tag} (with angle brackets and a slash). Elements can contain other elements. Elements can also bear {\attr{attributes}} (keyword-value combinations). %, such as the {\elem{PARAM}} elements above. The payload may be in two forms: parsed or unparsed character data. Examples are: \begin{verbatim} François \end{verbatim} In the first example, the sequence {\tt \&\#231;} is interpreted as part of the ISO/IEC 10646 character set, and translates to an accented character, so that the text is ``Fran\c{c}ois". The second example uses the special {\tt CDATA} sequence so that the characters {\tt <}, {\tt >}, and {\tt\&} can be used without interpretation; in this case, any ASCII characters are allowed except the terminating sequence {\tt]]>} For more information, see any book on XML. \subsection{Syntax policy} Following the general XML rule, element and attribute names are case-sensitive and have to be used with the specified capitalisation. For VOTable, we have adopted the convention that element names are spelled in uppercase and attribute names in lowercase (with an exception for the {\attr{ID}} attribute). Element and attribute names are further distinguished in this paper by being shown in a {{\tt fixed-width}} font. \section{Data Model} In this section we define the data model of a VOTable, and in the next sections its syntax when expressed as XML. The data model of VOTable can be expressed as: \medskip \begin{tabular}{rrcp{0.7\textwidth}} \hspace{3em}&{\bf VOTable} &=& hierarchy of {\bf Metadata} + associated {\bf TableData}\\ &{\bf Metadata} &=& {\bf Parameters} + {\bf Infos} + {\bf Descriptions} + {\bf {\fg{black}Links + Fields + Groups}}\\ &{\bf Table} &=& list of {\bf Fields + TableData}\\ &{\bf TableData} &=& stream of {\bf Rows}\\ &{\bf Row} &=& list of {\bf Cells}\\ &{\bf Cell} &=& $\left\{ \begin{tabular}{l} {\bf Primitive} \\ or variable-length list of {\bf Primitives} \\ or multidimensional array of {\bf Primitives}\\ \end{tabular} \right.$ \\ &{\bf Primitive} &=& integer, character, float, floatComplex, etc (see \Tref{primitives}{table of primitives} below). \end{tabular} \medskip \par\noindent Metadata is divided into that which concerns the table itself (parameters), and the definitions of the fields (or column attributes) of the table. Each \elem{FIELD} represents the metadata that can be found at the top of the column in a paper version of the table: in the example introduced in \Aref{example1}{the section} below, the first \elem{FIELD} has its \attr{name} attribute set to \value{RA}. The Field can be thought of as a class definition, and the table cells below it are the instances of that class. A parameter ({\elem{PARAM}}) is similar to a {\elem{FIELD}}, except that it has a \attr{value} attribute. Parameters can be seen as ``constant columns'', containing for instance FITS keywords or any other information pertaining to the table itself or its environment, as the {\tt Epoch} parameter in the above example. \label{elem:INFO} An informative parameter ({\elem{INFO}}) is a restricted form of the {\elem{PARAM}} -- it has only the \attr{name} / \attr{value} pair of attributes. The ordered list of Fields at the top of the table thus provides a template for a Row object (also called a {\it record}). The template allows interpretation of the data in the Row. In VOTable, there is no advance specification of the number of rows in the table: this is to allow streaming of large tables, as discussed above. The record is a set of Cells, with the number of Cells the same for each Row, and the same as the number of Fields defined in the Metadata. From Version 1.1, columns may be logically grouped, so that it is possible to define table substructures made of column associations. Such an association is declared as a \elem{GROUP}, which typically contains columns (\elem{FIELD}) and associated parameters (\elem{PARAM}). \subsection{Primitives} \ifhtx\label{primitives} \begin{center}\Beg{tabular}{CELLSPACING=4 CELLPADDING=4}{|rlrl|} \else \begin{table}[hbt] \begin{center}\begin{tabular}{|r|l|c|r|} \fi\hline {\attr{datatype}} & Meaning & \attr{FITS} & { Bytes} \\ \hline \value{boolean} & Logical &\value{L}& 1 \\ \value{bit} & Bit &\value{X}& * \\ \value{unsignedByte} & Byte (0 to 255) &\value{B}& 1 \\ \value{short} & Short Integer &\value{I}& 2 \\ \value{int} & Integer &\value{J}& 4 \\ \value{long} & Long integer &\value{K}& 8 \\ \value{char} & ASCII Character &\value{A}& 1 \\ \value{unicodeChar} & Unicode Character& & 2 \\ \value{float} & Floating point &\value{E}& 4 \\ \value{double} & Double &\value{D}& 8 \\ \value{floatComplex} & Float Complex &\value{C}& 8 \\ \value{doubleComplex}& Double Complex &\value{M}& 16 \\ %logical & 1 \\ %bit & * \\ %byte & 1\\ %short & 2 \\ %int & 4 \\ %long & 8 \\ %char & 1 \\ %unicodeChar & 2 \\ %float & 4 \\ %double & 8 \\ %floatComplex & 8 \\ %doubleComplex & 16 \\ \hline\end{tabular}\end{center} \ifhtx\par \else\caption{\label{primitives}List of the Primitives {\em(details in \Aref{sec:datatypes}{})}}\end{table} \fi Each Cell is composed from Primitives, each of which is a datatype of fixed-length binary representation, as listed in \Tref{primitives}{the accompanying table}. %(detailed in %\Aref{sec:datatypes}{the section below}). Cells may consist of a single Primitive (this is the default), or of a multidimensional array of Primitives (see \Aref{array}{the next section}). Except for the Bit type, each primitive has the fixed length in bytes given in \Tref{primitives}{the table}. Bit scalars and arrays are stored in the minimum number of bytes feasible (so that $b$ bits take the integer part of $(b+7)/8$ bytes). %It is this fixed size that allows efficiency %in storage, so that the memory used is minimized. These primitives are described in more detail in \Aref{sec:datatypes}{section 7}. VOTables support two kinds of characters: ASCII 1-byte characters and Unicode 2-byte characters. Unicode is a way to represent characters that is an alternative to ASCII. It uses two bytes per character instead of one, it is strongly supported by XML tools, and it can handle a large variety of international alphabets. Therefore VOTable supports not only ASCII strings ({\attrval{datatype}{char}}), but also Unicode ({\attrval{datatype}{unicodeChar}}). Note that strings are not a primitive type: strings are represented in VOTable as an array of characters. %in an characters are. %In VOTable, characters are Primitives, either one byte for an %ASCII character or two bytes for a Unicode character. \subsection{Multidimensional Arrays}\label{array} \label{sec:dim} A table cell can contain an array of a given primitive type. The array is specified by a sequence of dimensions, with the first dimension changing fastest, and the last dimension that may be variable in length. For example, the following \elem{FIELD} definition declares a table cell which may contain a set of up to 10 images, each 64x64 bytes: \elemdef{FIELD}{ \attrval{ID}{thumbs} \attrval{datatype}{unsignedByte} \attrval{arraysize}{64x64x10*}\slash} The string in the \attr{arraysize} attribute expressed these dimensions, each integer separated by the {\tt x} character, except the last. The last (slowest-varying) subscript of a multidimensional array may have variable length, meaning that the dimensionality of the final subscript may be different for different rows of the table. In this case, there may be just an asterisk, in which case the array may be arbitarily large; or a number followed by an asterisk, meaning that this subscript is guaranteed not to exceed this value. \iffalse Variable-length arrays are more efficient in storage and data transfer, but less efficient computationally, because extra pointer dereferencing is required to access the data. %The reason that only %the slowest-varying subscript can be variable is effectively a demand %that the data provider pack the data into a small number of %fixed-size containers as much as is possible, rather than a large %number of small containers. \fi Strings can therefore be represented in VOTable as a fixed- or variable-length array of characters: \elemdef{FIELD}{ \attrval{name}{unboundedString} \attrval{datatype}{char} \attrval{arraysize}{*}\slash} %\begin{verbatim} % %\end{verbatim} A 1D array of strings can be represented as a 2D array of characters, but given the logic above, it is possible to define a variable-length array of fixed-length strings, but not a fixed-length array of variable-length strings. A convention to express an array of variable-length strings was proposed (see \Aref{sec:arraystring}{in the appendix}) but is not part of this standard. \subsection{Compatibility with FITS Binary Tables} VOTable is closely compatible with the FITS Binary Table format. Henceforth, we shall abbreviate ``FITS Binary Table and its Conventions" simply by the word ``FITS". Given a FITS file that represents a binary table, the header may be converted to VOTable, with a pointer to the original file, or with the original file included directly in VOTable. Since the original file is still present, it is clear that no data has been lost. A {\elem{PARAM}} element can be used to hold any FITS keyword with its value and comment string. We might ask two more significant questions, about how much of the FITS header and data can be represented in VOTable. The answer is that there is considerable overlap. For instance, the recommended formatting of the data for an edition of the data is expressed by the non-mandatory TDISP keyword: %FITS has semantics for how data is to be %represented when printed, expressed by the non-mandatory TDISP keyword: for example F12.4 means 12 characters are to be used, and 4 decimal places. This has been converted in VOTable as the attributes {\attr{width}} and {\attr{precision}} which, connected with {\bf {\attr{datatype}}}, are semantically identical to the TDISP keyword. \iffalse Note that error estimation and the number of digits to print are rather different semantically. \fi \subsubsection*{What can FITS do but not VOTable?} FITS has a complex semantics (the ``Substring Array" convention) for structuring a single string as a collection of substrings, and VOTable 1.1 does not support this {\em(see however the \Aref{sec:arraystring}{appendix} which proposes a compatible VOTable definition)}. The current version of VOTable allows fixed and variable-length strings, as well as variable-length arrays of fixed length strings. \subsubsection*{What can VOTable do but not FITS?} VOTable supports separating of data from metadata and the streaming of tables, and other ideas from modern distributed computing. It bridges two ways to express structured data: XML and FITS. It tries (through the UCD -- see \Aref{sec:ucd}{below}) to express formally the semantic content of a parameter or field. It has the hierarchy and flexibility of XML: using \elem{GROUP} elements introduced in version 1.1, a VOTable can represent structures of arbitrary complexity; and the ID attribute can be used in XML to enable what are essentially pointers. FITS does not handle Unicode (extended alphabet) characters. \bigskip \noindent{\fg{black}It should be noticed that the transformation of FITS to VOTable is meant to be reversible: any FITS table can be converted to a VOTable without loss of information and the resulting VOTable can be converted back to a FITS table also without loss of information. However, it is possible to create new VOTables which cannot be converted to FITS tables without loss of information. \iffalse the conversion of a FITS file into VOTable does not lose any information, and a transformation back into FITS is possible. It will however not be possible to transform any VOTable into a FITS file without losing some information. \fi} \section{The VOTable Document Structure} \label{elem:VOTABLE} The overall VOTable document structure is described and controlled by its \A{http://vizier.u-strasbg.fr/xml/VOTable-1.1.xsd}{XML Schema} referenced at its top. % of the document. %The XML Schema corresponding to this version of VOTable %is accessible from %http://vizier.u-strasbg.fr/cgi-bin/VizieR %however hardly human-readable, and is replaced in this document %by its equivalent This schema actually represents the VOTable definition, which means that documents claiming to represent VOTables should pass through W3C XML Schema validators without error. An illustration of the XML Schema is given in \Aref{dtd}{section 7}. An example %(presented in \Aref{example1}{below}) is used here to illustrate the components of a VOTable document described in the following sections. Basically, a VOTable document consists of a single all-containing element called {\elem{VOTABLE}}, which contains descriptive elements ({\elem{DESCRIPTION}}, \elem{DEFINITIONS}, \elem{INFO}), followed by one or more {\elem{RESOURCE}} elements. Each Resource element contains one or more \elem{TABLE} elements, and possibly other \elem{RESOURCE} elements. The \elem{TABLE} element, the actual heart of VOTable, contains a description of the columns and parameters (described in \Aref{sec:field}{the next section}) followed by the data values (described in \Aref{sec:data}{the following section}). \subsection{Example} This simple example of a VOTable document lists 3 galaxies with their velocity with its error, and the estimated distance. % \label{example1} \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small \fi \begin{verbatim} Velocities and Distance estimations Distance of Galaxy, assuming H=75km/s/Mpc
010.68+41.27N 224-29750.7
287.43-63.85N 6744839610.4
023.48+30.66N 598-18230.7
\end{verbatim} \ifhtx\End{TABULAR} \else %\caption{\label{example1}A simple VOTable example} \endgroup \fi This simple \elem{VOTable} document shows a single \elem{RESOURCE} made of a single \elem{TABLE}; the table is made of 6 columns, each described by a \elem{FIELD}, and has one additional \elem{PARAM} parameter (the Epoch). The actual rows are listed in the \elem{DATA} part of the table, here in XML format (introduced by \elem{TABLEDATA}); each cell is marked by the \elem{TD} element, and follow the same order as their \elem{FIELD} description: {\sl RA, Dec, Name, RVel, e\_RVel, R}. \subsection{ID and name attributes} \label{sec:name} Most of the elements defined by VOTable may or have to bear {\em names}, like a \elem{RESOURCE}, a \elem{TABLE}, a \elem{PARAM} or a \elem{FIELD}. Naming an element is generally possible by means of one of or both \attr{ID} and \attr{name} attributes. {\attr{ID}} and {\attr{name}} attributes have a different role in VOTable: the ID is meant as a {\em unique identifier} of an element seen as a VOTable component, while the name is meant for presentation purposes, and need not to be unique throughout the VOTable document. %The value of each of \attr{ID} and \attr{name} attributes defaults %to the other value. \iffalse defaults to the same as the \elem{ID} value if not present. The ID's of the fields and parameters {\em must be unique throughout the document} -- this is part of the XML specification, and we intend that eventually ID's can be used to tie together data sources and applications that read and write them. The name of an object, when not present, defaults to the ID value, and when it is present is intended for presentation to humans. \fi The {\attr{ID}} attribute %(as defined by Xpointer standard) is therefore required in the elements which {\em have to be referenced}, but in principle any element may have an {\attr{ID}} attribute. According to the XML standard, the attribute {\attr{ID}} is a string beginning with a letter or underscore ({\tt{\_}}), followed by a sequence of letters, digits, or any of the punctuation characters {\tt.} (dot), {\tt-} (dash), {\tt\_} (underscore), or {\tt:} (colon). %and each ID {\it must be unique} in the XML document. For example %{\attrval{ref}{apple}} %refers to the element that contains {\attrval{ID}{apple}} %in the current XML document. The {\attr{ID}} attribute is required %for the elements which have to be referenced, but in principle any element %may have an {\attr{ID}} attribute. %Elements that support the {\attr{ref}} %attribute (and can point to those with ID) are: {\elem{FIELD}}, %{\elem{PARAM}}, and {\elem{TABLE}}. In summary, the {\attr{ID}} is different from the {\attr{name}} attribute in that (a) the ID attribute is made from a restricted character set, and must be unique throughout a VOTable document %(or else the document is considered invalid in the XML sense), whereas names are standard XML attributes and need not be unique; and (b) there should be support in the parsing software to look up references and extract the relevant element with matching ID. \subsection{DEFINITIONS element} This element may contain a definition of a coordinate system, stored in a {\elem{COOSYS}} element. The \elem{COOSYS} element provides attributes for equinox and epoch, as well as a specification of the celestial coordinate system. The \elem{COOSYS} element being the only astronomy specific part of VOTable, it may be deprecated in the future, as it is expected that a more formal structuring of the coordinate system will be designed, which would encompass conventions used in space science or solar physics. Its current definition is given below. The \elem{DEFINITIONS} element may also include one or more {\elem{PARAM}} elements (\Aref{sec:field}{section 4}) that may contain user-specific data. Each of these may have an \attr{ID} attribute, that can be referenced with the \attr{ref} attribute of other elements. \subsubsection*{The \elem{COOSYS} element} \label{elem:COOSYS} This element defines a celestial coordinate system, to which the components of a position on the celestial sphere refer. It has an \attr{ID} attribute --- required if the \elem{COOSYS} element has to be referred via the \attr{ref} attribute of the position components, which is generally the case --- a \attr{system} attribute which specifies the coordinate system among \value{ICRS}, \value{eq\_FK5}, \value{eq\_FK4}, \value{ecl\_FK4}, \value{ecl\_FK5}, \value{galactic}, \value{supergalactic}, \value{barycentric}, \value{geo\_app} and a user-defined \value{xy} value. \attr{equinox} is the parameter required to fix the equatorial or ecliptic systems (as e.g. \value{J2000} as the default \value{eq\_FK5} or \value{B1950} as the default \value{eq\_FK4}), and \attr{epoch} specifies the epoch of the positions if necessary. As mentioned above, the \elem{COOSYS} may be deprecated in the future in favor of a more generic way of describing the conventions used to define the positions. \subsection{RESOURCE element} \label{sec:resource} \label{elem:RESOURCE} A VOTable document contains one or more {\elem{RESOURCE}} elements, each of these providing a description and the data values of some logically independent data structure. Each \elem{RESOURCE} may include the descriptive elements {\elem{DESCRIPTION}}, {\elem{INFO}}, {\elem{COOSYS}} and {\elem{PARAM}}; it may also contain {\elem{LINK}} elements to provide URL-type pointers that give further information. The main component of a \elem{RESOURCE} is typically one or more \elem{TABLE} elements -- in other terms a \elem{RESOURCE} is basically a set of related tables. The \elem{RESOURCE} is recursive (it can contain other \elem{RESOURCE} elements), which means that the set of tables making up a \elem{RESOURCE} may become a complex structure. A \elem{RESOURCE} may have one or both of the \attr{name} or \attr{ID} attributes (see \Aref{sec:name}{above}); it may also be qualified by \attrval{type}{meta}, meaning that the resource is {\em descriptive} only (does not contain any actual data in any of its sub-elements). %\section{5 Parameters and Hierarchy} %The VOTable document is a hierarchy of {\elem{RESOURCE}} %elements, each of which can contain other {\elem{RESOURCE}}s. %The root of the tree is the single {\elem{VOTABLE}} %element that constitutes the entire XML document, and the leaves of %the tree are {\elem{TABLE}}s. %The resource elements may also contain parameters ({\elem{PARAM}}). \iffalse The {\elem{RESOURCE}} may also contain contain other {\elem{RESOURCE}}s: the following is a complete VOTable which contains no tables, only a hierarchy of parameters. \begin{verbatim} \end{verbatim} The main ingredient of the {\elem{RESOURCE}} element is one or more {\elem{TABLE}}s. These are described in \Aref{sec:field}{section 5} of this document. \fi \subsection{LINK element} \label{sec:link} \label{elem:LINK} The {\elem{LINK}} element is to provide pointers to other documents or data servers on the Internet through a URL. In VOTable, the {\elem{LINK}} element may be part of a {\elem{RESOURCE}}, {\elem{TABLE}}, \elem{GROUP} or {\elem{FIELD}} elements. The {\attr{href}} attribute of the {\elem{LINK}} element can comprise any arbitrary protocol, for example \value{http://server/file} or \value{bizarre://server/file}. VOTable parsers are not required to understand arbitrary protocols, but are required to understand the following three common protocols: \value{file:}, \value{http:} and \value{ftp:}. \iffalse %%% OLD is meant to provide a URL that is at least valid syntactically, even though there need be no assurance that the link will actually connect and deliver data. It may be that a strange protocol is implied that the parser does not know about, for example {\value{httpg://server/file}}. However, parsers are expected to understand at least the {\value{file}}, {\value{http}} and {\value{ftp}} protocols. \fi %%% OLD The {\attr{gref}} attribute is meant for a higher-level protocol of some type, perhaps a logical name for a data resource, perhaps a GLU reference [5]. In the Astrores format, from which VOTable is derived, there is additional semantics for the {\elem{LINK}} element; the \elem{href} attribute is used as a template for creating URL's. This behavior is explained in \Arefx{LINK}{Appendix A}, and it represents %a further proposal, a possible extension of VOTable. In addition to the referencing \attr{href} and \attr{gref} attributes and to the naming \attr{name} and \attr{ID} attributes (see \Aref{sec:name}{name and ID}), the \elem{LINK} element may announce the mime type of the data it references with a \attr{content-type} attribute (e.g. \attrval{content-type}{image/fits}), and specify the role of the link by a \attr{content-role} attribute (e.g. \attrval{content-role}{doc} for an access to a documentation). \subsection{TABLE element} \label{elem:TABLE} The \elem{TABLE} element represents the basic data structure in VOTable; it is made of a description of the table structure (the {\em metadata}) essentially in the form of \elem{PARAM} and \elem{FIELD} elements (detailed in \Aref{sec:field}{the next section}), followed by the {\em values} of the described fields in a \elem{DATA} element (detailed in \Aref{sec:data}{the section below}). The \elem{TABLE} element is always contained in a \elem{RESOURCE} element: in other terms any \elem{TABLE} element has a single father made of the \elem{RESOURCE} element in which the table is embedded. The \elem{TABLE} element contains a {\elem{DESCRIPTION}} element for descriptive remarks, followed by a mixed collection of \elem{PARAM}, \elem{FIELD} or \elem{GROUP} elements which describe a parameter (constant column), a field (column) or a group of columns respectively. \elem{PARAM} and \elem{FIELD} elements are detailed in \Aref{sec:field}{the next section}, and the \elem{GROUP} element %introduced in Version 1.1 is presented in \Aref{sec:group}{the following section}. Furthermore the \elem{TABLE} element may contain {\elem{LINK}} elements that provide URL-type pointers, exactly like the {\elem{LINK}} elements existing within a \elem{RESOURCE} element (see \Aref{sec:link}{above}). The last element included in a \elem{TABLE} is the optional \elem{DATA} element (see \Aref{sec:data}{below}): a table without any actual data is quite valid, and is typically used to supply a complete description of an existing resource e.g. for query purposes. The \elem{TABLE} element may have the naming attributes \attr{name} and/or \attr{ID} (see \Aref{sec:name}{name and ID conventions}). A \elem{TABLE} may also have a \attr{ref} attribute referencing the ID of another table previously described, which is interpreted as {\em defining a table having a structure identical to the one referenced}: this facility avoids a repetition of the definition of tables which may be present many times in a VOTable document. \iffalse \section{XML} VOTable is constructed with XML (extensible Markup Language), a powerful standard for structured data throughout the Internet industries. It derives %through simplification from SGML, %which has been a standard used in the publishing industry and for technical documentation for many years. XML consists of {\it elements} and payload, where an element consists of a {\it start tag} (the part in angle brackets), the payload, and an {\it end tag} (with angle brackets and a slash). Elements can contain other elements. Elements can also bear {\attr{attributes}} (keyword-value combinations), such as the {\elem{PARAM}} elements above. The payload may be in two forms: parsed or unparsed character data. Examples are: \begin{verbatim} François \end{verbatim} In the first example, the sequence {\tt \&\#231;} is interpreted as part of the ISO/IEC 10646 character set, and translates to an accented character, so that the text is ``Fran\c{c}ois". The second example uses the special {\tt CDATA} sequence so that the characters {\tt <}, {\tt >}, and {\tt\&} can be used without interpretation; in this case, any ASCII characters are allowed except the terminating sequence {\tt]]>} For more information, see any book on XML. \subsection{Lists} Within a table cell, multiple Primitives can be formatted by separating them by whitespace, to express compound primitives (complex numbers) and multidimensional arrays. Text tokens are separated by contiguous whitespace (ASCII space {\tt{0x20}}, tab {\tt{0x9}}, carriage-return {\tt{0xD}}, newline {\tt{0xA}}, vertical tab {\tt{0xB}}). There are no null tokens, comments, quote characters, or separators. However, it is possible to include special characters through an escape mechanism as in HTML: for example the string {\tt New York} would be encoded as {\tt New\ York}. Thus a table cell that contains an array of three complex numbers could be represented as: \begin{verbatim} 1.0 0.0 -0.5 0.866 -0.5 -0.866 \end{verbatim} However, it should be noted that in a character array (a string) no space is needed to separate each element (a character). \subsection{Syntax policy} Following the general XML rule, element and attribute names are case-sensitive and have to be used with the specified capitalisation. For VOTable, we have adopted the convention that element names should be in uppercase and attribute names in lowercase (with an exception for the {\attr{ID}} attribute). Element and attribute names are further distinguished in this paper by being in {{\tt fixed-width}} font. \subsection{Xlink and STREAM} The \elem{STREAM} element is used to point to remote table data, and as such it closely follows the W3C specification called ``Xlink". The {\elem{STREAM}} implements the interface defined by Xlink; in particular it is an Xlink with {\attrval{type}{locator}}. However, {\elem{STREAM}} has more attributes than Xlink allows for: a {\attr{rights}} attribute for authentication information; and {\attr{expires}} attribute for when the link may cease to be valid; and an {\attr{encoding}} attribute if the data is filtered, (for example compression or binary-to-ascii filtering). Therefore we will wait until a future release to formalize the relationship between Xlink and {\elem{STREAM}}. \subsection{Location of the DTD} A VOTable document, like all XML documents, should be {\it well-formed}, meaning that it obeys the syntax rules of XML: for example, elements should start and end properly and be properly nested. The document may be further constrained to be {\it valid}, meaning that it follows the VOTable syntax rules, as defined in the DTD of \Aref{dtd}: for example the data type ``float" is valid, but not the datatype ``real". Access to the DTD is necessary to check validity. Valid XML documents may employ certain advanced features of XML, features that can significantly improve the usability of a document, including: linking mechanisms, entities and attributes. Valid XML documents offer much more to the document process than those that may be simply well-formed. Document authoring, processing, storage and display are made easier because documents exist in a structured environment. Authors create documents against a pre-defined structure and benefit from a clear document model. There are three ways to give the document access to its DTD structure: by embedding the DTD directly into the XML file, or by referencing a local file, or by referencing a remote file. We should point out that many parsers will simply stop if the DTD reference cannot be resolved, rather than falling back to a non-validated document. Any XML book will explain the syntax of these options. \begin {itemize} \item Embedding the DTD means that the document is longer, since it includes all the text of the DTD (\Aref{dtd}{below}). It also means that the document is more portable, since it does not rely on external references that may not be available. However, the receiver of such a document does not know that it is a proper VOTable, since the DTD may be different from the official version. \item If the DTD is a local file, it means that the document processing still works even when the machine is disconnected from the internet. However, when the XML file is moved to a new directory or sent by email, the DTD must move with it. \item Referencing a DTD at a standard place on the internet makes the XML document portable, and files can be moved at will. However, the document processing will fail when not connected to the internet. We expect the VOTable DTD to be mirrored at several sites, including: \begin{tabular}{l} {\bf\fg{DarkRed}\Plain{http://ivoa.net/xml/VOTable.dtd}}\\ {\bf\fg{DarkRed}\Plain{http://cdsweb.u-strasbg.fr/xml/VOTable.dtd}}\\ \end{tabular} \end{itemize} \fi \section{FIELDs and PARAMeters} \label{sec:field} The atoms of the table structure are represented by \elem{FIELD} and \elem{PARAM} elements, where \elem{FIELD} represents the description of an actual table column, while \elem{PARAM} supplies a value which remains constant over the whole table, like the \attr{Epoch} in \Aref{example1}{the example}. A \elem{PARAM} may therefore be viewed as a \elem{FIELD} which keeps a {\em constant value} over all the rows of a table, and the only difference between the two elements is the existence of a \attr{value} attribute in a \elem{PARAM} which does not exist in a \elem{FIELD}. A {\elem{FIELD}} or \elem{PARAM} element may have several sub-elements, including the informational {\elem{DESCRIPTION}} and {\elem{LINK}} elements; it may also include a {\elem{VALUES}} element that can express limits and ranges of the values that the corresponding cell can contain, such as minimum (\elem{MIN}), maximum (\elem{MAX}), or enumeration of possible values (\elem{OPTION}). \subsection{FIELD attributes} \label{elem:FIELD} \label{elem:PARAM} The valid attributes of a \elem{FIELD} or \elem{PARAM} are: \begin{itemize} \item the \attr{name} and/or \attr{ID}. The \attr{ID} attribute is required if the field has to be referenced (see \Aref{sec:name}{the generic ID rule}). It may help to include the ordinal number of the column in the table in the value of the \attr{ID} attribute as e.g. \attrval{ID}{col3} when a single table is involved: the connection to the corresponding column would become more obvious, especially in the FITS data serialization which uses the ordinal column number in the keywords containing the metadata related to that column. \item the \attr{datatype}, which expresses the nature of the data that is described as one of the permitted primitives (see \Tref{primitives}{table above} and their exact meaning in \Aref{sec:datatypes}{section 7}). This attribute determines how data are read and stored internally; it is {\em required}, except when the \attr{ref} attributes exists in which case the \elem{FIELD} is just referenced (see \Aref{sec:group}{the GROUP definitions}) \item the \attr{arraysize} attribute exists when the corresponding table cell contains more than one of the specified datatype, as explained \Aref{sec:dim}{above}. Note that strings are are not a primitive type, and have to be described as an array of characters. \item {\fg{black}}the \attr{width} and \attr{precision} attributes define the numerical accuracy associated to the data (see \Aref{sec:form}{below}) \item the \attr{unit} attribute specifies the units in which the values of the corresponding column are expressed (see \Aref{sec:unit}{below}) \item the \attr{ucd} attribute supplies a standardized classification of the physical quantity expressed in the column (see \Aref{sec:ucd}{below}). \item the \attr{utype} attribute, introduced in VOTable 1.1, is meant to express the role of the column in the context of an external data model (see \Aref{sec:utype}{below}). \item the \attr{ref} attribute defines the field as being a {\em reference} to a column having the referenced \attr{ID} attribute. This attribute normally exists alone: if present, it precludes the existence of any other attribute except a \attr{utype} attribute, and a \attr{value} attribute for \elem{PARAM} elements. \item The \attr{type} attribute is {\em not} part of this standard, but is reserved for future extensions. In Astrores it was used to express some pecularities of the column in the table as \attrval{type}{hidden} (see \Arefx{query}{link substitutions}) and \attrval{type}{no\_query} (see \Arefx{query}{Query Extension}); an additional \attrval{type}{location} value is proposed to express columns containing parts of URIs (see \Arefx{location}{fields as pointers}). The \attr{type} is not part of this standard, but is reserved for future extensions. \end{itemize} In addition, in the \elem{PARAM} element only: \begin{itemize} \item the \attr{value} attribute which exists only in the \elem{PARAM} element; this attribute is moreover {\em required}, even when the \elem{PARAM} contains the \attr{ref} attribute. \end{itemize} \subsection{Numerical Accuracy} \label{sec:form} The VOTable format is meant for transferring, storing, and processing tabular data, and is not intended for presentation purposes: therefore (in contrast to Astrores) we generally avoid giving rules on presentation, such as formatting. Inevitably however some at least of the data will have to be presented -- either as actual tables, or in forms or graphs, etc... Two attributes were retained for this purpose: \begin{itemize} \item the {\attr{width}} attribute %of the {\elem{FIELD}}, is meant as a hint to the application about the number of characters to be used for input or output of the quantity. \item the \attr{precision} attribute is meant to express the number of significant digits, either as a number of decimal places (e.g. \attrval{precision}{F2} or equivalently \attrval{precision}{2} to express 2 significant figures after the decimal point), or as a number of significant figures (e.g. \attrval{precision}{E5} indicates a relative precision of $10^{-5}$). \end{itemize} \iffalse But there is a semantic difference between a number written as \value{5.12} and one that is written \value{5.1200}, in that the former implies three significant digits of accuracy, and the latter five digits. Therefore the number of digits to show is not purely a presentation matter, but part of the metadata content of the number. \fi The existence and presentation of the special {\em null} value of a field (when the actual value of the field is unknown) is another aspect of the numerical accuracy, which is part of the \elem{VALUES} sub-element (see \Aref{sec:values}{below}). \subsection{Units} \label{sec:unit} The quantities in a column of the table may be expressed in some physical unit, which is specified by the {\attr{unit}} attribute of the {\elem{FIELD}}. The syntax of the {\em unit} string is defined in reference [3]; it is basically written as a string without blanks or spaces, where the symbols {\bf.} or {\bf*} indicate a multiplication, {\bf/} stands for the division, and no special symbol is required for a power. Examples are \attrval{unit}{m2} for m$^2$, \attrval{unit}{cm-2.s-1.keV-1} for cm$^{-2}$s$^{-1}$keV$^{-1}$, or \attrval{unit}{erg/s} for erg\,s$^{-1}$. The references [3] provides also the list of the valid symbols, which is essentially restricted to the {\em Syst\`eme International} (SI) conventions, plus a few astronomical extensions concerning units used for time, angular, distance and energy measurements. %\begin{verbatim} % units="cm-2.s-1.keV-1" % units="erg.s-1" %\end{verbatim} % %The syntax of this string is defined in reference [3]. \subsection{Unified Content Descriptors} \label{sec:ucd} The Unified Content Descriptors (UCD) can be viewed as a hierarchical glossary of the scientific meanings of the data contained in the astronomical tables. The initial version was created at CDS, but the UCD definition is currently evolving [4]. %The CDS in Strasbourg has used the metadata from thousands of %astronomical tables to make a hierarchical glossary of the scientific %meanings of the data in those tables [4]. Of 1600 entries in the %glossary, here are a few typical examples. \noindent A few typical examples taken from the original UCD design: \begin{tabular}{ll} {\value{PHOT\_INT-MAG\_B}} & Integrated total blue magnitude \\ {\value{ORBIT\_ECCENTRICITY}} & Orbital eccentricity \\ {\value{STAT\_MEDIAN}} & Statistics Median Value \\ {\value{INST\_QE}} & Detector's Quantum Efficiency \\ \end{tabular} %\noindent but the contents of the %{\attr{ucd}} attribute will follow the evolution of the UCD %standard. %of the {\elem{FIELD}} element %is to hold this information. \subsection{The \attr{utype} attribute} \label{sec:utype} In some contexts, it can be important that \elem{FIELD}s or \elem{PARAM}eters are explicitely designed as being {\em the} parameter performing some well-defined role in some external data model. For instance, it might be important for an application to know that a given \elem{FIELD} expresses {\em the} surface brightness processed by an explicit method. None of the existing \attr{name}, \attr{ID} or \attr{ucd} attributes can fill this role, and the \attr{utype} (usage-specific or {\em unique} type) attribute has been added in VOTable 1.1 to fill this gap. In order to avoid name collisions, the data model identification should be introduced following the XML namespace conventions, as \attrval{utype}{{\em datamodel\_identifier:role\_identifier}}. The mapping of \value{datamodel\_identifier} to an xml-type attribute is recommended, but not required. \iffalse The respective roles of these attributes are: \begin{itemize} \item \attr{name}: a name freely defined by the data server \item \attr{ID}: an XML identifier designating uniquely a field within a document \item \attr{ucd}: the characterisation of the field contents, with a scope essentially devoted to interoperability \item \attr{utype}: the relation to an explicit parameter described in a data model. The contents of this attribute is moreover proposed to include a {\em namespace} part, as \attrval{utype}{{\em namespace}:datamodel-{\em parameter\_name}} \end{itemize} It was effectively proposed during the discussions on UCDs that the \attr{ucd} attribute could be replaced by a pointer to some data model in the future; in practice it seems rather impossible to the UCD to play simultaneously a role of {\em global meaning} enabling global interoperability and to play the role of defining precisely which parameter is represents in the context of a data model. The \attr{utype} attribute is a simple solution to this dilemna. \elem{FIELD} and \elem{PARAM} sharing the same set of attributes (with the exception of the \attr{value} attribute), it is proposed that the \elem{PARAM} entity can also exhibit a \attr{utype} attribute. \fi \subsection{VALUES element} \label{sec:values} The {\elem{VALUES}} element of the {\elem{FIELD}} is designed to hold subsidiary information about the {\em domain} of the data. For instance, in the example (\Aref{example1}{above}) we could rewrite the RA field definition as: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small\fi \begin{verbatim} \end{verbatim} \ifhtx\End{TABULAR}\else\endgroup\fi \label{elem:VALUES} \label{elem:MIN} \label{elem:MAX} \label{elem:OPTION} The \elem{VALUES} element may contain {\elem{MIN}} and {\elem{MAX}} elements, and it may contain {\elem{OPTION}} elements. The latter may itself contain more {\elem{OPTION}} elements, so that a hierarchy of keyword-values pairs can be associated with each field. All three \elem{MIN}, \elem{MAX} and \elem{OPTION} sub-elements store their value corresponding to the minimum, maximum, or ``special value'' in a \attr{value} attribute. \elem{MIN} and \elem{MAX} elements can have an \attr{inclusive} attribute to specify whether the \attr{value} quoted belongs or not to the domain, and the \elem{OPTION} element can have a \attr{name} attribute to qualify the ``special'' quoted \attr{value}. The \elem{VALUES} element may also have a \attr{null} attribute to define a non-standard value that is used to specify {\em``non-existent data''} -- for example \attrval{null}{-32768}. When this value is found in the corresponding data, it is assumed that no data exists for that table cell; the parser may choose to use this also when unparsable data is found, and the null value will be substituted instead. \iftrue % Added FO 2002-03-21 In the \elem{TABLEDATA} data representation, the default representation of a ``null'' value is an empty column (i.e. \verb++); for fields containing arrays, individual ``null'' elements of the array can be specified either by the value specified in the \attr{null} attribute, or by the \value{NaN} or \value{nan} text in place of the expected numeric value. For the \elem{FITS} and \elem{BINARY} data representations, the {\em NaN} (not-a-number) patterns are recommended to represent floating-point ``null'' values. The ``null'' convention is therefore only necessary for primitive types that do not have a natural ``null'' value: long, int, short, and byte datatypes. \fi \iffalse %%% THIS IS WRONG -- does not match FITS rules For fields containing arrays or complex numbers, the values specified in the \attr{value} or \attr{null} attributes have to be compatible with their datatype, and contain as many numbers separated by white space as implied by the \attr{datatype} and \attr{arraysize} of the parent \elem{FIELD}. \fi The scope of the domain described by the \elem{VALUES} element can be qualified by \attrval{type}{actual}, if it is only applicable to the data enclosed in the parent \elem{TABLE}. The domain of a valid RA in the example above has the default \attrval{type}{legal} qualification. Finally the \attr{ref} attribute of a \elem{VALUES} element can be used to avoid a repetition of the domain definition, by referring to a previously defined \elem{VALUES} element having the referenced \attr{ID} attribute. When specified, the \attr{ref} attribute defines completely the domain without any other element or attribute, as e.g. \elemdef{VALUES}{ \attrval{ref}{RAdomain}\slash} \subsection{GROUPing FIELDs and PARAMeters} \label{sec:group} \label{elem:GROUP} The \elem{GROUP} element was added in VOTable 1.1, to group together a set of \elem{FIELDS} which are logically correlated, like a value and its error. Each field participating in a \elem{GROUP} can be defined either {\em physically} (the \elem{FIELD} contains a \attr{datatype} field), or {\em logically} (the \elem{FIELD} contains only a \attr{ref} attribute referencing a field defined in the same parent \elem{TABLE}). A physical field (i.e. a single column of the table) may therefore participate (logically) to several groups. A straightforward example of a group, based on the example of \Aref{example1}{above}, can be to replace the definitions of columns 4 and 5 by the following: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small\fi \begin{verbatim} Velocity and its error \end{verbatim}\ifhtx\End{TABULAR}\else\endgroup\fi A {\em logical} definition of this group could alternatively be achieved by inserting just before the \elem{DATA} element the following: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small\fi \begin{verbatim} Velocity and its error \end{verbatim}\ifhtx\End{TABULAR}\else\endgroup\fi The \elem{GROUP} element can have the \attr{name}, \attr{ID}, \attr{ucd}, \attr{utype} and \attr{ref} attributes. It can include a \elem{DESCRIPTION}, and any mixture of \elem{FIELD}s, \elem{PARAM}eters, and other \elem{GROUP}s -- this recursive grouping enabling a definition of arbitrary complex structures. The possibility of adding \elem{PARAM}eters in groups introduces also a possibility of associating parameter(s) to describe accurately the context of the data stored in the table: for instance, it is possible to associate the actual frequency of a radio survey with the following declaration: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||}\fi \begin{verbatim} Flux measured at 352MHz \end{verbatim}\ifhtx\End{TABULAR}\fi \par Similarly, the \elem{GROUP} can be used to associate several parameters to one or several \elem{FIELD}s: a filter may for instance be characterized by the central wavelength and the FWHM of its transmission curve; or several parameters of an instrument setup may be detailed. \section{Data Content} \label{sec:data} While the bulk of the metadata of a VOTable document is in the {\elem{FIELD}} elements, the data content of the table is in a single {\elem{DATA}} element. The data is organized in ``reading" order, so that the content of each row appears in the same order as the order of the {\elem{FIELD}} tags having a \attr{datatype} attribute, with each row having the same number of items as there are {\elem{FIELD}} tags having a \attr{datatype} attribute. Fields without a \attr{datatype} attribute have a \attr{ref} attribute, and represent references to ``true'' columns (see \Aref{sec:field}{FIELD attributes}). \label{Image1} \ifhtx\tag{IMG SRC="serial.gif" NAME="Image1" ALIGN=BOTTOM BORDER=0}\else \begin{figure}[hbt] \includegraphics[width=\textwidth]{serial.jpg} \caption{\label{fig:serialization}Data serialization} \end{figure} \fi Each \elem{DATA} part of the VOTable document can be viewed as a stream coming out of a pipeline. The abstract table is first serialized by one of several methods, then it may be encoded for compression or other reasons. The result may be embedded in the XML file ({\it local} data), or it may be {\it remote} data. \Fref{fig:serialization}{The figure} shows how the abstract table is rendered into the VOTable document. First the data is {\it serialized}, either as XML, a FITS binary table, or the VOTable Binary format. This data stream may then be {\it encoded}, perhaps for compression or to convert binary to text. Finally, the data stream may be put in a remote file with a URL-type pointer in the VOTable document; or the table data may be embedded in the VOTable. The serialization elements and their attributes are described in the next sections. %\subsection{Data Serialization} \subsection{TABLEDATA Serialization} \label{sec:TABLEDATA} \label{elem:TD} This element is a way to build the table in pure XML, and is the only serialization method that does not allow an encoding or a remote data stream. It contains {\elem{TR}} elements, which in turn contain {\elem{TD}} elements --- i.e. the same conventions as the familiar {\em HTML} ones. An example is contained in \Aref{example1}{section 3.1}, surrounded by in the \elemdef{TABLEDATA}{} and \elemdef{\slash TABLEDATA}{} delimiters. \iffalse \begin{verbatim} Procyon 114.827242 5.227506 Vega 279.234106 38.782992 \end{verbatim} \fi The number of {\elem{TD}} elements should be in number equal to the number of {\elem{FIELD}} elements having \attr{datatype} attributes declaring the table; when there are less {\elem{TD}}'s than expected, the corresponding values are set to \value{null}s; superfluous {\elem{TD}}'s are ignored. While this serialization has a high overhead in the number of bytes, it has the advantage that XML tools can manipulate and present the table data directly. Each item in the {\elem{TD}} tag is passed to a reader that is implicitly defined by the {\attr{datatype}} attribute of the corresponding {\elem{FIELD}}, which attempts to read the object from it. If it reads a value that is the same as the {\attr{null}} value for that field, then the cell will contain that value, and is therefore assumed to contain no data. Valid representations of a number in a cell, depending on their \attr{datatype}, are detailed in \Aref{sec:datatypes}{the complete description of datatypes}. \iffalse The reader may not succeed, for example if we try to parse the string \value{36.9H9} into a float, where the alphabetic character is obviously a problem. In this case, the parser may choose to insert the {\attr{null}} value (no data available), or it may use a NaN (not a number), or it may throw an exception. It might however be useful if the data provider would warn that invalid data patterns could be used to designate non-existing data -- via an \attr{invalid} attribute. \fi If a cell contains an array or complex number, it should be encoded as multiple numbers separated by whitespace. However in the case of character and Unicode strings, no separators are required. Here is an example of a table with a two rows, that has arrays in the table cells: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small\fi \label{example2} \begin{verbatim}
Apple1.62 4.56 3.44 67 1.57 4 3.14 77 -1.57
Orange2.33 4.66 9.53 39 0 46 3.14
\end{verbatim} \ifhtx\End{TABULAR}\else\endgroup\fi The first entry is a fixed-length array of 10 characters; since the value being presented ({\tt Apple}) has 5 characters, this is padded with trailing blanks. The second cell is an array of three floats. The last cell contains a variable array of complex numbers, each complex number being represented by its real part followed by at least a blank and its imaginary part -- hence 6 numbers for 3 complex numbers, or 4 numbers for 2 complex numbers. \subsection{FITS Serialization} \label{sec:FITS} \label{elem:FITS} The FITS format for binary tables [2] is in widespread in astronomy, and its structure has a major influence on the VOTable specification. Metadata is stored in a header section, followed by the data. The metadata is substantially equivalent to the metadata of the VOTable format. One important difference is that VOTable does not require specification of the number of rows in the table, an important freedom if the table is being created dynamically from a stream. The VOTable specification does not define the behavior of parsers with respect to this doubling of the metadata. A parser may ignore the FITS metadata, or it may compare it with the VOTable metadata for consistency, or other possibilities. The following code shows a fragment that might have been created by a FITS-to-VOTable converter. Each FITS keyword has been converted to a \elem{PARAM}, and the data itself is remotely stored and gzipped at an ftp site: \begin{plain}\small \elemdef{RESOURCE}{}\\ \hspace*{0.5em}\elemdef{PARAM}{ \attrval{name}{EPOCH} \attrval{datatype}{float} \attrval{value}{1999.987}}\\ \hspace*{1em}Original Epoch of the coordinates \hspace*{0.5em}\elemdef{\slash PARAM}{} \\ \hspace*{0.5em}\elemdef{PARAM}{ \attrval{name}{TELESCOP} \attrval{datatype}{char} \attrval{arraysize}{*} \attrval{value}{VTel} \slash}\\ \hspace*{0.5em}\elemdef{INFO}{ \attrval{name}{HISTORY}}\\ \hspace*{1em}The very first Virtual Telescope observation made in 2002 \\ \hspace*{0.5em}\elemdef{\slash INFO}{} \\ \hspace*{0.5em}\elemdef{TABLE}{} \hspace*{0.5em}\elemdef{FIELD}{\quad(insert field metadata here) }\\ \hspace*{0.5em}\elemdef{DATA}{}\elemdef{FITS}{ \attrval{extnum}{2}}\\ \hspace*{1em}\elemdef{STREAM}{ \attrval{encoding}{gzip} %Not for REMOTE data ? \attrval{href}{ftp://archive.cacr.caltech.edu/myfile.fit.gz}\slash}\\ \hspace*{0.5em}\elemdef{\slash FITS}{}\elemdef{\slash DATA}{} \\ \hspace*{0.5em}\elemdef{\slash TABLE}{}\\ \elemdef{\slash RESOURCE}{} \end{plain} The FITS file may contain many data objects (known as extensions, numbered from 1 up -- the main header being numbered 0), and the \attr{extnum} attribute allows the VOTable to point to one of these. \subsection{BINARY Serialization} \label{sec:BIN} The binary format is intended to be easy to read by parsers, so that additional libraries are not required. It is just a sequence of bytes, the length of each sequence corresponding to the {\attr{datatype}} and {\attr{arraysize}} attributes of the {\elem{FIELD}} elements in the metadata. The binary format consists of a sequence of records, with no header bytes, no alignment considerations, no block sizes. The order of the bytes in multi-byte primitives (e.g. integers, floating-point numbers) is Most Significant Byte first, i.e. it follows the FITS convention. Table cells may contain arrays of primitive types, each of which may be of fixed or variable length. In the former case, the number of bytes is the same for each instance of the item, as specified by the {\attr{arraysize}} attribute of the {\elem{FIELD}}. If all the fields have a fixed {\attr{arraysize}}, then each record of the binary format has the same length (the sum of {\attr{arraysize}} times the length in bytes of the corresponding {\attr{datatype}}). Variable-length arrays of primitives are preceded by a 4-byte integer containing the number of items of the array. The way the stream of bytes is arranged for the data of the example in \Aref{example2}{the above section} is illustrated in \Fref{fig:bin}{Figure 2}. The parser can then compute the number of bytes taken by the variable-length array by multiplying the size and number of the primitives. %Binary format becomes more complex. Fixed-length cells are stored %stored as is, has first a part for %the fixed-length fields, (as well as four bytes for each of the %variable-length fields), followed by a section for the variable %length fields. The four bytes for the variable-length field is %interpreted as a four-byte integer with %the number of items in %the variable-length array, as shown in the figure. The parser can %then read the data by computing appropriate offsets. This is done by %multiplying the size and number of the primitives in each table cell %to get length in bytes, then %adding these lengths from previous %variable-length sections of the record. \label{Image2} \ifhtx\begin{TABULAR}{c} \tag{IMG SRC="binary.gif" NAME="Image2" ALIGN=LEFT BORDER=0}\end{TABULAR} \else\begin{center} \begin{figure}[htb] \includegraphics[width=0.8\textwidth]{binary} \caption{\label{fig:bin}Data Storage in BINARY mode} \end{figure}\end{center} \fi \iffalse \par The figure shows the byte layout for this binary format for the same data as the example in \Aref{sec:TABLEDATA}{section 6.1.1}. Each record has a ten-byte character array, followed by an array of three four-byte floats, and then a variable number of complex numbers. The number of elements of these variable arrays are stored (3 and 2 in the two records shown), followed by the corresponding data (made of 24 and 16 bytes). \fi \subsection{Data Encoding} \label{elem:STREAM} As a result of the serialization, the table has been converted to a byte stream, either text or binary. If the {\elem{TABLEDATA}} serialization is used, then the table is represented as XML tags directly embedded in the document, document, and conventional tools can be used to encode the entire XML document. However, VOTable also provides limited encoding of its own. A VOTable document may point to a remote data resource that is compressed; rather than decompressing before sending on the wire, it can be dynamically decoded by the VOTable reader. We might also use the encoding facilities to convert a binary file to text (through base64 encoding), so that binary data can be used in the XML document. In this version (1.1) of VOTable, it is not possible to encode individual columns of the table: the whole table must be encoded in the same way. The possibility of encoding selected table cells is however being examined for future versions of VOTable (see \Arefx{sec:b64}{appendix below}). In order to use an encoding of the data, it must be enclosed in a {\elem{STREAM}} element, whose attributes define the nature of the encoding. The {\attr{encoding}} attribute is a string that should indicate to the parser how to undo the encoding that has been applied. Parsers should understand and interpret at the following values: \begin{itemize} \item {\attrval{encoding}{gzip}} [RFC1952] implies that the data following has been compressed with the {\em gzip} filter, so that {\em gunzip} or similar should be applied. \item {\attrval{encoding}{base64}} [RFC2045] implies that the {\em base64} filter has been applied, to convert binary to text. \item {\attrval{encoding}{dynamic}} implies that the data is in a remote resource (see below), and the encoding will be delivered with the header of the data. This occurs with the http protocol, where the MIME header indicates the type of encoding that has been used. \end{itemize} \noindent The default value of the encoding attribute is the null string, meaning that no encoding has been applied. In future releases, we might allow more complex strings in the encoding attribute, allowing combinations of encoding filters and a way for the parser to find the software needed for the decoding. \subsection{Remote Data} If the encoding of the data produces text, or if the serialization is naturally text-based, then it can be directly embedded into the XML document, as for instance: \begin{plain} \hspace*{0.5em}\elemdef{DATA}{}\elemdef{BINARY}{}\\ \hspace*{1em}\elemdef{STREAM}{ \attrval{encoding}{base64}}\\ \hspace*{1.5em} \verb+AAAAAj/yVZiDGSSUwFZ6ypR4yGkADwAcQV0euAAIAAJBmMzNwZWZmkGle4tBR3jVQT9ocwAA+\\ \hspace*{1.5em} $\cdots\cdots\cdots\cdots\cdots\cdots\cdots\cdots$\\ \hspace*{1em}\elemdef{\slash STREAM}{}\\ \hspace*{0.5em}\elemdef{\slash BINARY}{}\elemdef{\slash DATA}{} \end{plain} However, if the data is very large, it may be preferable to keep the data separate from the metadata. The \attr{href} attribute of the {\elem{STREAM}} element, if present, provides the location of the data in a URL-type syntax, for example: \begin{plain} \elemdef{STREAM}{ \attrval{href}{ftp://server.com/mydata.dat}\slash} \par\elemdef{STREAM}{ \attrval{href}{ftp://server.com/mydata.dat} \attrval{expires}{2004-02-29T23:59:59}\slash} \par\elemdef{STREAM}{ \attrval{href}{httpg://server.com/mydata.dat} \attrval{actuate}{onLoad}\slash} \par\elemdef{STREAM}{ \attrval{href}{file:///usr/home/me/mydata.dat}\slash} \end{plain} The examples are the well-known anonymous ftp, and http protocols. \value{httpg} is an example of a Grid-based access to data through httpg; \value{file} finally a reference to a local file. VOTable parsers are not required to understand arbitrary protocols, but are required to understand the three common protocols \value{file:}, \value{http:} and \value{ftp:}. There are further attributes of the {\elem{STREAM}} element that may be useful. The {\attr{expires}} attribute indicates the expiration time of the data: this is useful when data are dynamically created and stored on some staging disk where files only persist for a specified lifetime and are then automatically deleted. The {\attr{expires}} attribute expresses when a remote resource ceases to become valid, and is expressed in Universal Time in the same way as the FITS specification [2], itself conforming to ISO 8601 standard. %\begin{verbatim} % %\end{verbatim} The {\attr{rights}} attribute expresses authentication information that may be necessary to access the remote resource. If the VOTable document is suitably encrypted, this attribute could be used to store a password. The {\attr{actuate}} attribute is borrowed from the XML Xlink specification, expressing when the remote link should be actuated. The default is {\value{onRequest}}, meaning that the data is only fetched when explicitly requested (like a link on an HTML page), and the {\value{onLoad}} value means that data should be fetched as soon as possible (like an embedded image on an HTML page). \section{Definitions of Primitive Datatypes} \label{sec:datatypes} This section describes the primitives summarized in \Tref{primitives}{the table of primitives} and their representations in the \elem{BINARY} and in the \elem{TABLEDATA} serializations (see \Aref{sec:TABLEDATA}{above}). In the following, the term ``hexadigit'' designates the ASCII numbers \value{0} to \value{9}, or the ASCII lower- or upper-case letters \value{a} to \value{f} (i.e. a digit in an hexadecimal representation of a number). \begin{itemize} \item {\bf Logical}\quad If the value of the {\attr{datatype}} attribute specifies data type {\value{boolean}}, the contents of the field{ }shall consist in the \elem{BINARY} serialization of ASCII \value{T}, \value{t}, or \value{1} indicating true, ASCII \value{F}, \value{f}, or \value{0} indicating false; the ``null'' value is indicated by an sacii NULL (hexadecimal 00), a space (hexadecimal 20) or a question mark \value{?} (hexadecimal 3F). The acceptable representations in the \elem{TABLEDATA} serialization include in addition any capitalisation variation of the strings \value{true} and \value{false} (e.g. \value{tRUe} or \value{FalsE}); the default representation of a {\em null} value is an empty cell (see \Aref{sec:values}{VALUES definitions above}) \item {\bf Bit Array} \quad If the value of the {\attr{datatype}} attribute specifies data type {\value{bit}}, the contents of the field{ }in the \elem{BINARY} serialization shall consist of a sequence of bits starting with the most significant bit; the bits following shall be in order of decreasing significance, ending with the least significant bit. A bit field shall be composed of the smallest number of bytes that can accommodate the number of elements in the field. Padding bits shall be 0. The representation of a bit array in the \elem{TABLEDATA} serialization is made by a sequence of ASCII \value{0} and \value{1} characters. \item {\bf Byte}\quad If the value of the {\attr{datatype}} attribute specifies data type {\value{unsignedByte}}, the field shall contain in the \elem{BINARY} serialization a byte (8-bits) representing a number in the range 0 to 255. %no default {\em null} value exists, if necessary one %has to be defined via the \attr{null} attribute of the \elem{VALUES} element %(\Aref{sec:values}{section 4.6}). In the case of an array of bytes (\attrval{arraysize}{*}), also known as a ``blob", the bytes are stored consecutively. The representation of a Byte in the \elem{TABLEDATA} serialization can be its {\em decimal} representation (a number between {\tt0} and {\tt255}) or its {\em hexadecimal} representation when starting by {\tt0x} and followed by one or two hexadigits, (e.g. {\tt0xff}), separated by at least one space from the next one in the case of an array of bytes. %the default representation of a {\em null} value is an empty cell %(see \Aref{sec:values}{VALUES definitions above}) No default {\em null} value exists; if necessary one has to be defined via the \attr{null} attribute of the \elem{VALUES} element (\Aref{sec:values}{section 4.6}). \item {\bf Character}\quad if the value of the {\attr{datatype}} attribute specifies data type {\value{char}}, the field shall contain in the \elem{BINARY} serialization an ASCII (7-bit) character. The \attr{arraysize} attribute indicates a character string composed of ASCII text. The \elem{BINARY} serialization follows the FITS rules for character strings, and a character string may therefore be terminated by an ASCII NULL (hexadecimal code 00) before the length specified in the \attr{arraysize} attribute: in this case characters after the first ASCII NULL are not defined; and a string having the number of characters identical to the \attr{arraysize} value is not NULL terminated. Characters should be represented in the \elem{TABLEDATA} serialization using the normal rules for encoding XML text: %it is necessary to ensure %that special characters are not interpreted by XML: the ampersand (\&) can be written \verb+&+ (symbolic representation) or \verb+&+ (decimal representation) or \verb+&+ (hexadecimal representation); the less-than ($<$) and greater-then ($>$) symbols should be coded \verb+<+ and \verb+>+ or \verb+<+ and \verb+>+ and a blank which would be interpreted by XML as whitespace (e.g. several consecutive blanks) should be coded \verb+ + or \verb+ +. \item {\bf Unicode Character}\quad If the value of the {\attr{datatype}} attribute specifies data type {\value{unicodeChar}}, the field shall contain a Unicode character The \attr{arraysize} attribute indicates a string composed of Unicode text --- which enables representation of text in many non-Latin alphabets. Each Unicode character is represented in the \elem{BINARY} serialization by two bytes, using the big-endian UCS-2 encoding (ISO-10646-UCS-2). %; no byte order mark should appear. The representation of a Unicode character in the \elem{TABLEDATA} serialization follows the XML specifications, %: special ASCII characters are escaped as in %the case of Character datatype, and and e.g. the Cyrillic uppercase ``Ya'' can be written \verb+Я+ in UTF-8. \item {\bf 16-Bit Integer}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{short}}, the data in the \elem{BINARY} serialization shall consist of big-endian twos-complement signed 16-bit integers (the most significant byte first). The representation of a Short Integer in the \elem{TABLEDATA} serialization is either its decimal representation between -32768 and 32767 made of an optional {\tt-} or {\tt+} sign followed by digits, or its hexadecimal representation when starting by {\tt0x} and followed by 1 to 4 hexadigits. No default {\em null} value exists; if necessary one has to be defined via the \attr{null} attribute of the \elem{VALUES} element (\Aref{sec:values}{section 4.6}). \item {\bf 32-Bit Integer }\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{int}}, the data in the \elem{BINARY} serialization shall consist of big-endian twos-complement signed 32-bit integer --- contained in four bytes, with the most significant first, and subsequent bytes in order of decreasing significance. The representation of an Integer in the \elem{TABLEDATA} serialization is either its decimal representation between -2147483648 and 2147483647 made of an optional {\tt-} or {\tt+} sign followed by digits, or its hexadecimal representation when starting by {\tt0x} and followed by 1 to 8 hexadigits; No default {\em null} value exists; if necessary one has to be defined via the \attr{null} attribute of the \elem{VALUES} element (\Aref{sec:values}{section 4.6}). \item {\bf 64-Bit Integer}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{long}}, the data in the \elem{BINARY} serialization shall consist of big-endian twos-complement signed 64-bit integers --- contained in eight bytes, with the most significant byte first, and subsequent bytes in order of decreasing significance. The representation of a Long Integer in the \elem{TABLEDATA} serialization is either its decimal representation between -9223372036854775808 and 9223372036854775807 made of an optional {\tt-} or {\tt+} sign followed by digits, or its hexadecimal representation when starting by {\tt0x} and followed by 1 to 16 hexadigits; No default {\em null} value exists; if necessary one has to be defined via the \attr{null} attribute of the \elem{VALUES} element (\Aref{sec:values}{section 4.6}). %the default representation of a {\em null} value is an empty cell %(see \Aref{sec:values}{VALUES definitions above}) \item {\bf Single Precision Floating Point}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{float}}, the data in the \elem{BINARY} serialization shall consist of ANSI/IEEE-754 32-bit floating point numbers in big-endian order. All IEEE special values are recognized. The IEEE {NaN} pattern is used to represent ``null" values. The representation of a Floating Point number in the \elem{TABLEDATA} serialization is made of an optional {\tt-} or {\tt+}, followed by the ASCII representation of a positive decimal number, and followed eventually by the ASCII letter \value{E} or \value{e} introducing the base-10 exponent made of an optional {\tt-} or {\tt+} followed by 1 or 2 digits. The number must be within the limits of the IEEE floating-point definition (around $\pm3.4\cdot10^{38}$; numbers with absolute value less than about $1.4\cdot10^{-45}$ are equated to zero); the default representation of a {\em null} value is an empty cell (see \Aref{sec:values}{VALUES definitions above}), and the special values \value{+Inf}, \value{-Inf}, and \value{NaN} are accepted. \item {\bf Double Precision Floating Point}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{double}}, the data in the \elem{BINARY} serialization shall consist of ANSI/IEEE-754 64-bit double precision floating point numbers in big-endian order. All IEEE special values are recognized. The IEEE {NaN} pattern is used to represent ``null" values. The representation of a Double number in the \elem{TABLEDATA} serialization is made of an optional {\tt-} or {\tt+}, followed by the ASCII representation of a positive decimal number, and followed eventually by the ASCII letter \value{E} or \value{e} introducing the base-10 exponent made of an optional {\tt-} or {\tt+} followed by 1 or 2 digits. The number must be within the limits of the IEEE floating-point definition (around $\pm1.7\cdot10^{308}$; numbers with absolute value less than about $5\cdot10^{-324}$ are equated to zero); the default representation of a {\em null} value is an empty cell (see \Aref{sec:values}{VALUES definitions above}), and the special values \value{+Inf}, \value{-Inf}, and \value{NaN} are accepted. \item {\bf Single Precision Complex}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{floatComplex}}, the data in the \elem{BINARY} serialization shall consist of a sequence of pairs of 32-bit single precision floating point numbers in big-endian order. The first member of each pair shall represent the real part of a complex number and the second member shall represent the imaginary part of that complex number. If either member contains a {NaN}, the entire complex value is ``null". The representation of a Floating Complex number in the \elem{TABLEDATA} serialization is made of two representations of a {\em Single Precision Floating Point} numbers separated by at least one blank, representing the real and imaginary part respectively; the default representation of a {\em null} value is an empty cell (see \Aref{sec:values}{VALUES definitions above}) \item {\bf Double Precision Complex}\quad If the value of the {\attr{datatype}} attribute specifies datatype {\value{doubleComplex}}, the data in the \elem{BINARY} serialization shall consist of a sequence of pairs of 64-bit double precision floating point numbers in big-endian order. The first member of each pair shall represent the real part of a complex number and the second member of the pair shall represent the imaginary part of that complex number. If either member contains a {NaN}, the entire complex value is ``null". The representation of a Double Complex number in the \elem{TABLEDATA} serialization is made of two representations of a {\em Double Precision Floating Point} numbers separated by at least one blank, representing the real and imaginary part respectively; the default representation of a {\em null} value is an empty cell (see \Aref{sec:values}{VALUES definitions above}) \end{itemize} %\clearpage \section{A simplified view of the VOTable 1.1 Schema} \label{dtd} \begin{quote} \em The XML Schema [8] defining the VOTable document is available from http://vizier.u-strasbg.fr/xml/VOTable-1.1.xsd \end{quote} \subsection{Element Hierarchy} The illustration of the XML schema uses the following conventions: {\em italicized} text represents {\em optional} elements; \order{} indicates that the order of the elements is mandatory, while the open bullet \unorder{} indicates that the elements may occur in any order; the \choice{} symbol marks a choice between alternatives. The dots $\cdots$ indicate than an element may be repeated. The \underline{underlined elements} are explained in a dedicated box. \ifhtx\Beg{TABULAR}{\bg{LavenderBlush} CELLPADDING=4}{||l||} \else\begin{quote}\small \fi %\inputverbatim{VOTable_v1.0.dtd} \input{VOTable-1.1.elem.tex} %\verbatiminput{VOTable_v1x.dtd}\normalsize \ifhtx\End{TABULAR}\else\end{quote} \fi \iffalse %\clearpage %\section{Schema Diagram for VOTable} %\ifhtx\tag{IMG SRC="diagram_v1.gif" NAME="VOTable Schema" % ALIGN=BOTTOM BORDER=0}\else %%\begin{figure}[htbp] %%\hspace*{-1cm} %\includegraphics[width=\textwidth]{diagram_v1.jpg} %%height=0.75\textheight %%\caption{Schema Diagram for VOTable} %%\end{figure} \fi \subsection{Attribute summary} The list of the attributes is summarized in the table below; attributes written in bold are \requiredattr{required attributes}, while the attributes written in a {fixed font} are \attr{optional}. The {\em italicized} attributes are mentioned in the Appendix, and are not part of VOTable 1.1 \ifhtx\Beg{TABULAR}{\bg{LavenderBlush} CELLPADDING=4}{||l||} \else\begin{quote}\small \fi \input{VOTable-1.1.attr.tex} \ifhtx\End{TABULAR}\else\end{quote} \fi %\clearpage \section{Differences between versions 1.0 and 1.1} \label{diff} The differences between version 1.1 of VOTable and the preceding version 1.0 are: \ifhtx\Beg{TABULAR}{\bg{LavenderBlush} CELLPADDING=4}{||p||} \fi \begin{itemize} \item the introduction of \elem{GROUP} element (\Aref{sec:group}{description}) \item the introduction of the \attr{utype} attribute in the \elem{FIELD}, \elem{PARAM} and \elem{GROUP} elements (\Aref{sec:utype}{description}) \item generalisation of the description of a table as an unordered mixture of \elem{FIELD}, \elem{PARAM} and \elem{GROUP} elements \item the \elem{INFO} elements may exist in \elem{TABLE} as well as \elem{RESOURCE} % the \attr{value} attribute is required in a \elem{PARAM} element \item the \elem{VALUE} element can have a \attr{ref} attribute \item the usage and scope of the \attr{null} attribute was clarified \item in the \elem{BINARY} serialization, there is no requirement of having fixed-length columns first. \end{itemize} \ifhtx\End{TABULAR}\fi \section{References} \noindent [1] Accomazzi {\it et. al, Describing Astronomical Catalogues and Query Results with XML }\\ \hspace*{1em}{{http://vizier.u-strasbg.fr/doc/astrores.htx}} \noindent [2] {\it FITS: Flexible Image Transport Specification}, specifically the Binary Tables Extension \\ \hspace*{1em}{{http://fits.gsfc.nasa.gov/}} \noindent [3] {\it Standards for Astronomical Catalogues: Units, CDS Strasbourg} \\ \hspace*{1em}{{http://vizier.u-strasbg.fr/doc/catstd-3.2.htx}} \\ \hspace*{1em} {\it See also Section 4 in} {Greisen and Calabretta} 2002, A\&A {\bf 395}, 1061; and the IAU Recommendations concerning Units from the {\em IAU Style Manual} by G.A. Wilkins (1989) available at http://www.iau.org/IAU/Activities/nomenclature/units.html \noindent [4] {\it Unified Content Descriptors} \\ \hspace*{1em}{{http://vizier.u-strasbg.fr/doc/UCD.htx}} (UCD1) \\ \hspace*{1em}{{http://www.ivoa.net/twiki/bin/view/IVOA/IvoaUCD}} \noindent [5] {\it GLU: G\'{e}n\'{e}rateur de Liens Uniformes, CDS Strasbourg} \\ \hspace*{1em}{{http://simbad.u-strasbg.fr/glu/glu.htx}} \noindent [6] {\it ASU: Astronomical Server URL, CDS Strasbourg} \\ \hspace*{1em}{{http://vizier.u-strasbg.fr/doc/asu.html}} \noindent [7] {\it XDF: Extensible Data format, ADC} \\ \hspace*{1em}{{http://xml.gsfc.nasa.gov/XDF/XDF\_home.html}} \noindent [8] {\it XML Schema: W3C Document} \\ \hspace*{1em}{{http://www.w3.org/XML/Schema}} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%% A P P E N D I C E S %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \ifhtx\par\thickrule\par\else\clearpage\fi \appendix \noindent {\bf\LARGE Appendices} \bigskip %\begin{quote} %\em\fg{DarkBlue} \section{Possible VOTable extensions} The definitions enclosed in this appendix are {\bf not} part of VOTable 1.1, but are considered as candidates for VOTable improvements. %This section is a short explanation on how Astrores defines %the set of parameters and fields which can be qualified for a query -- %what could be defined as the contents of a {\bf form}. %VOTable currently does not define the parameters available for a query; %such definitions are delayed to the next version of VOTable, and could %make use of the Web Services Description Language (WSDL) %\end{quote} \subsection{VOTable LINK substitutions} \label{LINK} \begin{quote}\em \fg{DarkBlue} The \elem{LINK} element in Astrores [1] contains a mechanism for string substitution, which is a powerful way of defining a link to external data which adapts to each record contained in the table \elem{DATA}. \end{quote} When a {\elem{LINK}} element appears within a \elem{RESOURCE} or a {\elem{TABLE}} element, extra functionality is implied. The {\attr{href}} or {\attr{gref}} attributes may not be a simple link, but instead a template for a link. If, in the example of \Aref{example1}{myFavouriteGalaxies}, we add the link \begin{verbatim} \end{verbatim} \noindent a substitution filter is applied in the context of a particular row. For the first row of the table, the substitution would result in the URL \begin{verbatim} http://ivoa.net/lookup?Galaxy=N++224&RA=010.68&DE=%2b41.27 \end{verbatim} Whenever the pattern {\tt{\$\{...\}}} is found in the original link, the part in the braces is compared with the set of {\attr{ID}} (preferably) or \attr{name} attributes of the fields of the table. If a match is found, then the value from that field of the selected row is used in place of the {\tt{\$\{...\}}}. If no match is found, no substitution is made. Thus the parser makes available to the calling application a value of the {\attr{href}} and {\attr{gref}} attributes that depends on which row of the table has been selected. Another way to think of it is that there is not a single link associated with the table, but rather an implicitly defined new column of the table. This mechanism can be used to connect each row of the table to further information resources. %The {\attr{action}} attribute is related to the Query mechanism described in %the \Aref{query}{next section}. The purpose of the link is defined by the {\attr{content-role}} attribute. The allowed values are {\value{query}} (see \Aref{query}{query mechanism}), {\value{hints}} for information for use by the application, and {\value{doc}} for human-readable documentation. %The first implies that string substitution should be used as defined %above, and the latter two imply first that no substitution is needed, %and that the link points to either information for use by the %application ({\value{hints}}) %or human-readable documentation ({\value{doc}}). The column names invoked in the pattern of the \attr{href} attribute of the \elem{LINK} element should exist in the document to generate meaningful links. In the common case where the VOTable was generated from a query of a database and contains only some of the columns in that database, it might be necessary to include columns additional to those requested in order to ensure that the LINKS in the VOTable are operational. \iffalse This necessity may imply that a \elem{TABLE} contains columns which were not asked, but have been included to ensure that the quoted \elem{LINK}s are operational. \fi Such a \elem{FIELD} included ``by necessity'' is marked with by the attribute \attrval{type}{hidden}. The primary key of a relational table is a typical example of a \elem{FIELD} which would carry the \attrval{type}{hidden} attribute. \subsection{VOTable Query Extension} \label{query} \begin{quote}\em\fg{DarkBlue} The metadata part included in a \elem{RESOURCE} contains all the details necessary to create a {\em form} for querying the resource. The addition of a link having the \attr{action} attribute can turn VOTable into a powerful query interface. \end{quote} \noindent In Astrores [1], the details on the input parameters available in queries are described by the {\elem{PARAM}} and {\elem{FIELD}} elements, and the syntax used to generate the actual query is described in the ASU [6] procotol: the {\elem{FIELD}} or \elem{PARAM} elements are paired in the form {\it name}{{\tt=}}{\it value}, where {\it name} is the contents of the \attr{name} attribute of a \elem{FIELD} or \elem{PARAM}, and {\it value} represents a constraint written with the ASU conventions (e.g. \value{$<8$} or {\value{12.0..12.5}} which denotes a range of values). Such pairs are appended to the {\attr{action}} specified in the {\elem{LINK}} element contained in the {{\elem{RESOURCE}}}, separated by the ampersand (\&) symbol -- in a way quite similar to the HTML syntax used to describe a {\elem{FORM}}. A special \attrval{type}{no\_query} attribute of the \elem{PARAM} or \elem{FIELD} elements marks the fields which are {\em not} part of the form, i.e. are ignored in the collection of {\it name}{{\tt=}}{\it value} pairs. The following is an example of a transformation of the VOTable in \Aref{example1}{the example} into a form interface: \label{form1} \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\begingroup\small \fi \begin{verbatim} Maximal number of records to retrieve Velocities and Distance estimations Distance of Galaxy, assuming H=75km/s/Mpc
\end{verbatim} \ifhtx\End{TABULAR} \else %\caption{\label{example1}A simple VOTable example} \endgroup \fi \noindent Note that the {\elem{RESOURCE}} displaying the parameters accessible for a query has the {\attrval{type}{meta}} attribute; it is also assumed that only one {\elem{LINK}} having the {\attrval{content-role}{query}} attribute together with an {\attr{action}} attribute exists within the current {\elem{RESOURCE}}. The \elem{PARAM} with \attrval{name}{-out.max} has been added in this example to control the size of the result. A valid query generated by this VOTable could be: \begin{verbatim} myQuery?-source=myGalaxies&-out.max=50&R=10..100 \end{verbatim} %\subsection{Additional Propositions} \subsection{Arrays of variable-length strings} \label{sec:arraystring} Following the FITS conventions, strings are defined as arrays of characters. This definition raises problems for the definition of arrays of strings, which have then to be defined as 2D-arrays of characters -- but in this case only the slowest-varying dimension (i.e. the number of strings) can be variable. %According to this %limitation, the list of references given in the example above %(\elemdef{FIELD}{\attrval{name}{ references}}) was assigned an arraysize %of 20 to take into account the blank which separates two references %made of 19 characters each. This limitation becomes severe when a table column contains a set of remarks, each being made of a variable number of characters as it occurs in practice. FITS invented the {\em Substring Array} convention (defined in an appendix, i.e. not officially approved) which defines a {\em separator} character used to denote the end of a string and the beginning of the next one. In this convention ($r${\tt A:SSTR}$w$/$ccc$) the total size of the character array is specified by $r$, $w$ defines the maximum length of one string, and $ccc$ defines the separator character as its ASCII equivalent value. The possible values for the separator includes the space and any printable character, but excludes the control characters. Such arrays of variable-length strings are frequently useful e.g. to enumerate a list of properties of an observed source, each property being represented by a variable-length string. A convention similar to the FITS one could be introduced in VOTable in the \attr{arraysize} attribute, using the {\bf s} followed by the separator character; an example can be \attrval{arraysize}{100s,} indicating a string made of up to 100 characters, where the comma is used to separate the elements of the array. \subsection{FIELDs as data pointers} \label{location} Rather than requiring that all data described in the set of \elem{FIELD}s are contained in a single stream which follows the metadata part, it would be possible to let the \elem{FIELD} act as a {\em pointer} to the actual data, either in the form of a URI or of a reference to a component of a multipart document. Each component of the data described by a \elem{FIELD} may effectively have different requirements: while text data or small lists of numbers are quite efficiently represented in pure XML, long lists like spectra or images generate poor performances if these are converted to XML. The method available to gain efficiency is to use a binary representation of the {\em whole data stream} by means of the \elem{STREAM} element -- at the price of delivering data in a totally non-human readable format. %\subsection{The \attrval{type}{location} attribute} The following options would allow more flexibility in the way the various \elem{FIELD}s can be accessed: \begin{itemize} \item a \elem{FIELD} can be declared as being a {\em pointer} with the addition of a \attrval{type}{location} value, meaning that the field contains a way to access the data, and not the actual data; \item a \elem{FIELD} can contain a \elem{LINK} element marked \attrval{type}{location} which contains in its \attr{href} attribute the partial URI to which the contents of the column cell is appended in order to generate a fully qualified URI. \end{itemize} Note that the \elem{LINK} is not required -- a \elem{FIELD} declared with \attrval{type}{location} and containing no \elem{LINK} element is assumed to contain URIs. An example of a table describing a set of spectra could look like the following: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||} \else\small\fi \begin{verbatim} Spectrum absolutely calibrated
NGC6543SWS06202801301903
NGC6543SWS07254401302004
\end{verbatim}\ifhtx\End{TABULAR}\else\normalsize\fi \noindent The reading program has therefore to retrieve the data for this first row by resolving the URI \begin{plain} {\tt http://ivoa.spectr/server?obsno=01301903} \end{plain} \noindent The same method could also be immediately applicable to {\em Content-ID}s which designate elements of a multipart message, using the protocol prefix {\tt cid:} [RFC2111] Note that the {\em VOTable LINK substitution} proposed in \Aref{LINK}{Appendix A} fills a similar functionality: generate a pointer which can incorporate in its address components from the \elem{DATA} part for the VOTable. \subsection{Encoding individual table cells} \label{sec:b64} Accessing binary data improves quite significantly the efficiency both in storage and CPU usage, especially when one compares with the XML-encoded data stream. But binary data cannot be included in the same stream as the metadata description, unless a dedicated coding filter is applied which converts the binary data into an ASCII representation. The base64 is the most used filter which does this conversion, where 3 bytes of data are coded as 4 ASCII characters, which implies an overhead of 33\% in storage, and some (small) computing time necessary for the reverse transformation. In order to keep the full VOTable document in a unique stream, VOTable 1.0 introduced the \attr{encoding} attribute in the \elem{STREAM} element, meaning that the data, stored as binary records, are converted into some ASCII representation compatible with the XML definitions. One drawback of this method is that the entire data contents become non human-readable. %it should also be noted that the %binary encoding of the full records can result in a waste of storage %when the data contains arrays which size can vary widely from record %to record. The addition of the \attr{encoding} attribute in the \elem{TD} element allows the data server to decide, at the cell level, whether it is more efficient to distribute the data as binary-encoded or as edited values. The result may look like the following: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||}\fi \begin{verbatim}
NGC6543SWS062028 QJKPXECHvndAgMScQHul40CSLQ5ArocrQLxiTkC3XClAq0OWQKQIMUCblYFAh753QGij10BT Em9ARKwIQExqf0BqbphAieuFQJS0OUCJWBBAhcrBQJMzM0CmRaJAuRaHQLWZmkCyhytAunbJ QLN87kC26XlA1KwIQOu+d0DsWh1A5an8QN0m6UDOVgRAxO2RQM9Lx0Din75A3o9cQMPfO0C/ dLxAvUeuQKN87kCXQ5ZAjFodQH0vG0B/jVBAgaHLQI7Ag0CiyLRAqBBiQLaXjUDYcrBA8p++ QPcKPUDg7ZFAwcKPQLafvkDDlYFA1T99QM2BBkCs3S9AjLxqQISDEkCO6XlAmlYEQKibpkC5 wo9AvKPXQLGBBkCs9cNAuGp/QL0euEC4crBAuR64QL6PXEDOTdNA2987QN9T+EDoMSdA8mZm QOZumEDDZFpAmmZmQGlYEEBa4UhAivGqQLel40Dgan9A4WBCQLNcKUCIKPZAk1P4QNWRaEEP kWhBKaHLQTkOVkFEan9BUWBCQVyfvg==
\end{verbatim} \ifhtx\End{TABULAR}\fi \par \noindent When decoded, the contents of the last column is the binary representation of the spectrum, as defined in \Aref{sec:BIN}{the BINARY serialization}; no length prefix is required here, the total length of the array being implicitely defined by the length of the encoded text. \subsection{Additional \elem{TABLE} attributes} % Suggested TomMcGlynn % Nov.2003, Mails 7978 + 8073 The \elem{GROUP} introduced in version 1.1 can be qualified by \attr{ucd} and \attr{utype} attributes. These attributes could similarly bring useful additional metadata to qualify the \elem{TABLE} structure. \subsection{A new \elem{XMLDATA} serialization} % Following discussions Tony Linde / Roy Williams % in January 2004 on the VOTable group In order to facilitate the usage of the standard XML query tools which are easier to handle when each parameter has its individual tag, the \elem{XMLDATA} serialization introduces the designation of each \elem{FIELD} by a dedicated tag. An example could look like the following: \ifhtx\Beg{TABULAR}{\bg{LightCyan} CELLPADDING=5}{||l||}\fi \begin{verbatim} Messier Number Common name used to designate the Messier object Classification (galaxy, glubular cluster, etc) 3205.5+28.4 Globular Cluster 31010.7+41.3Andromeda GalaxyGalaxy
\end{verbatim} \ifhtx\End{TABULAR}\fi \par \noindent The full document would need an XML-Schema definition of the tags \elem{M}, \elem{RA}, \elem{DE}, \elem{N} and \elem{T}; these being derived directly from the \attr{ID} attribute of the \elem{FIELD} element, their definition can be generated automatically from the set of \elem{FIELD} definitions. \end{document}