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\makebox[10em][l]François Ochsenbein
Observatoire Astronomique de Strasbourg, France
\makebox[10em][l]Roy Williams
California Institute of Technology, USA
\makebox[10em][l]Clive Davenhall
University of Edinburgh, UK
\makebox[10em][l]Daniel Durand
Canadian Astronomy Data Centre, Canada
\makebox[10em][l]Pierre Fernique
Observatoire Astronomique de Strasbourg, France
\makebox[10em][l]David Giaretta
Rutherford Appleton Laboratory, UK
\makebox[10em][l]Robert Hanisch
Space Telescope Science Institute, USA
\makebox[10em][l]Tom McGlynn
NASA Goddard Space Flight Center, USA
\makebox[10em][l]Alex Szalay
Johns Hopkins University, USA
\makebox[10em][l]Mark B. Taylor
Physics, Bristol University, UK
\makebox[10em][l]Andreas Wicenec
European Southern Observatory, Germany
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.
The VOTable format is a 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 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.
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 interoperable and 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). The XML fabric allows applications to easily validate an input document, as well as facilitating transformations through XSLT (eXtensible Style Language Transformation) engines.
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. 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 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 (expires). Data on the net may require authentication for access, so VOTable allows expression of password or other identity information (the `rights' attribute).
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.
VOTable is constructed with XML (extensible Markup Language), a powerful standard for structured data throughout the Internet industries. It derives from SGML, a standard used in the publishing industry and for technical documentation for many years. XML consists of elements and payload, where an element consists of a start tag (the part in angle brackets), the payload, and an end tag (with angle brackets and a slash). Elements can contain other elements. Elements can also bear attributes (keyword-value combinations).
The payload may be in two forms: parsed or unparsed character data. Examples are:
<text>François</text> <text><![CDATA[ a <= (b & c) ]]></text>
In the first example, the sequence ç is interpreted as part of the ISO/IEC 10646 character set, and translates to an accented character, so that the text is ``François". The second example uses the special CDATA sequence so that the characters <, >, and & can be used without interpretation; in this case, any ASCII characters are allowed except the terminating sequence ]]> For more information, see any book on XML.
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 ID attribute). Element and attribute names are further distinguished in this paper by being shown in a fixed-width font.
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:
Metadata is divided into that which concerns the table itself
(parameters), and the definitions of the fields (or column
attributes) of the table.
Each 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 section example1 below, the first FIELD has its name attribute
set to "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 (PARAM)
is similar to a FIELD,
except that it has a 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
Epoch parameter in the above example.
An informative parameter (INFO)
is a restricted form of the PARAM – it has only the
name / 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 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 GROUP, which typically
contains columns (FIELD) and associated parameters (PARAM).
Each Cell is composed from Primitives, each of which is a datatype
of fixed-length binary representation, as listed in
Table primitives. Cells may consist of a single Primitive (this is
the default), or of a multidimensional array of Primitives (see
section array).
Except for the Bit type, each primitive has the fixed length in
bytes given in Table primitives.
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). These primitives
are described in more detail in section sec:datatypes.
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 (datatype="char"),
but also Unicode (datatype="unicodeChar").
Note that strings are not a primitive type: strings are
represented in VOTable as an array of characters.
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 FIELD definition
declares a table cell which may contain a set of up to 10 images,
each 64x64 bytes:
<FIELD ID="thumbs" datatype="unsignedByte"
arraysize="64x64x10*"/>
The string in the arraysize attribute expressed these
dimensions, each integer separated by the 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.
Strings can therefore be represented in VOTable as a fixed- or variable-length
array of characters:
<FIELD name="unboundedString" datatype="char"
arraysize="*"/>
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 section sec:arraystring) but is not
part of this standard.
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 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:
for example F12.4 means 12 characters are to be used, and 4 decimal
places. This has been converted in VOTable as the attributes width
and precision
which, connected with datatype,
are semantically identical to the TDISP keyword.
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
(see however the section sec:arraystring 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.
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 section sec:ucd)
to express formally the semantic
content of a parameter or field. It has the hierarchy and flexibility
of XML: using 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.
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.
The overall VOTable document structure is described and controlled
by its XML Schema
referenced at its top. 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 section dtd.
An example 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 VOTABLE,
which contains descriptive elements (DESCRIPTION,
DEFINITIONS, INFO),
followed by one or more RESOURCE elements.
Each Resource element contains one or more TABLE elements,
and possibly other RESOURCE elements.
The TABLE element, the actual heart of VOTable, contains
a description of the columns and parameters
(described in section sec:field)
followed by the data values
(described in section sec:data).
This simple example of a VOTable document lists 3 galaxies with their
velocity with its error, and the estimated distance.
This simple VOTable document shows a single RESOURCE made of a single TABLE;
the table is made of 6 columns, each described by a FIELD, and has
one additional PARAM parameter (the Epoch). The actual rows are
listed in the DATA part of the table, here in XML format
(introduced by TABLEDATA); each cell is marked by the TD element,
and follow the same order as their FIELD description:
RA, Dec, Name, RVel, e_RVel, R.
Most of the elements defined by VOTable may or have to bear names,
like a RESOURCE, a TABLE, a PARAM or a FIELD.
Naming an element is generally possible by means of one of or
both ID and name attributes.
ID and name attributes have a different role in
VOTable: the ID is meant as a 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 ID attribute is therefore required in the elements which have to be referenced,
but in principle any element may have an ID attribute.
According to the XML standard, the attribute ID
is a string beginning with a letter or underscore (_),
followed by a sequence of letters, digits, or any of the
punctuation characters . (dot), - (dash), _ (underscore),
or : (colon).
In summary,
the ID is different from the name
attribute in that (a) the ID attribute is made from a restricted character
set, and must be unique throughout a VOTable document
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.
This element may contain a definition of a coordinate system,
stored in a COOSYS
element. The COOSYS element
provides attributes for equinox and epoch, as well as a
specification of the celestial coordinate system.
The 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 DEFINITIONS element may also include one or more PARAM
elements (section sec:field)
that may contain user-specific data. Each of these may have
an ID attribute, that can be referenced with the ref attribute
of other elements.
As mentioned above, the COOSYS may be deprecated in the future
in favor of a more generic way of describing the conventions used to define
the positions.
A VOTable document contains one or more RESOURCE
elements, each of these providing a description and the
data values of some logically independent data structure.
Each RESOURCE may include the descriptive elements DESCRIPTION,
INFO, COOSYS and PARAM;
it may also contain LINK
elements to provide URL-type pointers that give further information.
The main component of a RESOURCE is typically one or more TABLE
elements – in other terms a RESOURCE is basically a set
of related tables. The RESOURCE is recursive (it can contain other
RESOURCE elements), which means that the set of tables making up
a RESOURCE may become a complex structure.
A RESOURCE may have one or both of the name or ID
attributes (see section sec:name); it may also be qualified by
type="meta", meaning that the resource is descriptive
only (does not contain any actual data in any of its sub-elements).
The LINK element is to provide pointers to other documents
or data servers on the Internet through a URL. In VOTable, the LINK
element may be part of a RESOURCE,
TABLE, GROUP or FIELD elements. The href
attribute of the LINK element can comprise any arbitrary protocol,
for example "http://server/file" or "bizarre://server/file".
VOTable parsers are not required to understand arbitrary protocols,
but are required to understand the following three common protocols:
"file:", "http:" and "ftp:".
The 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 LINK
element; the href attribute is used as a template for creating
URL's. This behavior is explained in appendix LINK, and it represents
a possible extension of VOTable.
In addition to the referencing href and gref attributes
and to the naming name and ID attributes
(see section sec:name), the LINK element
may announce the mime type of the data it references
with a content-type attribute (e.g. content-type="image/fits"),
and specify the role of the link by a content-role attribute
(e.g. content-role="doc" for an access to a documentation).
The TABLE element represents the basic data structure in VOTable;
it is made of a description of the table structure (the metadata)
essentially in the form of PARAM and FIELD elements
(detailed in section sec:field),
followed by the values of the described fields in a DATA
element (detailed in section sec:data).
The TABLE element is always contained in a RESOURCE element:
in other terms
any TABLE element has a single father made of the
RESOURCE element
in which the table is embedded.
The TABLE element contains
a DESCRIPTION element for descriptive remarks, followed
by a mixed collection of PARAM, FIELD or GROUP elements
which describe a parameter (constant column), a field (column) or a group of
columns respectively. PARAM and FIELD elements are detailed in
section sec:field, and the GROUP element
is presented in section sec:group.
Furthermore the TABLE element may contain LINK elements
that provide URL-type pointers, exactly like the LINK elements
existing within a RESOURCE element (see section sec:link).
The last element included in a TABLE is the optional DATA
element (see section sec:data): 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 TABLE element may have the naming attributes name and/or
ID (see section sec:name). A TABLE
may also have a ref attribute referencing the ID of another
table previously described, which is interpreted as
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.
The atoms of the table structure are represented by FIELD and
PARAM elements, where FIELD represents the description
of an actual table column, while PARAM supplies a value
which remains constant over the whole table, like the Epoch
in section example1. A PARAM may therefore be
viewed as a FIELD which keeps a constant value over all
the rows of a table, and the only difference between the two elements
is the existence of a value attribute in a PARAM
which does not exist in a FIELD.
A FIELD or PARAM element may have several sub-elements,
including the informational DESCRIPTION
and LINK elements; it may also include a VALUES element
that can express limits and ranges of the values that the
corresponding cell can contain, such as minimum (MIN),
maximum (MAX), or
enumeration of possible values (OPTION).
In addition, in the PARAM element only:
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:
The quantities in a column of the table may be expressed in
some physical unit,
which is specified by the unit
attribute of the FIELD.
The syntax of the unit string is defined in reference [3];
it is basically written as a string without blanks or spaces,
where the symbols . or * indicate a multiplication,
/ stands for the division, and no special symbol is required
for a power.
Examples are unit="m2" for m2,
unit="cm-2.s-1.keV-1" for cm–2s–1keV–1,
or unit="erg/s" for erg s–1.
The references [3] provides also the list of the valid symbols,
which is essentially restricted to the Système International
(SI) conventions, plus a few astronomical extensions concerning
units used for time, angular, distance and energy measurements.
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].
A few typical examples taken from the original UCD design:
In order to avoid name collisions, the data model identification
should be introduced following the XML namespace conventions,
as utype="datamodel_identifier:role_identifier".
The mapping of "datamodel_identifier" to an xml-type attribute
is recommended, but not required.
The VALUES element of the FIELD
is designed to hold subsidiary information about the domain of the
data. For instance, in the example (section example1)
we could rewrite the RA field definition as:
Primitive
or variable-length list of Primitives
or multidimensional array of Primitives
\right.
Primitive = integer, character, float, floatComplex, etc
(see Table primitives below).
2.1 Primitives
|
datatype |Meaning |FITS
| Bytes |
|
"boolean" |Logical |"L" |1 |
|"bit" |Bit |"X" |* |
|"unsignedByte" |Byte (0 to 255) |"B" |1 |
|"short" |Short Integer |"I" |2 |
|"int" |Integer |"J" |4 |
|"long" |Long integer |"K" |8 |
|"char" |ASCII Character |"A" |1 |
|"unicodeChar" |Unicode Character | |2 |
|"float" |Floating point |"E" |4 |
|"double" |Double |"D" |8 |
|"floatComplex" |Float Complex |"C" |8 |
|"doubleComplex" |Double Complex |"M" |16 |
|2.2 Multidimensional Arrays
2.3 Compatibility with FITS Binary Tables
What can FITS do but not VOTable?
What can VOTable do but not FITS?
3 The VOTable Document Structure
3.1 Example
3verbatim
<?xml version="1.0"?>
<VOTABLE version="1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://vizier.u-strasbg.fr/xml/VOTable.xsd">
<DEFINITIONS>
<COOSYS ID="J2000" equinox="2000." epoch="2000." system="eq_FK5"/>
</DEFINITIONS>
<RESOURCE name="myFavouriteGalaxies">
<TABLE name="results">
<DESCRIPTION>Velocities and Distance estimations</DESCRIPTION>
<PARAM name="Epoch" datatype="float" ucd="TIME_EPOCH"
value="2003.875/">
<FIELD name="RA" ID="col1" ucd="POS_EQ_RA_MAIN" ref="J2000" datatype="float"
width="6" precision="2" unit="deg"/>
<FIELD name="Dec" ID="col2" "POS_EQ_DEC_MAIN" ref="J2000" datatype="float"
width="6" precision="2" unit="deg"/>
<FIELD name="Name" ID="col3" ucd="ID_MAIN" datatype="char" arraysize="8*"/>
<FIELD name="RVel" ID="col4" ucd="VELOC_HC" datatype="int"
width="5" unit="km/s"/>
<FIELD name="e_RVel" ID="col5" ucd="ERROR" datatype="int"
width="3" unit="km/s"/>
<FIELD name="R" ID="col6" ucd="PHYS_DISTANCE_TRUE" datatype="float"
width="4" precision="1" unit="Mpc">
<DESCRIPTION>Distance of Galaxy, assuming H=75km/s/Mpc</DESCRIPTION>
</FIELD>
<DATA>
<TABLEDATA>
<TR>
<TD>010.68</TD><TD>+41.27</TD><TD>N 224</TD><TD>-297</TD><TD>5</TD><TD>0.7</TD>
</TR>
<TR>
<TD>287.43</TD><TD>-63.85</TD><TD>N 6744</TD><TD>839</TD><TD>6</TD><TD>10.4</TD>
</TR>
<TR>
<TD>023.48</TD><TD>+30.66</TD><TD>N 598</TD><TD>-182</TD><TD>3</TD><TD>0.7</TD>
</TR>
</TABLEDATA>
</DATA>
</TABLE>
</RESOURCE>
</VOTABLE>
3.2 ID and name attributes
3.3 DEFINITIONS element
The COOSYS element
This element defines a celestial coordinate system, to which the
components of a position on the celestial sphere refer.
It has an ID attribute — required if the
COOSYS element has to be referred via the ref attribute
of the position components, which is generally the case —
a system attribute which specifies the coordinate system
among "ICRS", "eq_FK5", "eq_FK4", "ecl_FK4",
"ecl_FK5", "galactic", "supergalactic",
"barycentric", "geo_app" and a user-defined "xy"
value. equinox is the parameter required to fix the
equatorial or ecliptic systems (as e.g. "J2000" as
the default "eq_FK5" or "B1950" as the default
"eq_FK4"), and epoch specifies the epoch of the positions
if necessary.
3.4 RESOURCE element
3.5 LINK element
3.6 TABLE element
4 FIELDs and PARAMeters
4.1 FIELD attributes
The valid attributes of a FIELD or PARAM are:
4.2 Numerical Accuracy
The existence and presentation of the special 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
VALUES sub-element (see section sec:values).
4.3 Units
4.4 Unified Content Descriptors
"PHOT_INT-MAG_B" Integrated total blue magnitude
"ORBIT_ECCENTRICITY" Orbital eccentricity
"STAT_MEDIAN" Statistics Median Value
"INST_QE" Detector's Quantum Efficiency
4.5 The utype attribute
In some contexts, it can be important that FIELDs or PARAMeters
are explicitely designed as being 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 FIELD expresses the surface brightness
processed by an explicit method. None of the existing name, ID
or ucd attributes can fill this role, and
the utype (usage-specific or unique type) attribute has
been added in
VOTable 1.1 to fill this gap.
4.6 VALUES element
3verbatim
<FIELD name="RA" ID="col1" ucd="POS_EQ_RA_MAIN" ref="J2000" datatype="float"
width="6" precision="2" unit="deg">
<VALUES ID="RAdomain">
<MIN value="0"/>
<MAX value="360" inclusive="no"/>
</VALUES>
</FIELD>
All three MIN, MAX and OPTION sub-elements store their value corresponding to the minimum, maximum, or ``special value'' in a value attribute. MIN and MAX elements can have an inclusive attribute to specify whether the value quoted belongs or not to the domain, and the OPTION element can have a name attribute to qualify the ``special'' quoted value.
The VALUES element may also have a null attribute to define a non-standard value that is used to specify ``non-existent data'' – for example 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. In the TABLEDATA data representation, the default representation of a ``null'' value is an empty column (i.e. <TD></TD>); for fields containing arrays, individual ``null'' elements of the array can be specified either by the value specified in the null attribute, or by the "NaN" or "nan" text in place of the expected numeric value.
For the FITS and BINARY data representations, the 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.
The scope of the domain described by the VALUES element can be qualified by type="actual", if it is only applicable to the data enclosed in the parent TABLE. The domain of a valid RA in the example above has the default type="legal" qualification.
Finally the ref attribute of a VALUES element can be used to avoid a repetition of the domain definition, by referring to a previously defined VALUES element having the referenced ID attribute. When specified, the ref attribute defines completely the domain without any other element or attribute, as e.g. <VALUES ref="RAdomain"/>
The GROUP element was added in VOTable 1.1, to group together a set of FIELDS which are logically correlated, like a value and its error. Each field participating in a GROUP can be defined either physically (the FIELD contains a datatype field), or logically (the FIELD contains only a ref attribute referencing a field defined in the same parent 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 section example1, can be to replace the definitions of columns 4 and 5 by the following:
2verbatim
<GROUP name="Velocity" ucd="VELOC_HC">
<DESCRIPTION>Velocity and its error</DESCRIPTION>
<FIELD name="RVel" ID="col4" ucd="VELOC_HC" datatype="float"
width="5" unit="km/s"/>
<FIELD name="e_RVel" ID="col5" ucd="ERROR" datatype="float"
width="3" unit="km/s"/>
</GROUP>
A logical definition of this group could alternatively be
achieved by inserting just before the DATA element the following:
2verbatim
<GROUP name="Velocity">
<DESCRIPTION>Velocity and its error</DESCRIPTION>
<FIELD ref="col4"/>
<FIELD ref="col5"/>
</GROUP>
The GROUP element can have the name, ID, ucd, utype and ref attributes. It can include a DESCRIPTION, and any mixture of FIELDs, PARAMeters, and other GROUPs – this recursive grouping enabling a definition of arbitrary complex structures.
The possibility of adding PARAMeters 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:
2verbatim
<GROUP name="Flux" ucd="VELOC_HC">
<DESCRIPTION>Flux measured at 352MHz</DESCRIPTION>
<FIELD name="Flux" ucd="PHOT_FLUX_RADIO_400M" datatype="float"
width="6" precision="1" unit="mJy"/>
<PARAM name="Freq" ucd="OBS_FREQUENCY" unit="MHz" datatype="float" value="352"/>
<FIELD name="e_Flux" ucd="ERROR" datatype="float" width="4"
precision="1" unit="mJy"/>
</GROUP>
Similarly, the GROUP can be used to associate several parameters to one or several FIELDs: 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.
While the bulk of the metadata of a VOTable document is in the FIELD elements, the data content of the table is in a single 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 FIELD tags having a datatype attribute, with each row having the same number of items as there are FIELD tags having a datatype attribute. Fields without a datatype attribute have a ref attribute, and represent references to ``true'' columns (see section sec:field).
Each 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 (local data), or it may
be remote data.
Figure fig:serialization shows how the abstract table is rendered into the VOTable document. First the data is serialized, either as XML, a FITS binary table, or the VOTable Binary format. This data stream may then be 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.
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 TR elements, which in turn contain TD elements — i.e. the same conventions as the familiar HTML ones. An example is contained in section example1, surrounded by in the <TABLEDATA> and </TABLEDATA> delimiters.
The number of TD elements should be in number equal to the number of FIELD elements having datatype attributes declaring the table; when there are less TD's than expected, the corresponding values are set to "null"s; superfluous 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 TD tag is passed to a reader that is implicitly defined by the datatype attribute of the corresponding FIELD, which attempts to read the object from it. If it reads a value that is the same as the 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 datatype, are detailed in section sec:datatypes.
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:
2example2The first entry is a fixed-length array of 10 characters; since the value being presented (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.<TABLE> <FIELD ID="aString" datatype="char" arraysize="10"/> <FIELD ID="Floats" datatype="float" arraysize="3"/> <FIELD ID="varComplex" datatype="floatComplex" arraysize="*"/> <DATA><TABLEDATA> <TR> <TD>Apple</TD><TD>1.62 4.56 3.44</TD> <TD>67 1.57 4 3.14 77 -1.57</TD> </TR><TR> <TD>Orange</TD><TD>2.33 4.66 9.53</TD> <TD>39 0 46 3.14</TD> </TR> </TABLEDATA></DATA> </TABLE>
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 PARAM, and the data itself is remotely stored and gzipped at an ftp site:
<RESOURCE>
<PARAM name="EPOCH" datatype="float" value="1999.987">
Original Epoch of the coordinates </PARAM>
<PARAM name="TELESCOP" datatype="char" arraysize="*" value="VTel" />
<INFO name="HISTORY">
The very first Virtual Telescope observation made in 2002
</INFO>
<TABLE> <FIELD (insert field metadata here) >
<DATA><FITS extnum="2">
<STREAM encoding="gzip" href="ftp://archive.cacr.caltech.edu/myfile.fit.gz"/>
</FITS></DATA>
</TABLE>
</RESOURCE>
The FITS file may contain many data objects (known as extensions, numbered from 1 up – the main header being numbered 0), and the extnum attribute allows the VOTable to point to one of these.
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 datatype and arraysize attributes of the 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 arraysize attribute of the FIELD. If all the fields have a fixed arraysize, then each record of the binary format has the same length (the sum of arraysize times the length in bytes of the corresponding 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 section example2 is illustrated in Figure fig:bin. The parser can then compute the number of bytes taken by the variable-length array by multiplying the size and number of the primitives.
2IMG SRC="binary.gif" NAME="Image2" ALIGN=LEFT BORDER=0
As a result of the serialization, the table has been converted to a byte stream, either text or binary. If the 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 appendix sec:b64).
In order to use an encoding of the data, it must be enclosed in a STREAM element, whose attributes define the nature of the encoding. The 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:
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.
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:
<DATA><BINARY>
<STREAM encoding="base64">
AAAAAj/yVZiDGSSUwFZ6ypR4yGkADwAcQV0euAAIAAJBmMzNwZWZmkGle4tBR3jVQT9ocwAA
························
</STREAM>
</BINARY></DATA>
However, if the data is very large, it may be preferable to keep the data separate from the metadata. The href attribute of the STREAM element, if present, provides the location of the data in a URL-type syntax, for example:
<STREAM href="ftp://server.com/mydata.dat"/>
<STREAM href="ftp://server.com/mydata.dat" expires="2004-02-29T23:59:59"/>
<STREAM href="httpg://server.com/mydata.dat" actuate="onLoad"/>
<STREAM href="file:///usr/home/me/mydata.dat"/>
The examples are the well-known anonymous ftp, and http protocols. "httpg" is an example of a Grid-based access to data through httpg; "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 "file:", "http:" and "ftp:".
There are further attributes of the STREAM element that may be useful. The 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 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.
The 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 actuate attribute is borrowed from the XML Xlink specification, expressing when the remote link should be actuated. The default is "onRequest", meaning that the data is only fetched when explicitly requested (like a link on an HTML page), and the "onLoad" value means that data should be fetched as soon as possible (like an embedded image on an HTML page).
This section describes the primitives summarized in Table primitives and their representations in the BINARY and in the TABLEDATA serializations (see section sec:TABLEDATA). In the following, the term ``hexadigit'' designates the ASCII numbers "0" to "9", or the ASCII lower- or upper-case letters "a" to "f" (i.e. a digit in an hexadecimal representation of a number).
The XML Schema [8] defining the VOTable document is available from
2<!-- **Error Opening File: VOTable_v1.0.dtd --> [missing file: VOTable_v1.0.dtd]
|
<VOTABLE> | |⊕ <DESCRIPTION> | |⊕ <DEFINITIONS> | | ○ <COOSYS>··· | | ○ <PARAM>··· | |⊕ <INFO>··· | |⊕ <RESOURCE>··· | |</VOTABLE>| ||
<RESOURCE>| |⊕ <DESCRIPTION> | |⊕ <INFO>··· | |⊕ <COOSYS>··· | |⊕ <PARAM>··· | |⊕ <LINK>··· | |⊕ <TABLE>··· | |⊕ <RESOURCE>··· | |</RESOURCE>| ||
<TABLE> | |⊕ <DESCRIPTION> | |○ <FIELD>··· | |○ <PARAM>··· | |○ <GROUP>··· | |⊕ <LINK>··· | |⊕ <DATA> | |</TABLE> | ||
<DATA> | |--> <TABLEDATA> | | ⊕ <TR>··· | | ⊕ <TD>··· | |--> <BINARY> | | ⊕ <STREAM>| |--> <FITS>| | ⊕ <STREAM>| |</DATA>| ||
<GROUP> | |⊕ <DESCRIPTION> | |○ <FIELD>··· | |○ <PARAM>··· | |○ <GROUP>··· | |</GROUP>| ||
<PARAM> | |⊕ <DESCRIPTION> | |⊕ <VALUES> | |⊕ <LINK>··· | |</PARAM>| ||
<FIELD> | |⊕ <DESCRIPTION> | |⊕ <VALUES> | |⊕ <LINK>··· | |</FIELD>| ||
<VALUES> | |⊕ <MIN> | |⊕ <MAX> | |⊕ <OPTION>··· | | ○ <OPTION>···| |</VALUES>| |
2VOTable-1.1.attr.tex
2itemize
[2] FITS: Flexible Image Transport
Specification, specifically the Binary Tables
Extension
[3] Standards for Astronomical
Catalogues: Units, CDS Strasbourg
[4] Unified Content
Descriptors
[5] GLU: Générateur de Liens Uniformes, CDS
Strasbourg
[6] ASU: Astronomical Server URL, CDS
Strasbourg
[7] XDF: Extensible Data format, ADC
[8] XML Schema: W3C Document
When a LINK element appears within a RESOURCE or a
TABLE element,
extra functionality is implied. The href
or gref attributes may not be a simple link, but instead
a template for a link. If, in the example of
section example1, we add the link
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
Whenever the pattern ${...}
is found in the original link, the part in the braces is compared
with the set of ID (preferably) or 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
${...}. If no match is found, no substitution is made. Thus the
parser makes available to the calling application a value of the href
and 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 purpose of the link is defined by the content-role
attribute. The allowed values are "query"
(see section query),
"hints" for information for use by the application,
and "doc" for human-readable documentation.
The column names invoked in the pattern of the href attribute
of the 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.
Such a FIELD included ``by necessity'' is marked with
by the attribute type="hidden". The primary key of
a relational table is a typical example of a FIELD
which would carry the type="hidden" attribute.
In Astrores [1], the details on the input parameters available in
queries are described by the
PARAM and FIELD elements, and the syntax used
to generate the actual query is described in the ASU [6] procotol:
the FIELD or PARAM elements are
paired in the form name=value,
where name is the contents of the
name attribute of a FIELD or PARAM,
and value represents a constraint
written with the ASU conventions (e.g. "<8"
or "12.0..12.5"
which denotes a range of values).
Such pairs are appended to the
action specified in the LINK
element contained in the RESOURCE,
separated by the ampersand (&) symbol –
in a way quite similar to the HTML syntax used to
describe a FORM.
A special type="no_query" attribute of the
PARAM or FIELD elements marks the fields
which are not part of the form, i.e. are ignored
in the collection of name=value pairs.
The following is an example of a transformation of the VOTable
in section example1 into a form interface:
Note that the RESOURCE displaying the parameters accessible
for a query has the type="meta"
attribute; it is also assumed that only one LINK
having the content-role="query"
attribute together with an action
attribute exists within the current RESOURCE.
The PARAM with 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:
FITS invented the Substring Array convention (defined in an appendix,
i.e. not officially approved) which defines a separator character
used to denote the end of a string and the beginning of the next one.
In this convention (rA:SSTRw/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 arraysize
attribute, using the s followed by the separator character;
an example can be arraysize="100s,"
indicating a string made of up to 100 characters, where the comma
is used to separate the elements of the array.
Rather than requiring that all data described in the set of FIELDs
are contained in a single stream which follows the metadata part,
it would be possible to let the FIELD act as
a 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 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 whole data stream by means of the
STREAM element – at the price of delivering data in a totally non-human
readable format.
The following options would allow more flexibility in the way the
various FIELDs can be accessed:
An example of a table describing a set of spectra could look like the following:
The same method could also be immediately applicable to Content-IDs
which designate elements of a multipart message, using the protocol
prefix cid: [RFC2111]
Note that the VOTable LINK substitution proposed in
section LINK fills a similar functionality:
generate a pointer which can incorporate in its address components
from the DATA part for the VOTable.
In order to keep the full VOTable document in a unique stream,
VOTable 1.0 introduced the encoding attribute in the
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.
The addition of the encoding attribute in the 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:
When decoded, the contents of the last column is the binary representation
of the spectrum, as defined in section sec:BIN;
no length prefix is required here, the total length of the array being
implicitely defined by the length of the encoded text.
The full document would need an XML-Schema definition of the tags
M, RA, DE, N and T; these being
derived directly from the ID attribute of the FIELD
element, their definition can be generated automatically from the set of
FIELD definitions.
A Possible VOTable extensions
The definitions enclosed in this appendix
are not part of VOTable 1.1, but are considered as candidates
for VOTable improvements.
A.1 VOTable LINK substitutions
The 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 DATA.
<LINK href="http://ivoa.net/lookup?Galaxy=${Name}&RA=${RA}&DE=${DE}"/>
http://ivoa.net/lookup?Galaxy=N++224&RA=010.68&DE=%2b41.27
A.2 VOTable Query Extension
The metadata part included in a RESOURCE contains
all the details necessary to create a form for querying
the resource. The addition of a link having the action
attribute can turn VOTable into a powerful query interface.
2verbatim
<?xml version="1.0"?>
<VOTABLE version="1.1" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:noNamespaceSchemaLocation="http://vizier.u-strasbg.fr/xml/VOTable.xsd">
<DEFINITIONS>
<COOSYS ID="J2000" equinox="2000." epoch="2000." system="eq_FK5"/>
</DEFINITIONS>
<RESOURCE name="myFavouriteGalaxies" type="meta">
<PARAM name="-out.max" ucd="NUMBER" datatype="int" value="50">
<DESCRIPTION>Maximal number of records to retrieve</DESCRIPTION>
</PARAM>
<LINK content-role="query" action="myQuery?-source=myGalaxies2amp;" />
<TABLE name="results">
<DESCRIPTION>Velocities and Distance estimations</DESCRIPTION>
<PARAM name="Epoch" datatype="float" ucd="TIME_EPOCH"
value="2003.875/">
<FIELD name="RA" ID="col1" ucd="POS_EQ_RA_MAIN" ref="J2000" datatype="float"
width="6" precision="2" unit="deg"/>
<FIELD name="Dec" ID="col2" "POS_EQ_DEC_MAIN" ref="J2000" datatype="float"
width="6" precision="2" unit="deg"/>
<FIELD name="Name" ID="col3" ucd="ID_MAIN" datatype="char" arraysize="8*"/>
<FIELD name="RVel" ID="col4" ucd="VELOC_HC" datatype="int"
width="5" unit="km/s"/>
<FIELD name="e_RVel" ID="col5" ucd="ERROR" datatype="int"
width="3" unit="km/s"/>
<FIELD name="R" ID="col6" ucd="PHYS_DISTANCE_TRUE" datatype="float"
width="4" precision="1" unit="Mpc">
<DESCRIPTION>Distance of Galaxy, assuming H=75km/s/Mpc</DESCRIPTION>
</FIELD>
</TABLE>
</RESOURCE>
</VOTABLE>
myQuery?-source=myGalaxies&-out.max=50&R=10..100
A.3 Arrays of variable-length strings
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. 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.
A.4 FIELDs as data pointers
Note that the LINK is not required – a FIELD declared
with type="location" and containing no LINK element
is assumed to contain URIs.
2verbatim
<TABLE name="SpectroLog">
<FIELD name="Target" ucd="ID_TARGET" datatype="char" arraysize="30*"/>
<FIELD name="Instr" ucd="INST_SETUP" datatype="char" arraysize="5*"/>
<FIELD name="Dur" ucd="TIME_EXPTIME" datatype="int" width="5" unit="s"/>
<FIELD name="Spectrum" ucd="DATA_LINK" datatype="float" arraysize="*"
unit="mW/m2/nm" type="location">
<DESCRIPTION>Spectrum absolutely calibrated</DESCRIPTION>
<LINK type="location"
href="http://ivoa.spectr/server?obsno="/>
</FIELD>
<DATA><TABLEDATA>
<TR><TD>NGC6543</TD><TD>SWS06</TD><TD>2028</TD><TD>01301903</TD></TR>
<TR><TD>NGC6543</TD><TD>SWS07</TD><TD>2544</TD><TD>01302004</TD></TR>
</TABLEDATA></DATA>
</TABLE>
The reading program has therefore to retrieve the data
for this first row by resolving the URI
http://ivoa.spectr/server?obsno=01301903}
A.5 Encoding individual table cells
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.
3verbatim
<TABLE name="SpectroLog">
<FIELD name="Target" ucd="ID_TARGET" datatype="char" arraysize="30*"/>
<FIELD name="Instr" ucd="INST_SETUP" datatype="char" arraysize="5*"/>
<FIELD name="Dur" ucd="TIME_EXPTIME" datatype="int" width="5" unit="s"/>
<FIELD name="Spectrum" ucd="SPECT_FLUX_VALUE" datatype="float" arraysize="*"
unit="mW/m2/nm" precision="E3"/>
<DATA><TABLEDATA>
<TR><TD>NGC6543</TD><TD>SWS06</TD><TD>2028</TD><TD encoding="base64">
QJKPXECHvndAgMScQHul40CSLQ5ArocrQLxiTkC3XClAq0OWQKQIMUCblYFAh753QGij10BT
Em9ARKwIQExqf0BqbphAieuFQJS0OUCJWBBAhcrBQJMzM0CmRaJAuRaHQLWZmkCyhytAunbJ
QLN87kC26XlA1KwIQOu+d0DsWh1A5an8QN0m6UDOVgRAxO2RQM9Lx0Din75A3o9cQMPfO0C/
dLxAvUeuQKN87kCXQ5ZAjFodQH0vG0B/jVBAgaHLQI7Ag0CiyLRAqBBiQLaXjUDYcrBA8p++
QPcKPUDg7ZFAwcKPQLafvkDDlYFA1T99QM2BBkCs3S9AjLxqQISDEkCO6XlAmlYEQKibpkC5
wo9AvKPXQLGBBkCs9cNAuGp/QL0euEC4crBAuR64QL6PXEDOTdNA2987QN9T+EDoMSdA8mZm
QOZumEDDZFpAmmZmQGlYEEBa4UhAivGqQLel40Dgan9A4WBCQLNcKUCIKPZAk1P4QNWRaEEP
kWhBKaHLQTkOVkFEan9BUWBCQVyfvg==
</TD></TR>
</TABLEDATA></DATA>
</TABLE>
A.6 Additional TABLE attributes
The GROUP introduced in version 1.1 can be qualified
by ucd and utype attributes. These attributes
could similarly bring useful additional metadata to qualify the
TABLE structure.
A.7 A new XMLDATA serialization
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 XMLDATA serialization introduces the designation of
each FIELD by a dedicated tag. An example could look like
the following:
2verbatim
<TABLE name="Messier">
<FIELD name="Number" ID="M" ucd="ID_NUMBER" datatype="int" >
<DESCRIPTION>Messier Number</DESCRIPTION>
</FIELD>
<FIELD name="R.A.2000" ID="RA" ucd="POS_EQ_RA_MAIN" ref="J2000" unit="deg"
datatype="float" width="5" precision="1" />
<FIELD name="Dec.2000" ID="DE" ucd="POS_EQ_DEC_MAIN" ref="J2000" unit="deg"
datatype="float" width="5" precision="1" />
<FIELD name="Name" ID="N" ucd="ID_ALTERNATIVE" datatype="char" arraysize="*">
<DESCRIPTION>Common name used to designate the Messier object</DESCRIPTION>
</FIELD>
<FIELD ID="T" name="Classification" datatype="char" arraysize="10*"
ucd="CLASS_OBJECT">
<DESCRIPTION>Classification (galaxy, glubular cluster, etc)</DESCRIPTION>
</FIELD>
<DATA><XMLDATA>
<TR>
<M>3</M>
<RA>205.5</RA>
<DE>+28.4</DE>
<N/>
<T>Globular Cluster</T>
</TR>
<TR>
<M>31</M>
<RA>010.7</RA>
<DE>+41.3</DE>
<N>Andromeda Galaxy</N>
<T>Galaxy</T>
</TR>
</XMLDATA></DATA>
</TABLE>