Information Technology Reference
In-Depth Information
A FITS file containing a single astronomical image could be considered
Simple, and its Representation Information is the FITS specifications [
46
] with the
Representation Network shown in
Fig. 6.4
.
7.7.2 Composite
Composite digital objects are all those which are not Simple, which of course covers
a very large number of possibilities.
A FITS file such as that illustrated in
Fig. 4.2
has the same Representation
Information Network as for the Simple example above. Each of the components
would also be (essentially) a Simple FITS file. What would be missing is the expla-
nation of the relationship between the various components. That information would
have to be in an additional piece of Representation Information, for example a
simple text document or perhaps a more formal description using RDF.
7.7.2.1 NetCDF - Data Request Broker (DRB) Description
Network Common Data Format (NetCDF) [
45
] is a binary file format and data con-
tainer used extensively within the scientific community. The full DRB description is
an XML schema (Fig.
7.13
) consisting of XML schema elements with the addition
of extra SDF tags to describe the underlying data structures whether BINARY or
ASCII. For example the magic complex type, the first shown in the format diagram,
consists of a sequence of two elements CDF and VERSION_BYTE respectively and
can be expressed by the following code.
<xs:element name
=
"magic">
<xs:complexType>
<xs:sequence>
<xs:element name
=
"CDF">
<xs:annotation>
<xs:appinfo>
<sdf:block>
<sdf:length unit
=
"byte">3</sdf:length>
<sdf:encoding>ASCII</sdf:encoding>
</sdf:block>
</xs:appinfo>
</xs:annotation>
<xs:simpleType>
<xs:restriction base
=
"xs:string"/>
</xs:simpleType>
</xs:element>
<xs:element name
=
"VERSION_BYTE" type
=
"xs:unsignedByte">
<xs:annotation>
<xs:appinfo>
