Information Technology Reference
In-Depth Information
two images at corresponding points; that the co-ordinates associated with each
pixel in the two images were identical, for example the same latitude and longi-
tude; that the units associated with the numerical values were the same in both
images.
Science data is largely numerical or documentary. In a transformation the way
in which the numbers are encoded may change, for example from an IEEE real to
a scaled integer. In such a case a number in the old and the new formats should be
the same to within rounding errors or predefined accuracy. Additionally co-ordinate
system transformations may also require changes to the numerical values, which
however should be reversible. Thus the validity of the transformation in preserving
these significant data values is testable.
Alternatively the curator might simply document the fact that the trusted appli-
cation, which was widely believed to maintain these numerical values, had been
used in the transformation and thus implicitly those important values would auto-
matically be the same in the two versions. In that case details of the tool would
need to be available and the adequacy of its preservation of significant values can
be evaluated. Thus in these two cases, we can identify how the Performance of the
transformed format can be evaluated to test the Authenticity of preservation.
By analogy one can see that (some) Significant Properties of the data in this
case are the pixel data values, units and the co-ordinate values. However of course
this would not provide enough information to use the image. For example - what
frequencies of light were collected? what was the instrument response? when was
the data collected? All these, and more, would be needed to understand and use that
data and, unless very specific (restricted) definitions of the Designated Community
were used, which included this knowledge, all would therefore be required to be
described in appropriate Representation Information.
13.6.4 Significant Properties and Representation Information
Rendered objects such as JPEG images or audio files tend to be accompanied only
by structural information; in the OAIS terms this is equivalent to stating that the
knowledge base of the designated community includes whatever is needed to inter-
pret the contents of the JPEG image or audio file; as this can be anything, the
Designated Community is not explicitly defined. This is analogous to normal library
practice where the onus is on the reader to understand the printed document.
Scientific Data on the other hand tend to be numerical; even in the simplest case,
where the numbers are encoded in a document as text, although it may be acceptable
to assume that for an implicit Designated Community with a general knowledge of
standard Arabic numerals in decimal notation, they will be able to understand that
the sequence of characters “1” “2” means twelve. However it is not reasonable to
assume that the implicit Designated Community will understand what the twelve
signifies, for example 12
◦
C or 12 m or 12 apples (or even eighteen in hexadec-
imal). In order to fill in this missing information some Semantic Representation
Information must be provided.
