Information Technology Reference
In-Depth Information
media-type instance. Refreshment is also a Replication, but Replication may
require changes to the Archival Storage mapping infrastructure
.
2. Operations which change the bit sequences
•
Repackaging
: A Digital Migration where there is some change in the bits
of the Packaging Information.
•
Transformation
: A Digital Migration where there is some change in the
Content Information or PDI bits while attempting to preserve the full
information content.
This deserves some extended discussion, which follows.
12.3.1 Transformation
Transformation implies a change in the bit sequence of either the Content
Information or the PDI.
In many discussions of digital preservation the termMigration is used
when in fact what is meant is specifically Transformation because
the aim in those discussions is to change the digital encoding of the
information.
Given a certain piece of information there could be many different ways of
encoding it digitally. For example an image could be encoded as a TIFF file or a
JPEG; a document could be held as Word or PDF; a table containing scientific data
could be held as a FITS table or as a CSV (comma-separated values) file. Each of
these alternatives would need it their own, different, Representation Network.
However some Transformations make more sense than others. This will com-
monly be regarded as changing from one data format to another, but one must also
think about the associated
semantics
. Some formats have little or no room for the
semantics. Another consideration is the number and types of applications commonly
associated with the various formats.
For example an image could be regarded as a table where each of the cells con-
tains a number. However it would not make good sense to encode the image as a
CSV file because of the loss of semantics involved. Moreover the applications (e.g.
spreadsheet programmes) normally used to deal with a CVS file do not normally
display the data as one would expect an image to be displayed.
With regard to the semantics, one can supplement the capabilities of a particu-
lar format with something else e.g. the CSV file could have an associated text file
to supply the missing semantic information, such as the meanings of the columns,
which would otherwise be missing. In this case one would need the Representation
Information for (1) the CSV file (2) the text file and (3) the relationship between
them. While this is possible, the more attractive option would be to choose a
new format which can itself handle the required semantics, with available appli-
cations that supply the required functionality, at least as well as the original format.
Therefore given a piece of digitally encoded information that one needs to preserve,
the transformation which one should reasonably apply is not arbitrary.
