Information Technology Reference
In-Depth Information
There are deep reasons for making a careful choice and documenting that choice
appropriately. This is discussed in detail in Sect. 13.6 .
However there are a number of useful points which should be made here. For
example one can think of the ideal Transformation in which the new digital object
has the same information as the original. If this is the case then it should be pos-
sible to confirm this by means of another Transformation back to the original bit
sequence. If one can find this pair of Transformations then one can define (following
the revised version of OAIS):
Reversible Transformation : A Transformation in which the new represen-
tation defines a set (or a subset) of resulting entities that are equivalent to
the resulting entities defined by the original representation. This means that
there is a one-to-one mapping back to the original representation and its set
of base entities .
On the other hand if one looks at the other transformations mentioned above, for
example from FITS to CSV, then one would, without additional information, e.g.
the supplementary text file mentioned above, lose information and therefore not be
able to make the reverse transformation.
It is therefore reasonable to define:
Non-Reversible Transformation : A Transformation which cannot be guar-
anteed to be a Reversible Transformation .
An important point to note is that the definition of non-reversible is drawn as
broadly as possible. For example one does not need to have to prove there is no
backward transformation, only that one cannot guarantee that such a transformation
can be constructed.
We will come back to these definitions in Chap. 13 where they play an important
role in considerations of Authenticity.
12.4 Summary
This chapter has raced through a number of the basic preservation strategies and
techniques; it should be clear that each technique has its own strengths and weak-
nesses, and one must be careful to recognise these. The reader must be careful not
to be misled by the amount of material on emulation here; this was a useful loca-
tion for this material. Other preservation techniques are discussed in much more
detail throughout this topic. Other chapters are devoted to descriptive Representation
Information and also to Transformations.
In Part II we provide examples of many of these techniques with evidence to
support their efficacy when applied appropriately.
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