The OAIS Reference Model provides a number of models for repositories including a Functional Model, to which is relatively easy to map an existing archive system, an Information Model, which is rather more challenging, an Information Packaging Model and federation models, plus preservation perspectives including types of migration and a variety of software related processes. A number of overall strategies, processes and supporting infrastructures may be derived from these.
The Information Model provides the concepts to support the long-term understandability of the preserved data. This introduces the idea of Representation Information.
The UML diagram in Figure 1 means that
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an Information Object is made up of a Data Object and Representation Information
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A Data Object can be either a Physical Object or a Digital Object . An example of the former is a piece of paper or a rock sample.
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A Digital Object is made up of one or more Bits.
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A Data Object is interpreted using Representation Information
Representation Information is itself interpreted using further Representation Information
Figure 1 shows that Representation Information may contain references to other Representation Information. When this is coupled with the fact that Representation Information is an Information Object that may have its own Digital Object and other Representation Information associated with understanding each Digital Object, as shown in a compact form by the .interpreted using. association, the resulting set of objects can be referred to as a Representation Network. The question of where this recursion ends is answered by the concept of Designated Community, which is touched on further in sections 2.3.1.1 and 2.3.
Figure 2 shows more details and in particular breaks out the semantic and structural information as well as recognising that there may be “other” representation information such as software.
Figure 2. Representation information object
The types of Representation Information are very diverse and it is highly likely to be discipline dependent, although there will be some commonalities.
2.1.1 Role of Significant Properties
At this point it is worth comparing the concept of Significant Properties with that of Representation Information. The former is widely used in the library community but it is hard to see how it applies to, for example, science data.
Clearly Significant Properties focus on those aspects of digital objects which can be evaluated in some way and checked as to whether they have been preserved. In particular after a transformation of the digital object this is an important consideration. However, the meaning associated with a Significant Property is nowhere defined. Therefore it must be the case that the Significant Properties, while useful, do not contribute to Understandability. For example it a Significant Property might be that a text character is red, however the meaning of that redness is not defined.
The question then is what is their significance. Giaretta (2009) argues that the role that Significant Properties play is more closely related to authenticity. Essentially the data curator will check that the selected Significant Properties are unchanged after a transformation in order to assure him/herself that the new transformed version, to his/her satisfaction, may be used as an authentic copy.
This view of significant properties allows the concept to be included in the revision of OAIS which is being prepared, and related to Representation Information. It also allows Significant Properties of scientific data to be clearly defined; however that is the topic of another paper.