Significant Properties Definition (CPP-022)

Definition and scope

Significant properties is a widely-adopted concept in the analog and digital preservation sectors that can be defined as “essential characteristics of a digital object which must be preserved over time for the digital object to remain accessible and meaningful” (definition taken from the Digital Preservation Coalition, British Library, and JISC joint workshop, “What to preserve? Significant Properties of Digital Objects”). It acknowledges that digital Objects - just like analog Objects - are subject to intentional or unintentional changes and that memory organisations should identify which of their properties are significant, and should therefore be preserved by committing efforts and means. For a more in-depth presentation of the concept of significant properties, see Montague, Lynne, and Eleonora Nicchiarelli. “The Concept of Significant Properties.” The National Archives, Österreichischen Nationalbibliothek, May 26, 2010. Available at https://www.planets-project.eu/docs/reports/Planets_PC3-D23A_TheConceptOfSignificantProperties.pdf .

Significant properties are declared for a certain Object type, although this categorisation will vary. For example, one organisation can declare significant properties for all its still images, while another will specify different significant properties for digital photographs, graphic design works and screenshots. Significant properties are generally classified into five top-level categories:

• Content: the main signal(s) conveyed by the Object (e.g. textual, visual, audiovisual, etc.);
• Context: the information documenting the circumstances of creation and history of the Object;
• Appearance: the visual aspects of the Object (e.g. fonts, layouts, colours, etc.);
• Structure: the organisation of the content (e.g. pages, sections, ordering, etc.);
• Behaviour: all features beyond simple displaying (e.g. links, actions, navigation, etc.)

In addition to these categories, significant properties may be qualified by the possibility to be determined in an automated way, or only by manual observation. Significant properties that can be determined in an automated way are called “extractable”. For example, the visual aspect of source and target PDFs can be easily compared by means of tools like diffpdf. On the other hand, those significant properties that only a human action can assess are called “observational”. The step-by-step description below focuses on extractable properties. In cases where observational properties are considered significant by the TDA, it must define a manual determination method that is incorporated in its processes.

Guidance can be provided to memory organisations on which properties of certain Objects are generally considered significant, but the final decision of adopting these or not is still the responsibility of the TDA. Thus, the definition of significant properties is an institutional policy. Significant properties might even vary within the same organisation from one project to another. For example, the history of changes recorded as XMP internal Metadata in field xmpMM:History may be considered crucial for born-digital digital photographs, but negligible for album thumbnails.

The significant properties policy should be produced or at least confirmed by the TDA management, since it will influence the usability that the Objects will support. For this reason, significant properties are likely to be more high-level, organisational properties that will need to be mapped to technical extractable properties. For example, for a significant property “audio signal quality”, the technical properties associated will be sampling frequency, bit depth, etc. These properties might change (e.g. if a conversion from audio signal in DSD64 1 bit / 2.8224 MHz is made to PCM 24 bits / 96kHz) but their evolution should be controlled to ensure the audio signal quality is preserved.

In some cases, significant properties will be dictated by stakeholders (e.g. Producer, Consumer, etc.) - see the use case “Page Layout as Significant Property of Reflowable EPUBs” below. Community Watch (CPP-018) must therefore ensure that its significant properties policy is aligned with the needs of its designated community.