The term significant properties is used to refer to the properties of digital objects that must be preserved over time through preservation treatments such as migrations or emulations in order to ensure the continued usability and meaning of the objects. (Significant characteristics, essential characteristics, and essence are less commonly used synonyms). The definition and determination of these properties constitute a critical and mostly unsolved issue in the field of digital preservation.

Significant properties are usually categorized as pertaining to content, context, appearance, structure, and behavior. If, for example, the digital object in question were a chapter of a book in PDF format, the content might be the text and pictures, the context would be the bibliographic description of the book and chapter, the appearance would be the layout of the pages, the structure would include any metadata relating the chapter to the book as a whole, and the behaviors could include internal and external hyperlinks. For this particular PDF, it might be decided that the content, context, and structure must be preserved, but that the appearance and behaviors could be sacrificed in the course of preservation treatment.

Significant properties may adhere to formats, genres, or individual objects, and in some cases may be in the eye of the beholder—the actionable links that you consider expendable may be critical to me. Currently, most research into significant properties is focused on formats. The InSPECT project of the U.K. Arts and Humanities Data Service is investigating the significant properties of raster images, structured text, digital audio, and e-mail messages, and new awards were recently granted to study e-learning objects, software, vector images, and moving images.¹

A good understanding of digital formats is essential for the execution of nearly all preservation strategies. Unfortunately, the concept of format is anything but straightforward. Informally we tend to think of formats as generic file types such as PDF or QuickTime, denoted by MIME type or file extension. These distinctions are not particularly useful for preservation purposes, which require more specific and granular information about version, compression, profile, and bitstream encoding. Is it PDF 1.5 or 1.6? Is the codec used in this QuickTime file Apple Video, Cinepak or MPEG-1? Detailed representation information (as defined by OAIS), preferably linked to the official format specification, is critical for carrying out preservation strategies such as migration and emulation.

A related type of information is sometimes called “environment” information, and concerns the software capable of creating or rendering a format and the hardware capable of supporting that software. For example, a PDF 1.4 file can be created by Adobe Acrobat 5.0 and rendered by Acrobat Reader 5.0 (as well as dozens of other applications). Acrobat 5.0 can run under Windows 98, Windows 2000, Windows Me, and Windows 95. Windows 98 in turn requires a 486 DX2, 66 MHz or higher processor and at least 16 MB of RAM.

By this point it should be obvious that it takes a huge amount of research and expertise to determine both representation and environment information, and that everyone would benefit from having this information accessible from centrally maintained registries. Currently there are at least three registries in some stage of production or development, all with overlapping scopes and different but overlapping content:

• The PRONOM registry developed and maintained by The National Archives (U.K.) has elements of representation and environment information, and is expanding to include preservation planning information.
• The Format Descriptions database maintained by the Library of Congress has detailed representation information as well as an analysis of sustainability factors for many formats.
• The Representation Information Registry Repository under development by the Digital Curation Centre aims to have extensive representation information closely tied to the OAIS model.

PRONOM Registry www.nationalarchives.gov.uk/pronom

Format Descriptions Database www.digitalpreservation.gov/formats/fdd/descriptions.shtml

Representation Information Registry Repository http://registry.dcc.ac.uk/omar

Global Digital Format Registry http://hul.harvard.edu/gdfr/

The Global Digital Format Registry (GDFR) initiative aims to unify these and other registries by defining a common data model and a common network protocol for distributed registries to communicate with each other and synchronize their format representation information. Registries such as PRONOM would then become nodes in the global network. The GDFR was initiated by Harvard University and received a major grant supporting development work from the Andrew W. Mellon Foundation in 2005; the technical work will be done by OCLC.²

Another use for representation and environment information is to help preservation managers assess the risk of format obsolescence for files in their repositories. This is another time- and knowledge-intensive task that would benefit from centralized support. One approach is the Automatic Obsolescence Notification System (AONS) being developed by the National Library of Australia and its partners.³ AONS is designed to extract data from existing registries and the future GDFR and provide the data as decision-support information to repository managers. A beta pilot service was launched in 2007. Other studies concerned with methodologies for assessing the risk of format obsolescence are noted below.

Several Java-based open-source tools have been released in recent years to aid in format identification, validation, and characterization.