What is a Preservation Digital Object (PDO)?
The Cultural Heritage Community does not need an education in how quickly information can become obfuscated, obliterated, or impractical without proper preservation curation and migration – it is the very impetus for the existence of the community. The age of microfilm was a short blip on the historical timeline of Cultural Heritage Preservation, and already, much of the work done during this period is languishing in storage, inaccessible by its target audience with their modern expectations. Moreover, much of the work done on microfilm is now considered insufficient quality to be considered Preservation Grade.
Scanning an image and saving to a hard drive is trivially easy; creating a Preservation Digital Object (PDO) requires careful consideration and continuous vigilance. Technology rapidly evolves, especially those technologies which control how digital information is created, stored, and retrieved. The goal of a PDO is to capture, wrap, and describe data in such a way that enables migration between storage systems, with the specific ability to be indexed and deciphered by future access systems.
The PDO Itself: Elements Within the PDO
- Content is the abstract information that warrants creating the PDO in the first place. In Cultural Heritage, the content is a photographic representation of a real world object which can replace the original object for most types of access. To qualify as a PDO the content must adhere strictly to preservation-grade image quality guidelines such as FADGI 4-star. An extensive discussion of this can be found in The PDO Content: Image Quality.
- Container is the digital format, or the set of rules that dictates how the content is digitally represented. To qualify as a PDO the file format must be one widely acknowledged as preservation friendly (e.g. TIFF) and the data within the container must conform precisely and strictly to the rules of that format (i.e. passes validation using JHOVE). An extensive discussion of this can be found in The PDO Container: File Fidelity.
- Metadata describes the content and the container with respect to any relevant characteristics (e.g. size, type, identifier of original physical object). To qualify as a PDO the metadata must accurately link to the real world object it represents, and must contain sufficient metadata to aid in the discovery of the digital object within the collection and provide for understanding it within its context.
The PDO Substrate: External Elements the PDO relies on
- Access is the means by which the Content is viewed. Modern access to a PDO containing a TIFF file could be provided by programs like Adobe Photoshop, Preview, GIMP, or a home-brewed image reader. It may also be converted on the fly to a JPG derivative for access via an online portal.
- Storage is the digital media the PDO resides on (e.g. an on-site server or “cloud” storage solution).
It can be easily assumed that the PDO substrate (access and storage) will be in a near constant state of flux. Therefore, the PDO itself (content, container, and metadata) must be created in such a way that they are not dependent on any given type of access or storage.
The Ultimate Goal: Digital Surrogacy
A PDO is not just a digital copy of the original object. It is a complete package ready to function as a digital surrogate for the original physical object in as many use-cases as possible. A PDO is well structured, easy to access and understand, is well described, and contains high quality content. This means that a requesting entity (patron, researcher, administrator, marketing agent) who wishes to have access to the original object can be instead be offered access to the PDO, with no noticeable difference in value to that entity.