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Digital Archiving - Conclusion

data time migration term

University of Basel

Long-term archiving of photographic materials has always been a difficult task. The majority of photographic processes have not been designed with longevity in mind. In fact, photography is inherently unstable and all photographic materials will decay with time. While B & W photographs have a life expectancy of about 100 to 150 years (until the first effects of decaying become visible), color photography will usually begin its decay much faster. Even modern traditional photographic materials, which are chemically more stable, will decay with time. The decay rate can vary a lot and is dependent mainly on storage conditions, but also will be affected by the materials used, processing methods, handling, and so forth.

Since many objects in archives are unique artifacts that are irrecoverable in case of damage or total loss, each time they are handled poses great risks for damage or total loss. Often, the handling is also impractical and cumbersome. Therefore, many photographs are being digitized. In general, the digitized object has lost all “materiality;” it becomes an immaterial representation of part of the original object. In order to use photography again as a well-suited example, a digitized photograph can capture only the visual elements of the original content of the photograph, whereas most aspects of the material such as physical properties (thickness, surface properties, smell, and so forth) will generally be lost.

Most digitization projects have been launched or are conceived without long-term archiving in mind. These projects are started to improve access to the assets through databases and the Internet. But as soon as the first digital data arrives, the question of archiving arises: The digitization process is slow, expensive, and cumbersome and therefore will usually not be repeated in foreseeable time. As a consequence, anybody involved in digitization processes is suddenly facing the problem of long-term archiving of digital data.

To complicate the situation, most photographs created today are “born digital,” that is, they are digital in origin. And not only photographs, but also motion pictures, computer animations, and videos using modern technology result in “originals” of digital nature. This fact will increase the pressure to find solutions for digital long-term preservation very soon.

From our daily experience, digital data seems to be very volatile and unstable. Everybody working with computers of any scale has had the bad experience of data loss: a word-processor document becomes unreadable, an external storage medium cannot be accessed anymore, etc. It looks like “long-term archival” and “digital” are diametrically opposed concepts. However, the very properties of digital data offer the possibility of an unlimited storage time.


At the moment, for most cases the longevity of digital data can be best achieved by implementing a migration model based on the following rules:

  1. Redundancy
    Data must be kept with a high level of redundancy. At least three copies on a minimum of two different types of storage media (such as two copies on hard disk and one copy on magnetic tape) should be kept at geographically different locations.
  2. Checksums
    For all data files, checksums should be calculated and archived with the data files. This allows for checking data files at any time for aging-related changes or errors.
  3. Proofreading
    Every 12 to 24 months, the data should be proofread and the checksums compared. If errors are detected, a migration should be launched immediately.
  4. Migration
    Migrations have to be planned in advance, including financing. A bit stream migration is necessary about every 5 years. A format migration is advised if a new file format becomes standard and the conversion can be done without loss of data.
  5. Documentation
    Every step has to be documented in detail and all media must be labeled properly.

By following these rules, you may preserve digital data indefinitely. However, constant care is required. If this care is not possible for a certain length of time, the data will be lost, and only digital archaeology may possibly recover part of the data.

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