What if books and documents stored in archives and libraries suddenly changed their content or appearance? It would be unimaginable for properly stored material to do this. However this is the risk faced with digital items, where the physical storage medium can decay unseen and undetected.
The preservation of archives in digital format has many technological challenges. Digital archives may include records of organisations, websites, audio and video, photography and art, research data sets, or digital copies of records, where the depositor retains the original records. Data may be received on a variety of storage media, including magnetic tape, floppy discs, CDs and DVDs or memory sticks, which may require different hardware to access them. The data itself may be in a wide variety of file formats. It may also be stored in compressed formats or linked to other digital items (for example, websites or relational databases).
Unlike paper documents, all digital formats need computer technology to view and interpret them. This technology (hardware, software and formats) is constantly changing and may ultimately mean that data is unreadable unless steps are taken to address these problems.
Digital preservation is a still-developing science, but a number of strategies have been evolved to meet some of the challenges. These include refreshing the data (copying it from one storage medium to another) to avoid 'bit rot' (the gradual decay of storage media) which could lead to changes to the data. Migration involves copying data to newer system environments, which could include the conversion of resources from one file format to another. However does a change in file format maintain the authenticity of the file – might it mean the loss of formatting such as a font for example? Replication involves holding the data in more than one place to avoid risk of data loss, for example from fire or flood, but this multiplies the effort required to keep the data up to date.
Surrey History Centre's digital archive currently holds around 150Gb of data spread across all or part of 140 collections. Examples of material held include the photographic survey of Ottershaw, 1995 (ref. 6943); '100 images of Tatsfield', 2002 (ref. 7301); and photographs and text recording 'Bagshot at the Millennium' (ref. 7540).
These images are from 'Bagshot at the Millennium'.