The quality of digitization is a function of the application, how the digital copy will be used and by the amount of computer storage available .
The primary reasons for digitizing a map are:
- Dissemination & online publishing on the web, standard quality prints - to allow easy access to documents, to protect the physical originals and minimize handling of them, make standard quality prints -> 300 dpi or 600 dpi, lossy compression is possibly acceptable
- Substitution (the original document is fragile and likely to be lost in the near future, extremly valuable documents where redigitisation is problematic, requires high quality prints 1:1) - we need as perfect copy as possible -> usually 600 dpi or better 1200 dpi, with lighting and colour calibration (ICC profile), lossless compression
The choice depends on the character of the physical document. Very often documents are digitized to the highest possible quality (to avoid redigitisation) and digital copies are preserved using long term digital storage with backups. For online presentation a derived copy (a down-sampled lower quality version) is created from the master image.
Formats for storing the digitized masters of historical documents and maps:
- Uncompressed TIFF
- JPEG2000 part 1 (lossless and lossy)
- PNG 1.2
- TIFF LZW
- proprietary formats (MrSID, ECW) (not recommended)
For text oriented documents like newspapers or books, commonly used formats include DjVu or PDF-A.
Manufacturers of large format scanners for maps:
Introduction to digitization:
- Fleet, C.: The ABC of map digitization, Map Library, National Library of Scotland + local copy
- A very valuable document comparing the data formats available for storing master images:
Gillesse R., Rog J., Verheusen A.; Alternative File Formats for Storing Master Images of Digitisation Projects; National Library of the Netherlands
available at: http://www.kb.nl/hrd/dd/dd_links_en_publicaties/links_en_publicaties_intro.html + local copy
- Results of the survey for usage of JPEG2000 + best-practices + notes about the software tools used:
Presentation made by authors May 7, 2009 at the Society of Imaging Science and Technology's Archiving 2009 Conference, Arlington, VA. Authors' full report may be found at http://digitalcommons.uconn.edu/libr_pubs/19/ with raw survey results at http://digitalcommons.uconn.edu/libr_pubs/16/
Lowe, David and Bennett, Michael J., "A Status Report on JPEG 2000 Implementation for Still Images: The UConn Study" (2009). UConn Libraries Presentations. Paper 21. http://digitalcommons.uconn.edu/libr_pres/21
- Another example of best practice from Manuscriptorium project:
http://www.manuscriptorium.com/Download/Documentation/manuscriptorium_image_quality_ENG.pdf + local copy