Digitization of Maps and Atlases

These standards suggest best practices for digitizing maps, contemporary and historic, as well as atlases, that are intended for intensive visual study. Digitization standards for these types of material prioritize legibility, artistic creation, and historical significance, requiring deep zoom and resolution of fine detail and printing. While material digitized according to these standards may be in good condition, it may be vulnerable due to inherent vice in the medium (such as brittle paper or compromised binding). Conservation assessment is recommended prior to digitization. The result of the assessment may determine a need for treatment before digitization, after, or both.

Digitization standards for preservation

“Maps”, “Historic maps” and “Atlases’ are general categories of cartographic material and determination of contemporary, historic, rare or common maps is left to the discretion of experts. For digitization, contemporary maps or atlases may have been mass produced and show a distinct dot pattern. For this reason, capturing at higher than 400 PPI resolution may be excessive. Maps or atlases that are individually rendered should be captured at a minimum of 400 PPI/7200px on the long axis and higher resolution may be warranted for study of fine detail or script.

Because of the size of many maps, capturing in sections and stitching images may be necessary. This is a slow process and must be considered when faced with deadlines. This also yields very large files; a 24” x 24” map captured at 600 PPI is approximately 593 MB, a consideration when planning file storage needs. Sheet-feed scanners may be appropriate for certain maps but should not be used on any items without conservator approval. When photographing atlases, maintaining scale requires a consistent lens-to-subject distance.

Master File Format: All master files should be uncompressed, Tagged Image File Format (TIFF) version 6, in either “little endian” (IBM PC) or “big endian” (Mac) byte order. In addition, all files must pass JHOVE format validation.

Resolution: Image capture resolution is measured in pixels per inch (PPI). This should be a true optical resolution; the lens and pixel array in the capture device should be capable of creating an image file to the required resolution specification without interpolation.

Bit Depth:

Color (RGB):

• Images are captured natively in 24-bit RGB RAW or TIFF format and Master Files are exported as 24-bit TIFF files with the “Adobe RGB (1998)” color profile embedded.

Editing:

• All images should be cropped to include the entire item/object, leaving a small background border around the material to show the entirety of a map or atlas page. Black borders are preferred but there are exceptions, such as dark originals or backgrounds used to create visual contrast and enhance legibility.
• While sophisticated image viewers can easily rotate an image, the master image file should be oriented properly. For atlases with images of varying orientation, default to the orientation of the binding.