Scanning Specifications
The NAL bases its scanning specifications on guidelines from the National Archives and the Federal Agencies Digitization Guidelines Initiative. A brief overview is provided below.
Scanning must create a single Tagged Image File Format (TIFF) file per page, capturing all pages including those intentionally left blank. Scans should not be cropped or de-skewed. TIFF image files must conform to the TIFF 6.0 specification. The conversion of document pages must be as follows:
| Pages in Original Document ... | Resolution (pixels per inch/samples per inch)* |
Bit Depth | Compression |
|---|---|---|---|
| are blank or contain only text, charts, or line art | 600 | 1-bit bitonal | |
| contain color | 400 | 24-bit Red, Green, Blue Mode (RGB) | none |
contain half-tone prints in shades of gray or black and white photographs |
400 | 8-bit grayscale | none |
are larger than 11 by 16 inches and contain color |
300 | 24-bit RGB | none |
are larger than 11 by 16 inches and contain half-tone prints in shades of gray or black and white photographs |
300 | 8-bit grayscale | none |
When the TIFF files are processed for display in the NAL Digital Repository (NALDR), a machine-readable text file is created through optical character recognition (OCR). The OCR text is stored in an Extensible Markup Language (XML) file with the metadata for the image. The presence of the OCR text enables full-text searching of the images in the NALDR. The NALDR also provides users with the option to view the images in PDF. This PDF file is created on the fly and is not text searchable.
About the Resolution of Images Available in the NALDR
Images are scanned according to the specifications outlined above and stored on a server. When images are copied to the NALDR, the resolution for grayscale images is changed to 200 ppi/spi and for color images to 100 ppi/spi in order to reduce the size of the files stored in the repository. If you require a higher resolution image than the one available in the NALDR, please call 301-504-6503 or e-mail Special Collections.
