Scanning Cartographic Materials

The digital images were created by staff in the Geography and Map Division by scanning the original map on a large-format (24 x 36 inches) flatbed scanner using a RGB (red-green-blue) 24-bit CCD color sensor (16.8 million colors). The scanner is manufactured by Tangent Image Systems. The Geography and Map Division has two Tangent scanners. One scanner is on indefinite loan from Tangent Imaging.

Each raster image is produced by scanning the item at a resolution of 300 dots-per-inch and converting the resulting proprietary file format to TIFF format. These TIFF files, which average approximately 180Mb, are transferred over a network to a Unix server for temporary offline storage.

Following scanning, the TIFF files are enhanced using the WindowsNT version of image processing software Adobe Photoshop 5.0, to rotate, crop, adjust brightness or contrast, and stitch together TIFF images for items requiring multiple scans.

The enhanced TIFF files are compressed, using a wavelet-based image compressing software called Multi-Resolution Seamless Image Database, or MrSID. This software integrates multiple resolutions of an image into a single file which enables Internet users with a standard browser to zoom in, getting more and more detail. Although MrSID is a "lossy" compressor, the images were compressed at a ratio of 22:1 without experiencing any loss of information. LizardTech ( of Seattle, Washington donated the license for the compression software.

The final step is the use of Alchemy Software to create a small GIF file for use as the initial thumbnail display of the item along with the bibliographic information.

After completing the scanning, an Archive TIFF file along with the SID and GIF files are stored on the Library's RS6000 World Wide Web server.


Master image:
Scanning resolution: 300 dpi
Tonal resolution: 24 bit color, RGB separation
File format: TIFF
Compression: none

Reference image:
Scanning resolution: 300 dpi
Tonal resolution: 24 bit color, RGB separation
File format: SID
Compression: Wavelet, 22:1 ratio.

Thumbnail image:
Viewing resolution: 72 dpi
Tonal resolution: 8 bit color, RGB separation
File format: GIF

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