Thursday, April 2, 2009

Library of Congress digitized map collection

The Geography and Map Division of the Library of Congress holds more than 4.5 million items, many of which are digitized and available in copyright free formats for viewing or downloading from the Library of Congress Web site. These maps date back to the 1500s they are useful in locating old place names as well as boundaries of counties, cemetery locations (see the map above) and other uses. This collection comprises one of the largest high quality, freely accessible digital collections of historical maps presently on the Internet.

It is interesting to read the description of how the images were created. To quote the Library of Congress:

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.

Translated into English, this means that the images are high quality in the TIFF format, however, they are saved in JPEG 2000 format for use online, which may not be compatible with some software programs. I would suggest that the downloaded files be opened and saved in a more compatible format if necessary. It is also interesting to note that even though the files are initially saved in TIFF format for archive purposes, they are disseminated in JPEG format for actual use on the Internet.

You may also want to see over 6000 maps of Scotland, The University of Texas Libraries Perry-Castaneda Library Map Collection, The David Rumsey Map Collection, and Digital History. There are many other specialized collections as well as maps available from subscription Websites.

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