General comments on
digital reproductions of maps for American Memory
The maps selected for initial digitization by the Geography and Map Division for American
Memory are mainly from the 19th century. Many were designed as detailed views with
perspective and a pictorial approach rather than drawn strictly to scale. In particular, the
Panoramic Maps collection comprises bird's-eye views of American towns and cities. As of
November 1997, roughly 500 of these perspective views are available; batches of maps will be
added monthly for a total of around 1,000. Future Map Collections do include maps drawn to
scale that might be used for the cartographic information represented. These include several 20th
century maps of Yellowstone National Park.
The digitized maps included in American Memory have been digitized at 300 dpi and 24-bit
color. The archival uncompressed TIFF files are very large, running from 100 to 250
megabytes or more.
World-wide-web delivery of these very large files is accomplished by
using proprietary wavelet compression technology made available to the Library of Congress by
LizardTech of Seattle, Washington. The software in use consists of two general modules, one for
compression and another to support web-service including generation of GIF images on-the-fly.
Compression of the order of 22:1 has been achieved and the compressed format supports rapid
retrieval of any section of the image at different levels of resolution. The compression technology,
known as multiresolution seamless image database or MrSID, is derived from the research efforts
of Los Alamos National Laboratory, New Mexico.
For its web interface, the Library has adapted scripts (in perl) initially developed at Los
Alamos National Laboratory to support zooming in and out, choice of image size in pixels, and
selection of the center point for display. Routines provided by LizardTech generate the GIF
images for display over the web. For those with fast enough network connections to download
the compressed files (3 to 10 megabytes), free viewers for MrSID files (for Windows NT &
95 and Macintosh PowerPC) can be downloaded from the LizardTech web-site. The library is
not currently providing direct links to the MrSID files through American Memory. However, the
technical summaries for the map collections have links to sample MrSID files.
For each map, an item-level MARC record
exists. In an 856 field, the combination of
subfields $d and $f provides a unique identifier for the associated digital reproduction. Files
representing the images are available in a directory structure (known at the Library of Congress as
an "aggregate") for which $d identifies the root. Filenames for the different digital
versions of a map are created by combining the $f value (which identifies the item) with
distinguishing file extensions.