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.

Image formats

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.

Descriptive records

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.

Maps available for use in DLI - Phase II

The American Memory collections listed below, currently released or in an advanced state of production, include maps. Technical summaries with considerable detail and links to samples are available for each collection by clicking on the title.

Collection title:     Click for technical summary


Panoramic Maps, 1847-1929


Other Map Collections

Some pictorial, some to scale

Challenges faced by NDLP

How will users want to use these large images. What tools are needed to support effective exploration, manipulation, and printing?

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