Map lovers, rejoice! The United States Geological Survey, headquartered in Reston, Virginia, is about to complete a massive project to digitize its cache of approximately 200,000 historic topographic maps, previously available only in print or in some cases out-of-print, meaning that people searching for a special old topo had to go to the archive in Virginia to take a look.

Who cares? Geographers, geologists, hydrologists, demographers, engineers and urban planners, to be sure. Also people interested in local history and genealogy, says the USGS. And, if you ask me, travelers who want not only detailed maps for pursuits like walking and biking, but information about what a place looked like in the past. For instance, the course of rivers before impoundment by dams, villages that have grown into cities, vast empty spaces in the West now crossed by superhighways, mountain ranges reconfigured by volcanic eruption.

Some of the oldest maps in the USGS Historical Topographic Map Collection show the Chicago Loop in 1929; Tooele Valley, Utah, in 1885; New Bedford, Massachusetts, in 1888; Colorado’s Mosquito Mountains in 1886. When taken as a whole, the collection can be considered a National Map, a cartographic library of “last resort,” says archive manager Greg Allord, containing hard-to-find maps when all other sources fail. Allord says that scanning is now complete, though processing may take until September and some maps found in other libraries will eventually be added.

Meanwhile, it doesn’t take much computer-savvy to search the collection by state, scale or original map name. I just tried it, successfully downloading and printing a 1886 topo map of the Escalante River watershed in southern Utah. What will I do with it? I don’t quite know, but it’s free because the collection is in the public domain and making it broadly accessible is part of the program’s mandate.

A few definitions may be useful for laypeople who want to try it out: A topographical map shows physical features and elevations, usually with contour lines. Topo mapping done by the USGS generally divides the country into quadrants, or quads, bounded by two lines of longitude and two lines of latitude; the most popular are 1:24,000 in scale (one inch on the map representing 2,000 feet on the earth surface), available in sheets that show 64 square-mile areas.

Since the advent of digitized maps, new words have been added to the cartographic lexicon like georeferencing (a method of adapting old map information to contemporary computer-based geography, a study now known as Geographic Information System or GIS) and metadata (background map information, sometimes part of the legend), not to mention technical computer terms like Bagit, TIFF, GeoPDF—but let’s not even try to go there.

There was, of course, no such thing as georeferencing when the USGS was created by Congress in 1879, chiefly to locate and describe potential mineral resources in great swatches of the country that hadn’t been closely studied. By then the government had funded several surveys, marking what Clarence King, the first director of the USGS, saw as a turning point, “when science ceased to be dragged in the dust of rapid exploration and took a commanding position in the professional work of the country.”

John Wesley Powell, the great Colorado River explorer and second director of the USGS (1881-94), believed it was impossible to convey geological information without a topographic component, though he came under fire from Congress for the added expense it entailed. As a result, topographical surveying has long been intimately connected to geology in the U.S. (unlike Britain, which has separate divisions for topographical and geological mapping) and the USGS is part of the Department of the Interior. The oldest maps in the USGS collection come from Powell’s time.

It’s fitting to note that the Smithsonian Institution was a supporter of Powell’s surveying expeditions; indeed, he went on from the USGS to serve as director of the Bureau of American Ethnology, later folded into the Smithsonian Office of Anthropology. And even now the connection remains strong with the USGS and the Smithsonian cooperating on the Global Volcanism Program, which publishes a Weekly Volcanic Activity Report detailing geothermic events that may someday require new topos.