The industrial revolution entailed, among other things, the expansion of towns and cities, the continual reworking of urban infrastructures, and the increasing specialization of knowledge. Inevitably, there evolved several groups of professionals, each responsible for a particular portion of financing, building, and running the American city. And many of these groups made and used maps in support of their work. Property developers created "paper cities" to sell ambitious projects, some of which succeeded, but many of which remained imaginary (29). City planners and engineers made large-scale surveys as part of the construction of new roads, railway lines, sewer systems, and so on. Municipal officials added maps to the deeds and other records with which they assessed property taxes; because their interest lay primarily in property boundaries, their maps did not show buildings precisely, if at all (28). By the late 1800s, several companies flourished by selling specialized information. Most notably, the Sanborn Map Company took city plans and added detailed information about building size and materials, the uses to which buildings were put and how they were heated, and their locations with respect to fire hydrants; in short, everything an insurance underwriter needed to assess rates for fire insurance (30, 31). At the same time, the number of different professional groups needing detailed information had grown so that firms such as the Richards Map Company could sell "general purpose" city atlases (32).

All of these maps are marked by a utilitarian and functional look. They are not "treasures" in the usual sense. But to the social and urban historian, these maps are incredibly important. Together, they paint a detailed picture of American cities before the post-1945 programs of urban renewal changed them, often beyond recognition. For example, Sanborn's 1920 map of Bath reveals a neighborhood that has since disappeared to make room for the approaches to the Carleton Bridge (31). These maps are essential for preserving knowledge of our past. Yet they are threatened with extinction: because they are thought to be functional and not collectible, they are discarded when "out of date." It is only through foresight or plain luck that these old plans end up in the hands of historical societies and libraries that preserve our common history.