From Wikipedia: Sanborn maps are detailed maps of U.S. cities and towns in the 19th and 20th centuries. Originally published by The Sanborn Map Company, the maps were created to allow fire insurance companies to assess their total liability in urbanized areas of the United States. Since they contain detailed information about properties and individual buildings in approximately 12,000 U.S. cities and towns, Sanborn maps are valuable for documenting changes in the building environment of American cities over many decades.
Sanborn held a monopoly over fire insurance maps for the majority of the 20th century, but the business declined as US insurance companies stopped using maps for underwriting in the 1960s. The last Sanborn fire maps were published on microfilm in 1977, but old Sanborn maps remain useful for historical research.
From the Library of Congress: The Sanborn Fire Insurance Maps Online Checklist provides a searchable database of the fire insurance maps published by the Sanborn Map Company housed in the collections of the Geography and Map Division. The online checklist is based upon the Library’s 1981 publication Fire Insurance Maps in the Library of Congress and will be continually updated to reflect new acquisitions.
The online checklist also contains links to existing digital images from our collection and will be updated as new images are added. If you have any questions, comments, or are interested in obtaining reproductions from the collection, please Ask A Librarian.
The Sanborn maps are arranged by state, then city and release data. Currently there are over 25,000 sheets from over 3000 city sets online in the following states: AK, AL, AZ, CA, CO, CT, DC, DE, GA, ID, IL, IN, KY, LA, MA, MD, ME, MI, MO, MS, NC, ND, NE, NH, NJ, NV, OH, OK, PA, SD, TX, VA, VT, WY and Canada, Mexico, Cuba sugar warehouses, and U.S. whiskey warehouses.