The Sanborn map collection consists of a uniform series of large-scale maps, dating from 1867 to the 1950s and depicting the commercial, industrial, and residential sections of some twelve thousand cities and towns in the United States, Canada, and Mexico. The maps were designed to assist fire insurance agents in determining the degree of hazard associated with a particular property and therefore show the size, shape, and construction of dwellings, commercial buildings, and factories as well as fire walls, locations of windows and doors, sprinkler systems, and types of roofs. The maps also indicate widths and names of streets, property boundaries, building use, and house and block numbers. They show the locations of water mains, giving their dimensions, and of fire alarm boxes and hydrants. Sanborn maps are thus an unrivaled source of information about the structure and use of buildings in American cities.

The Library's Sanborn collection includes maps of many Sonoma County towns and cities, as well as a number of others cities and towns across California, on 11 microfilm reels, arranged alphabetically by city and chronologically within each city. The set was reproduced  from original copies in the Geography and Map Division of the Library of Congress and includes only maps published before 1950. Maps were originally published by Sanborn Map Company, Pelham, New York.

The library also has a small number of original printed maps available for Santa Rosa and Petaluma. Please see the Library's index of Sanborn Fire Insurance Maps for detailed holdings of both microfilm and print maps. Additional information can be found in the library catalog for details and locations.