Notes from the Field: Mapping History with Sanborn Maps

Notes from the Field: Mapping History with Sanborn Maps

Sanborn Map of the Theater District in New York City, 1911. Click for larger image. Photo courtesy Library of Congress Geography and Map Division.

For a historian, a good map can be a gold mine of valuable information. Beyond road names or topographical features, maps can show the historical arrangements of buildings, property owners, and even the location of a specific furnace or outbuilding on an industrial property. Few maps do this better than Sanborn fire insurance maps, which depict the commercial, industrial, and residential sections of more than 12,000 American cities and towns from ca. 1867 to 1970. Lest it sound like the company is defunct, Sanborn still offers mapping services.

Fire insurance maps were developed in the United States in the mid-19th century in response to the insurance industry’s need to assess fire risks. Underwriters had previously inspected properties themselves, but as cities expanded, this system became impractical (and probably impossible). As a result, several companies, among them the D. A. Sanborn National Insurance Diagram Bureau, began creating maps designed to assist insurance agents. By 1902, the newly renamed Sanborn Map Co. had monopolized the industry, making “Sanborn map” synonymous with “fire insurance map.”

The Sanborn Map Co. prepared its maps with an army of surveyors traveling from town to town, literally working from strangers’ backyards (although surveyors were told to contact the police station first, stories abound of surveyors being thrown in jail, attacked by dogs, or reported as spies). Relying heavily on their approximately 100 page “Surveyors’ Manual,” surveyors recorded careful notes and diagrams that were then developed into intricately drawn, hand-colored maps that have become invaluable to historians.

From this key of a Sanborn fire insurance map, researchers can glean how different buildings are represented in the series—sometimes as descriptive as “Frame building covered with asbestos.” Photos courtesy of The Library of Congress Geography and Map Division.

For some towns, the Sanborn Map Co. prepared new maps or map updates as often as every five years, making them one of our best sources for showing change over time at specific sites. Sanborn maps tell the careful observer everything from which way the prevailing wind blows, to the location of historical waste pits.

Sanborn maps are important to site-specific research, allowing historians to approximate when a particular building was torn down or how many oil tanks were on a site in a particular year. Sanborn maps can also be used for broader, investigative research: by looking through a range of maps in a particular region, historians can track the progression of a specific industry, identify economic development, and even gain new insight into social change.

Sanborn maps have their limitations, however, and those unfamiliar with them may make some false assumptions when using them as a resource. Here are a few of the common “myths” about Sanborn maps:

MYTH: Only towns of a certain size were mapped

While major cities were certainly covered by Sanborn surveyors, even tiny villages show up among the Sanborn annals. In fact, small towns were sometimes mapped more frequently than large cities. Syracuse, New York, for example, received no Sanborn updates at all between 1892 and 1910, while Manlius, a nearby village of about 1,000 residents, was mapped four times. One reason for this historical discrepancy may simply be that mapping a major city was time-consuming and expensive. Whatever the explanation, Sanborn surveys were not determined by population, but rather by the demands of the insurance industry.

MYTH: Every Sanborn map of an area is similar

While two different repositories may have a 1928 Sanborn map of Brooklyn, it is possible that one atlas may show a vacant lot where the other shows a shopping center. This is because during economic difficulties in the 1920s, the Sanborn Map Co. began updating maps by pasting correction slips on top of existing maps, rather than printing an entirely new map. For some cities, as many as 50 or 60 rounds of corrections were made during the next few decades.

Over the years, pages grew heavy with correction slips, making it impossible to tell which slips were added in which years. Among the Sanborn maps and atlases that exist in repositories today, some have been “corrected” while others have not. HAI has found that local repositories are particularly likely to have the “corrected” version of the maps, even though the Library of Congress holds over 700,000 pages of Sanborns, local is often best. By shining a light underneath the “corrected” map, researchers can literally see the pasted-in corrections.

MYTH: Sanborn maps are the only reliable fire insurance maps

Hexamer’s map of Philadelphia dating to 1915 shares many commonalities with Sanborn maps. Photos courtesy of The Library of Congress Geography and Map Division.

The Sanborn Fire Insurance Company certainly produced the most, but fire insurance maps date back to the end of the 1700s, pre-dating the Sanborn maps by nearly 100 years. Philadelphia, for example, was initially mapped by Ernest Hexamer’s business from the mid-1800s until the Sanborn company purchased it in 1916. In fact, the Library of Congress has a finding aid for non-Sanborn fire insurance maps in their holdings that is 342 pages long; you can find it here.

MYTH: Sanborn maps were only used by the fire insurance industry

Although insurance companies were the main consumers, city planners, government and municipal agencies, banks, and public utilities also purchased Sanborn maps and atlases. Occasionally, the Sanborn Map Co. even made specialized maps for private companies. In 1952, Business Week reported that the Sanborn Map Co. was working on a project to show Safeway food stores where to expand. In the early 1940s, the U.S. Census Bureau purchased 1,840 volumes of Sanborn atlases and maps to serve as the cartographic base for their statistical and sampling surveys. This collection was transferred to the Library of Congress in 1967, where it remains as the largest publicly accessible collection of Sanborns in the country.

Creative research can uncover a wealth of information in records that were intended to serve a different purpose, and historians often find valuable information from unexpected sources like the Sanborn maps. However, it is equally important to understand the context of that resource to ensure an accurate interpretation of the information once it is found. Sanborn maps, paid for by insurers, prioritized the more heavily developed areas and neglected predominantly Black communities.

Want to know what Sanborn maps might reveal about a site or property? Reach out to HAI with your burning research questions!