Sanborn Fire Insurance Maps are among the most remarkable historical maps ever produced in the United States. At first glance, they may look like ordinary street maps. Look more closely, however, and they become extraordinarily detailed records of the built environment: individual buildings, their construction materials, number of stories, occupants, doors, windows, fire walls, wells, hydrants, water mains, railroads, and even hazardous industrial activities can all appear on a single sheet.
Originally created for the insurance industry, Sanborn maps have become indispensable sources for historians, genealogists, preservationists, archaeologists, urban planners, and map enthusiasts. They can reveal how a town looked decades before aerial photography and can often show buildings that have completely disappeared.
What Are Sanborn Insurance Maps?
Sanborn maps are large-scale fire insurance maps produced primarily to help insurance companies evaluate the risks associated with buildings and properties. The maps were designed to answer a practical question: if a building caught fire, how likely was the fire to spread, and how difficult would it be to extinguish?
That required much more information than a conventional street map provided.
A typical Sanborn sheet might show the footprint of every building on a city block, together with its address, construction material, height, use, roof type, and relationship to neighboring structures. The maps also documented features important to firefighting, including hydrants, water mains, fire alarm boxes, and fire walls.
The result was a form of building-level cartography. Instead of simply representing streets and blocks, Sanborn maps documented the physical structure of the city itself.
The Library of Congress describes its Sanborn holdings as a series of large-scale maps covering approximately 12,000 cities and towns in the United States, Canada, and Mexico. Its collection contains roughly 50,000 editions and an estimated 700,000 individual sheets.
That makes Sanborn maps much more than insurance documents. Collectively, they form one of the great cartographic records of North American urban development.
A Brief History of the Sanborn Map Company
The story begins with Daniel Alfred Sanborn, a surveyor from Massachusetts. In 1866, Sanborn was engaged by the Aetna Insurance Company to prepare insurance maps for several Tennessee cities. The following year, he established the D. A. Sanborn National Insurance Diagram Bureau in New York City.
Sanborn had already produced an Insurance Map of Boston in 1867. That atlas, consisting of 29 large plates at a scale of 50 feet to the inch, is regarded by the Library of Congress as the earliest published Sanborn fire insurance map.
The company expanded rapidly. Its name changed several times as the business grew and acquired competitors, eventually becoming the Sanborn Map Company in 1902.
The maps were not simply produced once and forgotten. Sanborn maintained a system of continual updating. New editions documented changes in buildings, streets, businesses, and infrastructure. This makes them particularly valuable as a time series.
At the company’s peak during the early twentieth century, Sanborn employed hundreds of people, including field surveyors who traveled throughout the country documenting communities. Production peaked in the early 1930s, when the company reportedly employed as many as 300 field surveyors and more than 400 additional staff.
Although Sanborn mapping continued for decades, changing insurance practices eventually reduced the industry’s dependence on these maps. The last traditional fire insurance maps were published in the late twentieth century; the Library of Congress collection extends to 1999, while the company ceased producing fire insurance maps as insurers moved away from this method of underwriting.
What Information Do Sanborn Maps Contain?
This is where Sanborn maps become particularly fascinating for cartographers and historians.
The maps use a sophisticated visual language to compress enormous amounts of information into a relatively small space.
Color is particularly important. In many editions, reddish or pink coloring indicates brick or tile construction, while yellow generally represents wood-frame construction. Other colors and patterns can indicate concrete, adobe, metal, or fire-resistant construction.
A building may also contain numbers indicating its height or number of stories. Letters and abbreviations identify uses such as stores, dwellings, factories, warehouses, stables, and other structures.
You may find:
- Building footprints
- Street names and addresses
- Property boundaries
- Number of stories
- Construction materials
- Roof materials
- Windows and doors
- Fire walls
- Elevators
- Stairways
- Porches and additions
- Wells and cisterns
- Water mains
- Fire hydrants
- Fire alarm boxes
- Railroad tracks
- Industrial machinery
- Tanks and boilers
- Hazardous materials
- Commercial and industrial uses
The exact symbols and colors changed over time, so researchers should always consult the map key or legend for the particular edition being studied. The Library of Congress specifically cautions researchers not to assume that every symbol was used identically throughout the entire history of Sanborn mapping.
Why Are Sanborn Maps So Valuable?
The extraordinary value of Sanborn maps lies in their combination of scale, detail, and chronological coverage.
Consider an ordinary historical city map. It might tell you that Main Street existed in 1900 and show the location of several major buildings. A Sanborn map can tell you that a particular address contained a two-story wood-frame dwelling with a rear addition, a detached stable, a particular roof type, and perhaps a nearby hydrant.
That distinction is crucial.
Architectural history, Sanborn maps can document buildings that no longer exist.
Genealogy, they can help researchers understand the physical surroundings of an ancestor’s home or workplace.
Urban history, successive editions can show neighborhoods expanding, streets being widened, factories appearing, and residential areas being replaced by commercial development.
Archaeology, a Sanborn map can provide clues about where demolished structures, privies, workshops, wells, tanks, and industrial facilities once stood.
Historic preservation, the maps can establish the former footprint and use of a building or property.
They are also excellent sources for studying the historical geography of businesses and industries.
The Library of Congress notes that its Sanborn collection can be used for genealogy, environmental studies, urban planning, and many other forms of research.
Maps as a Time Series
One of the best ways to use Sanborn maps is not to examine a single edition but to compare several.
Suppose you are researching a downtown block. A map from 1885 might show several small wood-frame buildings. By 1900, those structures may have been replaced by a large brick commercial building. A 1915 edition might show an addition, a changed occupant, or alterations to the building.
The maps therefore allow researchers to reconstruct urban change through time.
The Library of Congress has specifically developed resources demonstrating how Sanborn maps can be used as historical time series. Changes to individual buildings can sometimes be dated to a relatively narrow period by comparing successive editions.
A Brief Tutorial: How to Use the Library of Congress Map Website
For cartographers, this is especially interesting because Sanborn maps are not simply snapshots. Taken together, they constitute a historical spatial database created long before the modern GIS era.
Where Can You Find Sanborn Maps?
The best place to begin is the Library of Congress.
Library of Congress Sanborn Maps Collection
The Library’s Geography and Map Division has one of the largest surviving collections of Sanborn maps, and many public-domain maps have been digitized and placed online. The collection can be searched by location and date, and individual map sheets can be viewed and downloaded.
The Library also provides a particularly useful Sanborn Fire Insurance Map Advanced Search, which is worth using when a simple keyword search does not produce the expected result.
Library of Congress Sanborn Map Research Guide
This guide includes instructions for searching the collection, interpreting symbols, understanding indexes, and locating individual sheets. It also provides a Sanborn Atlas Volume Finder for selected large cities.
Digital Sanborn Maps
Another major resource is Digital Sanborn Maps, 1867–1970, available through ProQuest.
Unlike the freely accessible Library of Congress material, this is generally a subscription resource. However, many public, academic, and research libraries provide access to it for their patrons. ProQuest describes the collection as containing enhanced digital images derived from the extensive Sanborn microfilm collection.
If your local library has access, Digital Sanborn Maps can be an excellent alternative to the Library of Congress interface, particularly for researchers who need extensive coverage.
Local Libraries and Archives
Do not overlook local institutions.
University libraries, state libraries, historical societies, municipal archives, and public libraries frequently hold Sanborn atlases or microfilm. The Library of Congress maintains information about institutions holding Sanborn collections, including a Sanborn Union List that can help researchers locate copies when an online edition is unavailable.
This becomes especially important for later editions that remain under copyright and therefore may not be freely viewable online.
A Few Tips for Using Sanborn Maps
If you are new to Sanborn maps, start with the index rather than immediately opening individual sheets.
Large atlases commonly include a graphic or “key” map showing which sheet covers which portion of the city. Street indexes can then help identify the sheet containing a particular address.
Next, find the legend. Do not assume that a symbol has the same meaning across every edition.
Finally, compare multiple dates whenever possible. One map is useful; three, four, or five editions can become a remarkably detailed reconstruction of urban change.
And remember what these maps were created to do. That is mapping the built environment for insurance purposes, not producing a neutral portrait of society. What was recorded, and how it was recorded, was shaped by the needs of fire insurers.
A Cartographic Window Into the Past
Sanborn Fire Insurance Maps occupy a unique position in the history of American cartography. They are simultaneously maps, architectural records, business directories, infrastructure surveys, and historical documents.
Their greatest achievement may be their scale. Traditional historical maps often tell us where a town was located. Sanborn maps tell us what the town was made of.
A street becomes a row of buildings. A block becomes a collection of individual structures. A building becomes a footprint, a construction type, a number of stories, a business, and a series of connections to streets, water systems, railroads, and neighboring properties.
For anyone interested in historical geography, urban history, genealogy, architecture, or maps themselves, Sanborn maps are therefore among the most rewarding primary sources available. And thanks to digitization by institutions such as the Library of Congress, much of this extraordinary cartographic record can now be explored from virtually anywhere.
In other words, if you want to know what an American town actually looked like a century ago, don’t just look for an old map. Look for its Sanborn map.