What are Sanborn Fire Insurance Maps?

Fire insurance maps were created to help insurance companies assess the potential risks involved in underwriting policies. They depict features that affect the vulnerability of properties to disasters like fire, earthquake, and flood. The first fire insurance maps were created in London in the late 18th century. In America, D.A. Sanborn, a civil engineer and surveyor, began working on fire insurance maps in 1866. That year, the Aetna Insurance Company contracted him to prepare maps of areas in Tennessee and Boston.

1The Sanborn Company mapped 12,000 cities between 1867 and 1970. The insurance industry determined what kind of information was conveyed on the map. The maps depicted commercial, industrial, and residential sections of cities and towns. Surveys were not determined by population, but by the needs of the insurance agency. The growth of a city or town required new surveys every few years. Armies of surveyors were dispersed to collect detailed information that were developed into hand-drawn and colored maps. Sanborn was not the only mapping company but came to dominate the industry. “Sanborn” has come to become synonymous with “fire insurance map.” Map production peaked in the 1930s, driven by growth in urbanization, industrialization, and immigration. Although created for the insurance industry, city planners, government and municipal agencies, banks, and public utilities also utilized the maps.

In the 1940s, the U.S. Census Bureau purchased 1,840 volumes to serve as base maps for statistical and sampling surveys. They later became the basis for the collection housed in the Library of Congress, the largest publicly accessible Sanborn collection in the country. Butte’s maps 1884-1891 are available online at https://www.loc.gov/collections/sanborn-maps/?q=butte,+montana.



Sanborn’s convey a complex amount of information very succinctly; one way they do this is through the use of color. A color key can convey information easily, freeing up space for more details, and creating uniformity throughout the maps (within a city, as well as nationally). The maps also employ detailed codes and graphics that impart information in the form of lines, tic marks, and abbreviations.


3The title page contains the name of the city and county, publication year, and sections of city. A block of text describes the fire services – staff, department, equipment: size, extent and pressure of water mains. The map scale is most often 1” = 50’ (1:600) which means objects are 600x smaller than in real life. The title page will show the legend with a code of colors, lines, and symbols; and a street index – an alphabetical listing with address ranges; and a list of “Specials.”

4Map pages show the blocks, streets, buildings, structures, and geographical features.

Buildings have their own codes:  yellow is wood frame construction, pink is brick construction, blue is concrete (or stone) construction, orange is fireproof construction.

Lines – solid is a wall, broken for doorways or passages; dashed are partitions or roof style (like mansard). Numbers usually indicate the number of stories (on a building), or address (on a street). Tic marks – indicate window locations in each story. Building use had a bearing on the likelihood of fire and was indicated by the following abbreviations:  D= dwelling, Sal = Saloon, Rest = Restaurant, S = Store.

5Specials were often located at the back of the bound book. A single page may contain multiple “specials,” separated by double-lined borders, and usually showed an industrial site (like a mine yard), college, factory, or outlying building

Paste-ups are corrections made to the maps between survey dates. Sanborn sent employees to cities to attach corrections to the maps




Sanborn maps can be compared from year to year to track suburban growth and the evolution of neighborhoods. Changes of time are evident on the maps, whether looking at individual structures, city blocks, or industrial sites. Changes include additions, new construction materials, and change in the use of a building. It’s easy to see the footprints of sites that may no longer exist – commercial, leisure, industrial, and residential area that have changed through time.

Sanborn Maps for Butte are accessible to the public in the Map Room of the Butte Archives. We have bound volumes covering the following years: 1900, 1907, 1914, 1916, 1927, 1942, and 1956. Come see for yourself how your neighborhood has changed over the years. 

Anaconda Road in 1916 (L) and 1956 (R)


For more information, please visit these sites:




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