What are Sanborn Maps?

The earliest Sanborn Fire Insurance Maps date back to the nineteenth century and served as a preventative measure for urban fires. Regularly updated, these maps detail business location, type, material, and other pertinent information that could contribute to a quick response to fire. Social scientists have utilized these detailed maps to track urban growth as well as how hazardous landscapes have been changed.

Some online sources of Sanborn Fire Insurance Maps --

Sanborn Maps are very detailed maps containing many "sheets", with one sheet representing only a few blocks of a city. Over time, as cities grew, the number of sheets in each Sanborn Map volume increased and the sheets were sometimes renumbered.  In addition to renumbering, maps were sometimes redrawn, meaning an area covered by one sheet may be divided among two or three sheets in a later volume. Cities also occasionally made wholesale changes in the layout of streets and street names (not to mention renumbering the addresses of individual buildings), thus making it difficult to find points of reference common to different editions. Where profound changes occurred, it can be difficult to relate the new structures to the ones that they replaced. This makes comparing the same area of a city over multiple Sanborn Map volumes challenging. The Graphic Index (or Key Map) will tell you which sheet number covers what area of town. 

Graphic Index (or Key Map)

In order to view the same area of a city in different maps over time, it's important to use the graphic index (also sometimes called the key map). The graphic index shows which areas of the city are covered by each sheet in the Sanborn Maps volume.  When a researcher wishes to examine the coverage for a portion of a city, the graphic index is more useful than the street or other indexes. Key maps serve two purposes: they show the areas encompassed by individual sheets and indicate the portions of a city or town that were mapped. Coloring was generally used on key maps to indicate the area covered by an individual sheet in the edition or atlas. Often the graphic index appears on sheet one of a Sanborn Maps volume.

Many Sanborn Maps volumes contain two indexes. The graphic index shows which sheets represent which areas of town. The street or special indexes (often combined and labeled simply as "index") gives street names and/or the names of businesses, public buildings, or other special properties along with their corresponding map sheet numbers.

For more information on interpreting indexes: https://www.loc.gov/rr/geogmap/sanborn/san2a10.html

See also the Sanborn Keyes and Colors page of the Library of Congress webiste, which explains the meaning of the various colors and other notations used in Sanborn Maps to represent types of building materials and other important details.

And for a very helpful tutorial on how to how to use Sanborn maps, visit Sanborn Fire Insurance Maps (G&M Reading Room, Library of Congress) (loc.gov) and especially take a look at the "interpretation" section.