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, residents, 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 --
Comparing maps of an area over time can be challenging for several reasons. Instead of merely updating the coverage of existing sheets the Sanborn Map Company often completely redrew and renumbered sheets from one edition to the next. This means that the area covered by one sheet may be divided among two or three sheets in a later edition. 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. The Graphic Index (or Key Map) will tell you which sheet number covers what area of town.
Graphic Index (or Key Map)
For researchers, an important part of the prefatory material is the graphic index that portrays the areas of the city covered by each sheet in the edition. In the case of multi-volume editions, there is often a "Graphic Map of Volumes" that shows the portions of the city covered by each volume.
The graphic index is frequently referred to as a "key map." 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. There are usually compas roses, often highly decorative, to orient the user, but not always scales.
For more information on interpreting indexes: https://www.loc.gov/rr/geogmap/sanborn/san2a10.html
Fire insurance maps are distinctive because of the sophisticated set of symbols that allows complex information to be conveyed clearly. In working with insurance maps, it is important to remember that they were made for a very specific use, and that although they are now valuable for a variety of purposes, the insurance industry dictated the selection of information to be mapped and the way that information was portrayed. Knowledge of the keys and colors is essential to proper interpretation of the information found in fire insurance maps.
Color plays an important role in Sanborn map reading. In Example 1, we see that brick and tile are represented with a reddish/pink color. Several advantages demonstrate themselves when using color: a) the mapmaker can easily and quickly convey information; b) space formerly used to convey this information can now be used to convey more detailed information; and c) uniformity across all the maps is achieved and maintained.
The use of yellow indicates frame, or wood, structures. Example 2 shows the use of framing on the inside as well as outside of buildings. Along with the color indicators, the map uses basic abbreviations to convey other information. S = store, D = dwelling, and ASB. CL. = asbestos clapboards.
Other colors employed by Sanborn mapmakers included an olive green to demark fire resistive construction and gray for adobe construction material. Blue denotes concrete and cinder block construction. Gray is also used to indicate metal or iron building materials. The tenant indicator "loft" is shown in color to indicate that it can be seen in any of the construction color indicators.