Sanborn Fire Insurance Maps

How to Read Sanborn Fire Insurance Maps

Sanborn maps use a shorthand of colors and symbols to represent a wealth of information. Although many Sanborn maps were hand-drawn and colored, they all use a standard scheme related in keys found in every map folio. Though new symbols were added as technology changed, most symbols remained constant since the founding of the Sanborn Company in the late 1800s.

Most Sanborn Maps are scaled one inch for 50 feet (1:600) on sheets 21 inches by 25 inches, although many maps are also scaled to 100 feet per inch (1:1200). Surveyors worked first from court documents and real estate notes, but were encouraged to survey areas themselves if documentation did not present itself easily. Only built-up parts of towns were surveyed. Often, Sanborn surveyors ignored sections of town not of interest to fire insurers, namely poor or predominantly African-American residential areas. Therefore, Sanborn maps often do not record an entire city or town.

There are several layers of labeling on Sanborn Maps that apply to a number of urban features important for insurers.

First, city areas are divided into numerous sheets, which appear on specific pages as designated by an index map. The image at left shows a map sheet number.

City blocks, numbered as shown on the right, were often assigned numbers that appear on individual sheets, and which can facilitate comparison across a series of maps. If block numbering changed from one map edition to another, numbers in parentheses indicate old block numbers that have changed from previous map editions.

A city block number, showing an older block number from previous map editions. Such numbering allowed surveyors to compare areas of town from survey year to survey year.

Streets are always labeled by name and have block numbers designated on each side of traffic. Street widths between blocks are also marked.

Under streets, Sanborn maps indicate where and what type of water and gas manes exist, including those for fire fighting, with their hydrants

Notataions indicating utilities.

Buildings on Sanborn Maps have their own elaborate system of symbols as well. A color code designates the type of material from which a building was constructed. Green or gray indicates fireproof or adobe construction, blue a stone or concrete structure, red a brick structure, gray an iron structure, and yellow a wood frame structure. Brick or stone veneers are also noted by colors, and buildings with mixed construction materials are labeled blue and have notation as to their materials on the drawing itself.

Building types:

a fireproof building	a stone building
a brick building with notations for windows, a skylight, wall thicknesses, and an interior fire wall	building of mixed construction (stone and brick)
a wood frame with an iron facade	a wood frame building

Skylights, windows, fire escapes, the thickness of walls, garages, elevators, sprinkler systems, asbestos shingles, and building height are all commonly noted on building drawings as well.

Buildings are also labeled as to their function. Civic or prominent buildings like schools, theatres, churches, businesses and offices are often named on Sanborn Maps. For those not named outright, the letter D indicates a dwelling, F a flat, S a store, and A an auto garage. Often notes like "Apts" for apartments will also appear.

A variety of more specific notations often appear in drawings as well, most pertaining to facts that would pertain to a buildings tolerance to fire, as these maps were intended for use by fire insurance companies first and foremost.