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Fire Insurance Maps at the Library of Congress: A Resource Guide

Interpreting Sanborn Maps

Sanborn Fire Insurance Maps all share a number of common elements that are useful to understand when working with the collection. Becoming familiar with these elements will make working with the Sanborn maps easier for the researcher.

Use the menu below to jump quickly to each section of this page:

Abbreviations | Block Numbering | Colors and Symbols | Congested or Business Districts | Fire Protection and Water System Reports | Indexes | Line Styles | Publication Dates | Scales | Sheet numbering | Title Pages


Fire insurance maps often relied on abbreviations to convey the type of activity that took place in a structure, since that information had some bearing on the likelihood of fires. The most common abbreviations are as follows: "D" or "Dwg" for a dwelling, "F" for a flat or apartment, "S" for a store, "Sal" for a saloon. Other abbreviations added more information about a structure: numerals were used to indicate the number of stories in a building, and the letter "B" indicated the presence of a basement. These could be combined, so that "2B" for instance, indicates that a building has two stories and a basement. Multiple symbols for a single structure could reflect the use or nature of different parts of a building; hence a two-story building with a basement might be marked "2B" for the main portion of the structure while an addition on the front had the number "1" by itself to indicate a single story. Look at the map index for abbreviations found within a particular volume.

Block Numbering

Example of a Block Number Table. Library of Congress Geography and Map Division.

Many editions include a listing of block numbers somewhere in the introductory material. Block numbers are usually used in real estate descriptions, and although that was not a function that fire insurance maps were designed to support, the presence of these listings indicates their value to insurance agents also. Sometimes there is also a table showing changes in the numbering system used in a city.

Colors and Symbols

Example of Sanborn map key. Library of Congress Geography and Map Division.

Fire insurance maps are distinctive due to 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 the key to the right, we see that brick and tile are represented with a reddish/pink color while the use of yellow indicates frame, or wood, structures. Other colors employed by Sanborn mapmakers included an olive green to show 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.

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 these symbols on fire insurance maps, especially those done by the Sanborn Map Company, was prescribed by company manuals and can usually be interpreted with a considerable degree of confidence. However, the symbols, abbreviations, and annotations are not always easily interpreted. Therefore, it is important for researchers to consult the keys to symbols. It is also important to remember that over time new symbols were added and that there were variations in the way in which symbols were applied.

Congested or Business Districts

Sanborn Map Company. Map of Congested District, Waikiki, Honolulu Hawaii (excerpt). View full size map. 1914. Library of Congress Geography and Map Division.

A valuable feature in the introductory material of editions for large cities is the map of the "congested" or "business" district. This kind of map was drawn at scales intermediate between the large scale of a single sheet and the small scale of the key map for a volume. A typical map of this type might cover an area twenty blocks by thirty blocks in size. Generally, such maps served as a frame of reference for understanding the relationship of buildings and places in the heavily developed portions of towns. In some cases, they were annotated to show the limits of coverage by fire departments (in the early years of fire protection, fire company service was not provided to all parts of a town or city) or some other salient feature of the city.

Fire Protection and Water System Reports

Example of a portion of a water system report. Library of Congress Geography and Map Division.

To the insurance industry, a valuable component of the introductory material was a description of the fire-fighting equipment and water system of a city or town. The description could be a simple statement of a line or two in the coverage of a small town, or an elaborate report of many paragraphs for the larger cities. Reports would address such issues as the number and types of fire-fighting equipment and the number and training background of firefighters. The size, extent, and pressure of water mains was a critical factor that was often addressed in such reports. The predominant wind patterns were usually noted as well.

Also in many editions of fire insurance atlases of large towns or cities, there was a separate map, frequently an inset but sometimes rather substantial in size, showing the water distribution system. Such a map might be accompanied by a description of pumping station equipment, pressure, sizes of mains and water lines, the types and distribution of fire hydrants, and so forth.


Graphic Index (or Key Map)

For researchers, an important part of the introductory 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 compass roses, often highly decorative, to orient the user, but not always scales.

Street Indexes

One important component of a fire insurance map or atlas is the street index. In theory, street indexes are rather simple. Street names are listed alphabetically, including numbered streets and avenues, which are spelled out in full.

Example of a street index. Library of Congress Geography and Map Division.

Subheadings under the name of a street frequently indicate address ranges covered by individual sheets in the edition or volume. For some large editions and atlases, however, this index may also contain a separate listing for a special component of the city, such as wharves and piers. In using these indexes, it must be remembered that street names and building numbering systems could change several times in a given city during the century and a half in which these maps were prepared. Trying to find the historical equivalent of a current address can be rather difficult if researchers try to rely strictly on the street indexes. In many cases, it will be necessary to relate an address on a current map to the key map described in the previous section. On the other hand, when working only with a written record of an address, street indexes are essential for locating buildings for which the street or number has changed.

In the physical sheets, this index is typically at the front of the volume. In the Library's digital platform, this index can sometimes be found at the end of the set of scanned images.

Special Indexes

Example of a special index. Library of Congress Geography and Map Division.

An interesting look at the life of a city can be found in an index of specific properties that sometimes appears separately but may also be a continuation of the street index. The name of this index usually includes the word "Specials." It identified major businesses, public buildings, factories, or other large structures. Such indexes were prepared to facilitate quick location of these major features. For current researchers, however, the list of "specials" is the equivalent of an abridged city directory that provides insight into the economic and social landscape of a community. From one edition of a city to another, entries in these indexes will come and go, and a particular business or institution will change its name to reflect new owners, a new function, or a modernized sense of propriety.

In the physical sheets, this index is typically at the front of the volume. In the Library's digital platform, this index can sometimes be found at the end of the set of scanned images.

Line Styles

Much information could be conveyed through line types. A solid line indicated a solid wall. A break in a line showed doorways and other passages. Additionally, dashed lines could indicate some aspect of wall construction or the presence of a mansard roof. Extending solid lines beyond the edge of a building was a technique for indicating how high above the roof fire walls were built. When interpreting any fire insurance map, researchers should take care to consult the legend for that particular edition to ensure the correct interpretation of line symbology.

Publication Dates

For the first half century of fire insurance mapping, the dating of editions is generally straightforward. For small-town maps issued in loose-leaf format, the first page and generally each subsequent page carried the month and year of publication quite prominently in an upper corner of the map. The year of publication was usually repeated as part of the copyright notice that appeared as part of or adjacent to the title.

The dating of multi-volume editions of large-city maps is somewhat more complicated. The date on the first volume may be valid only for that particular volume, as the subsequent volumes may have been issued over a period of years. In several cases, a single volume within a multi-volume set was revised and reissued with a new date even before the whole edition was completed. In the Sanborn Fire Insurance Maps Checklist [see Searching for Sanborn Maps], date ranges are given for the coverage of multi-volume editions.

Corrected Editions

Beginning around 1920, the dating of an edition becomes rather complicated. In response to various economic factors around that time, the Sanborn Map Company began updating maps for its customers by issuing paste-on correction slips. These were generally applied by a company employee, who then annotated a chart, usually entitled "Correction Record," attached to the title page, recording the dates on which corrections had been applied. Such correction slips were intended to keep the map coverages current.

Sanborn Fire Insurance Company. Chicago, IL, Vol. 4, Sheet 81. 1950. Library of Congress Geography & Map Division.

Over the years, however, more and more correction slips were added, and it is impossible to determine which correction slips were applied in which years. For many cities, as many as fifty or sixty series of correction slips were applied over the course of thirty or forty years. In Fire Insurance Maps in the Library of Congress, the dates for such editions are the date of the original printed edition of the map and the latest date for which corrections were applied.

The latest date, however, is not always found in the "Correction Record." In many cases, additions to indexes pasted into an atlas are later than the last correction slip annotation indicates. In theSanborn Fire Insurance Maps Checklist, the latest date of an edition or volume reflects the latest date found on any material pasted into that volume or edition.

The vast majority of examples of paste-on correction sheets in the collections of the Library of Congress come from the maps produced by the Sanborn Map Company after the late 1920s. For example, this sheet is from Vol. 4 of Chicago. When looking at the catalog record or in the index, the date appears asVol. 4, 1912 - May 1950 which can cause some confusion in interpreting the correct date.

The first date, 1912, indicates the year of the original map. In some cases, this earlier date is still printed in the top corner with the page number. However, the second date, May 1950, is the date of the last revision and is usually found on the first page of the volume in the correction record. From this, it can be determined that the volume represents the city as it was in 1950, not 1912.


The vast majority of fire insurance maps were drawn at a scale of one inch to fifty feet (1:600) when expressed as a representative fraction. A smaller or less detailed scale of mapping was employed for suburban areas or large industrial sites. For these areas, scales of one inch to one hundred feet (1:1,200) or one inch to two hundred feet (1:4,800) were used. In rare instances, even smaller scales were used. Most map sheets contain a bar scale that facilitates the measurement of features or distances, and the scale is usually given in the title of the map as well.

Sheet Numbering

Considering that there were more than fifty thousand editions of fire insurance maps of more than ten thousand communities, it is not surprising to find inconsistencies in the way sheets in a given edition were numbered. In early atlases, numbering was often sequential across coverage for the whole city. In other cases, especially in later editions, numbering was sequential within each volume. In some early editions also, double-page plates were given a single number, but the most common pattern was to have a separate number for each page. Introductory material was not given page numbers, however.

In the checklist of maps,Fire Insurance Maps in the Library of Congress, the figure in the column that indicates the number of pages does not reflect the page numbering. The figure that is given includes any unpaginated prefatory material, even the inside of atlas covers to which indexes have been pasted. Where applicable, the comments column in Fire Insurance Maps in the Library of Congress indicates special numbering sequences, such as double-page plates.

Title Pages

Sanborn Map Compnay. Title Page, Richmond, Virginia Volume 2. 1925. Library of Congress Geography and Map Division.

Most editions of fire insurance maps contained introductory material that is useful or necessary for interpreting the maps and which now has historic value in its own right. In particular, the map indexes and the descriptions of a city's fire protection services of a city provide insights into various aspects of the development of urban America. The amount and nature of such introductory material varies widely from city to city and over time, reflecting changes in mapping policies of the Sanborn Map Company, the size of the city, and the rate of change within the city.

Small-town maps that comprised only a few map sheets were generally issued in loose-leaf format and did not have separate title pages. The first page of such editions, however, would include the names of the city, county, and state. The county name was important for differentiating towns of the same name within a state. It is important to note that because of divisions of counties or changes in county boundaries, a city may now be part of a county different from that in which it was originally mapped.

For large-city maps issued in one or more volumes, the title page often presents a visual delight of ornate typography and design. As cities became denser and larger over time, the Sanborn Map Company occasionally altered the presentation of the coverage of a city. A single volume could be divided: for instance, volume 1 from an edition in the late 1800s might become volume 1, North and volume 1, South in the 1920s. Also, the coverage of the suburban portions of a city could be shifted from volume to volume. Researchers need to be aware of such changes reflected in the titles to insure that they are using the volume that covers the portion of the city in which they are interested.

An important component of title pages for large city atlases is the listing of incorporated and unincorporated places covered in the volume. This is particularly common on the fringe areas of towns from the 1920s onward, where suburban developments and new additions to the territory of a city were frequently given their own names in local usage. Such place names have been listed in the Checklist of Fire Insurance Maps in the Library of Congress.