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Interpreting Sanborn Indexes

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.

Street Indexes

A street index from a small town
A street index from a small town

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. 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.

Special Indexes

Example of a Special Index
Example of a Special Index

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.


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  March 23, 2022
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