Why are 19th century fire insurance maps of such historical value?

Sanborn maps. Author Tobia Black explains how the Sanborn maps were not only useful in fires back then, but continue to attract attention today as maps of great historical value.

From fire hazards to family trees: The Sanborn fire insurance maps — The Public Domain Review
https://publicdomainreview.org/essay/sanborn-fire-insurance-maps/

The modern oil industry began with the discovery of the Drake oil field In 1859, railroad worker Edwin Drake was the first person to extract oil mechanically in America. Lighting with petroleum-derived kerosene was cheaper than whale oil and brighter than candles, but it was highly flammable and caused many large fires in densely built-up American cities. The Great Chicago Fire of 1871 destroyed about 17,500 buildings and the Great Fire of Boston The following year, 776 buildings were destroyed, including most of the financial district, causing significant economic losses.

This is where insurance companies and the Sanborn map came in. The Sanborn map used a complex system of color-coding, symbols and abbreviations to display more information than a regular map, from building materials to street widths, from the location of water towers to the presence of flammable chemicals, from building heights to the number of skylights. So even if a major fire burned a city to the ground, insurance companies could still find the information they needed to set premiums.

Below you can see an example of a Sanborn map of the Library of Congress (LC) which shows town halls, theaters, factories and even amusement arcades. It also contains detailed information on whether the buildings had night watchmen.

The Sanborn Map Company eventually produced the Sanborn Map, which included more than 12,000 cities and towns in North America, as well as nearly every community with a population of more than 1,000. The company sent employees to the areas to be surveyed and spent several months there sketching and measuring all the streets and buildings to create the maps. There is even an anecdote that during World War I, a surveyor who created the maps was reported as a German spy.

Another feature of the Sanborn maps is that they were regularly updated by “pasters.” If there were changes to buildings or landscapes in an area already mapped, those changes were pasted at the top to keep the map up to date. Below is a Sanborn map that was updated by pasting from 1922 to 1951. A record of each update is available, so you can see the streetscape of each era.

The Sanborn Map Company experienced a significant drop in profits as the large, heavy Sanborn maps were made obsolete by more sophisticated fire risk assessment methods and new data storage formats. In 1961, the company stopped producing new maps and stopped pasting and updating maps in 1977. That same year, S. Greeley Wells, then president of the company, donated 45 of the old Sanborn atlases to the Library of Congress.

Although the Sanborn Maps went out of business, they were gradually re-evaluated after they were donated to the Library of Congress. City officials refer to the Sanborn Maps when deciding whether to preserve buildings, and historians and historical novelists refer to the Sanborn Maps when they want to understand what a particular street looked like at a particular time. Demographers also use the Sanborn Maps to study the growth and decline of cities by “pasting” updates. The image below shows a detailed view of