SANBORN FIRE INSURANCE MAPS AS ACTUAL DATA SOURCES FOR GEOSCIENTISTS

Originally published for fire insurance underwriters, D.A. Sanborn Company fire insurance map books have been used for decades by genealogists, historians, and environmentalists. Geoscientists, however, have largely neglected this potentially important source of data. The Sanborn map books pictorially document building inventory in over 12,000 towns and cities in the United States, Canada, and Mexico from 1867 to 1970. Covering an entire town or city, a Sanborn book gives city street layout and index, followed by 1:50- or 1:100-scale map pages showing all buildings. The books were reissued periodically; in the interim, they were updated by pasting in cutouts from the publisher's revision sheets. The Sanborns show street location and address of public, industrial, commercial, and residential buildings, and they indicate construction type (wood frame or masonry); some construction details (e.g., firewalls, steel reinforcement, porches, and number of stories); use of building; name and/or type of business; size of primary and auxiliary buildings and precise footprints, with measured setbacks, on lots; fire cisterns and hydrants; water towers; and water and sewer lines. Later editions contain more detail.

Sanborn maps are available to the public. Some have been microfilmed. Digital images can be purchased on the Web. Academic libraries are increasingly offering their Sanborn collections on the Web or providing subscription access to the public. Through the San José, California, Public Library system, for instance, users have access to a digitized collection of Sanborn maps for California as well as to microfilmed maps of San José.

Applications of these maps include estimation of losses from historical earthquakes, detection of hazardous materials, and location of structures no longer in existence. A hazardous waste site may be traced back through successively older Sanborn volumes to find an urban industrial polluter from decades or a century ago. Based on geographic and construction details from Sanborn volumes and on historical damage data, current investigation suggests that it is possible to distinguish spatial variation in shaking intensity in downtown San José, California, during the April 18, 1906 earthquake.