From Penn St. U.:
Recently Penn State’s University Libraries completed the cataloging and digitization of the entire collection of Sanborn maps that span the late 19th and early 20th centuries — 31,036 sheets, representing 585 large and small communities across the Commonwealth. Titles published before 1923 and available online to the public are presented in an alphabetical index of Pennsylvania communities a at www.libraries.psu.edu/psul/digital/sanborn.html. As copyright restrictions are lifted for post-1922 titles, the scanned images of those communities also will be mounted on the Maps Library’s Sanborn Fire Insurance Maps website for use.
Surveyor D.A. Sanborn founded the company in New York City in 1867 to create special maps, most often at a scale of 1:600, or 50 feet to the inch, to aid companies in assessing liability for fire insurance coverage of buildings in communities across the United States. Teams of surveyors infused the maps with a remarkable level of detail, and for most cities and towns new editions were published every few years, providing a fascinating and unique snapshot into community and industrial growth. The map sheets that cover each community accurately show the location of buildings and outbuildings, including details such as where windows and doors are positioned, as well as property boundaries, the names of land and company owners, the width of streets and sidewalks, building use and, most importantly, the construction materials used for homes, businesses, and other buildings. Key local industries were sometimes highlighted with inset maps, and textual notes added useful information such as the type of water supply and firefighting equipment and companies available. Other useful tidbits of information found on these maps include population figures, street and building indexes, and even prevailing winds!
Originally called the D.A. Sanborn National Insurance Diagram Bureau, the company went through several name changes. It was incorporated in 1876 as the Sanborn Map and Publishing Company, and after acquiring rival Perris and Browne in 1899, it was known as the Sanborn-Perris Map Company, Ltd., thus allowing it to trace its lineage to the year 1852. The name was later shortened in 1902 to its best-known moniker, the Sanborn Map Company.