A Visual Expression of History. Produced for over a century, more than 660,000 Sanborn maps chart the growth and development of more than 12,000 American towns and cities. Sanborn maps are large-scale plans of a city or town, drawn at a scale of 50 feet to an inch. They were created to assist fire insurance companies as they assessed the risk associated with insuring a particular property. The maps list street blocks and building numbers including numbers in use at the time the map was made and previous numbers.