Best practices for georeferencing large scale historical fire insurance maps of the USA

There is increasing interest in transforming historical print maps from passive archival documents into value-added digital geospatial data through the process of digitization and georeferencing. Central to this activity is the quantitative evaluation and documentation of the quality of the original map and resulting data product(s) produced. Best practices have been proposed for small scale (i.e. large area) historical print maps, while a similar set of recommendations for large scale (i.e. small area) maps are lacking. The current study presents methods for and the results of a project to digitize and georeference historic fire insurance maps produced by the SanbornTM map company during the late 19th and early 20th centuries in the state of Pennsylvania, USA. A random sample of 202 out of approximately 29,000 individual map sheets was evaluated for their horizontal positional accuracy against contemporary reference data. Results were summarized into a set of proposed best practices for georeferencing large scale historic fire insurance maps of the USA.