Old Maps for New Apps: Making and Using Georeferenced Sanborn Maps at Scale
Georeferencing historical Sanborn fire insurance maps transforms them from scanned atlas pages into proper geospatial datasets, allowing for geographical exploration, comparison, and analysis. This is a common task for researchers of all stripes, and since 2022 OldInsuranceMaps.net has provided open, browser-based tools for not only processing individual sheets from the Library of Congress digital collection, but going further to create seamless, city-wide mosaic layers. Numerous institutional research teams and enterprising hobbyists have used this “georeferencing commons” for their own work, yielding hundreds of publicly available mosaics each comprising dozens (sometimes hundreds) individual layers; it is arguably the largest crowdsourced Sanborn map georeferencing effort to date. This presentation will describe how the open source software behind OIM is modeled to facilitate a complex, collaborative workflow, and how historical map layers from the project have been integrated directly into other digital history projects and web applications using open standards like WMS, XYZ tiles, and PMTiles.