is a Charter Project of OpenStreetMap US that provides an open, online workflow for creating georeferenced layers of historical maps, with a special focus on making seamless mosaics from multi-page atlases in the Sanborn map collection at the Library of Congress.
1,439 maps/atlases... comprising 62,631 pages/sheets... georeferenced by 455 contributors... via 308,325 ground control points... over the course of 74,778 sessions... resulting in 56,787 individual layers.
Find content: by location... by state/county/city... by map name.
Digital scans of Sanborn maps are available through the Library of Congress and are pulled into this site through their JSON API , generating a "Map Summary" page (Baton Rouge, 1885).
Users prepare each sheet in the volume, sometimes splitting it into multiple regions, each to be georeferenced individually (Baton Rouge, 1885, page 1).
Next, each document must be georeferenced by creating ground control points, linking features on the old map with latitude/longitude coordinates to create a geospatial layer (Baton Rouge, 1885, page 1, part 3).
As they are georeferenced, layers slowly build a collage of all the content from a given volume, and their overlapping margins must be trimmed to create a seamless mosaic.
Finally, all volume mosaics for a given locale are automatically aggregated into a simple web viewer so you can easily compare different years and current maps (Baton Rouge viewer).
Here's how some organizations have used this platform to support their research and community mapping efforts.
Since 2023, the Institute for Social Research at the University if Michigan has supported OIM and used the platform to create new data to explore the effects of place-based factors on health in the United States. This work is carried out in conjunction with the Digital Scholarship Lab at the University of Richmond.
Building on their research into redlining maps and spatial inequality, the Digital Scholarship Lab is assembling a dataset of environmental hazards in mid-century urban America. To support this effort, the lab has trained more than 30 student researchers on OIM to georeference over 30,000 Sanborn sheets.
Many users have have georeferenced maps specifically in order to use them as reference data for adding content to OpenHistoricalMap. OIM provides web services that can be used directly in the OHM editor. Checkout hayleox's work in 1880s Cincinnati to see how historical data can be traced directly into OHM.
HistoryForge is a citizen history platform that facilitates transcription of historical census records. From the maps created during New Orleans georeference-a-thons, OIM provides web services that integrate directly into both the Southeast Lousiana HistoryForge and Schenectady, NY web applications, aiding the process of finding historical buildings.
In early 2025 the Community Mapping Lab at the University of Georgia held an event to georeference maps of Athens, GA.
Rowan University's Center for Digital Humanities Research hosted the largest georeferencing event that OIM has seen to date; in one afternoon, 50 people georeferenced over 250 layers, covering a number of towns around Gloucester County, N.J. Read more in Rowan's student newspaper, the Whit.
The Technical Assistance for Brownfields (TAB) group at Kansas State University uses OIM to create historical layers for numerous small towns and cities across the nation where they help communities identify and manage their brownfield sites.
To support their community history platform, Yesterdays, MapRVA creates georeferenced mosaics on OIM and integrates them into their own mapping interface, allowing contributors to spatially locate historical photos of Richmond, VA while using Sanborn maps as reference layers.
Since fall of 2024, OIM has worked with researchers and students at Morgan State University's Center for Urban Health Equity to georeference FHA Block Data maps of over 100 cities across the US. These maps are crucial materials for understanding the history of redlining in the US, and we look forward to making them available here soon!
The OSU libraries have hosted multiple community events using OIM to involve students and citizens in the work of georeferencing maps of Columbus and nearby cities. The team published a paper in 2023 discussing their preparation for the events, and the positive engagement they had from attendees.
For GIS Day in 2024, we held a georeferencing event at the Healthy Regions & Policies Lab, and about 20 people from across campus and beyond put together mosaics of Champaign, Urbana, and even Homer, IL!
The Chicago Urban Heritage Project creates mosaicked layers of Chicago in order to extract historically accurate building footprint data layers for visualization and analysis.
Haywood Mapping Service in Macon, GA, has georeferenced many small towns in the surrounding region, using the aggregated viewers on OIM to facilitate historic downtown walking tours.
Kendall County, Illinois is committed to developing the rich history of their area and uses OIM to georeference maps to get their communities into OpenHistoricalMap. They also work to involve local history groups with both OHM and Yesterdays and encourage collaborative contribution to historical knowledge.
Subterrix is an exploration platform designed for the metal detecting community, and georeferenced historical maps are a foundational piece of the toolbox they provide.
OldInsuranceMaps.net is funded in part by the National Institutes of Health (National Institute on Aging: R01AG080401) through a partnership with University of Michigan Institute for Social Research, University of Richmond Digital Scholarship Lab, and the National Community Reinvestment Coalition.
We have undertaken funded collaborations with:
Ongoing support also comes from Subterrix, premium tools for historical research and terrain analysis.
Special thanks goes out to individuals who have donated to the project:
Kevin H., Andrew M., Peter M., Pete Z., Chris P., Hayden S., Mike O., John P., Josiah W., Meagan B., Ian S., Zack R., Jacob R., Jeff M., Daniel P., Mike B.
Thank you!