OldInsuranceMaps.net
A crowdsourcing site for creating and viewing georeferenced mosaics of historical fire insurance maps from the Library of Congress. See how it works or visit the about or FAQ pages to learn more.
How it Works
Digital scans of Sanborn maps are available through the Library of Congress and are pulled into this site through the LOC JSON API , generating a "Volume Summary" page (Baton Rouge, 1885).
Contributors prepare each sheet in the volume, sometimes splitting it into multiple documents, 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).