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Online Historical Map Georeferencer (OHMG) is an open source application that facilitates the creation of georeferenced historical map mosaic layers through a collaborative, public process.


OHMG is an attempt to make a more generic application from the platform built for, a crowdsourcing site for georeferencing historical fire insurance maps. While that project was built specifically for the Sanborn Map collection at the Library of Congress, aspects of the workflows and tools developed can be valuable for a much wider range of maps. If this interests you, please get in touch.

Handle Complex Content

Multi-document items, like atlases, are first-class citizens, with full support for multiple maps on each sheet and multiple content scales across the atlas. Learn more...

Collaborative Model

An iterative, componentized workflow provides direct access for any user to contribute at any stage in the overall process. Learn more...


Seamless Mosaics

Margins are removed from adjacent map layers by creating a "multimask" across the entire item, rather than trimming each layer independently. Learn more...


Transformations & Projections

Any standard transformation algorithms can be used for GCPs, and any projection can be targeted. Learn more...


Web Services

Layers are immediately published as web services and can easily be integrated into third-party platforms, like OpenHistoricalMap and Felt. Learn more...


Data API

A simple API can provide researchers with programmatic access to GCPs, cutlines, and GeoTIFF download links. Learn more...