Georeference with MapWarper

TODO: write this section about using MapWarper, a tool created and hosted by Tim Waters, to upload and georeference a scanned map. This means to properly position a scanned map based on standard coordinates, so that you can place it as an overlay on an interactive map, such as Leaflet Storymaps with Google Sheets.

Anyone can upload and georectify a map on the developer’s public site at, and also see how it’s used by organizations such as the New York Public Library at

Warning: MapWarper is a wonderful open-source tool and platform, but service may be interrupted. As of July 2020, the site warns: “Ran out of disk space. Maps older than 2 years will need re-warping to work. Downtime will happen again.”

MODIFY TEXT FROM CH 13: To add a historical map overlay, it first must be georeferenced (also called georectified), which means to digitally align the static map image with a more precise present-day interactive map. If you have a high-quality static image of a historical map, use the Mapwarper tool as described in Chapter 14 to align several known points with those on a present-day interactive map. Mapwarper transforms the static map image into interactive map tiles, and publicly hosts them online with a link in Google/OpenStreetMap format, similar to{z}/{x}/{y}.png. Or you can search for historical maps that have already been georeferenced and transformed into tiles, or contribute to crowdsourcing efforts to align maps, on platforms such as Mapwarper, the New York Public Library Mapwarper, and the David Rumsey Map Collection. [TODO: confirm how Rumsey links work] Although map tile links are not viewable in a normal browser, they can be displayed by the Leaflet Storymaps code by entering them into these columns in the Chapters tab of your Google Sheet template: