Finding Georectified Maps using Map Warper

Georectified images are images that have been linked to a coordinate system and therefore can be overlayed onto an interactive map. This process usually involves identifying control points on both the image and the coordinate system; the more control points, the more accurately the image can overlay on the coordinate system. This process is commonly used to overlay historical maps on modern maps. Due to differences in map projection, the historical map ends up stretched and warped in places to overlay correctly on modern geographic information systems (GIS).

There are resources online to find maps that have been georectified. This guide uses Map Warper (mapwarper.net). Map Warper is open source and includes maps from institutions across the world. By using a WMS Overlay URL, it is possible to use a historical map in Omeka S without downloading and reuploading the map itself.

1. Go to mapwarper.net

2. Search for a georectified map that meets your needs. There are multiple ways to search by keyword, date, and location. For this guide, we’ll search by location and date. Select the Find Maps by Location tab.

   

3. Use the slider along the top of the Find Rectified Maps page to create a date range to search; use the map box to zoom in to a certain region to search. In this example, we’re searching for maps of Europe between 1935 – 1945. Results will automatically appear as you make changes to location/date.

   

4. When you identify a map you think you might want to use, Select Open Map under the title of the map.

   

5. Review the map to make sure it’ll work for your purposes. Be sure to look in the top bar to check for georectification control points. The more control points, the better the chances the map can stretch to overlay modern coordinates.

   

6. When you find a map that you want to use in your exhibit, select the Export tab.

   

7. Locate WMS: WMS Capabilities URL (for JOSM OpenStreetMap Editor) under the list of Map Services; right click on the WMS Capabilities URL link and copy the link address. You will add this link you just copied to Omeka S. See the next section for instructions to add the URL to an Omeka map.