Adding WMS Overlays

Omeka S allows you to geolocate items in your collection and build interactive maps that you can add to pages or the Timeline-Map plugin. Omeka S provides many standard basemaps that you can choose from in order to customize the look and information displayed on the map. For example, you can include a basemap showing topography, political borders, or even an artistic watercolor depiction. The basemap you choose will depend on your needs and project. These maps, however, all represent modern coastlines and political boundaries. For some projects, you might want to show your items mapped onto historical or customized maps. To do this, you will use World Map Service (WMS) Overlays.

This guide will describe how to add an existing WMS Overlay onto the Timeline-Map page. This process will also work for adding historical maps to maps on individual pages.

Step 1: 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.

   

Step 2. Adding Historical Maps to the Timeline-Map Plugin using WMS Overlays

The Timeline-Map plugin allows you to create an interactive map and timeline from your collection item set and added items. If you would like this plugin, let us know, and we will set it up for you. When you get access to your Omeka S site, you will find this feature in the pages section.

Navigate to the Timeline-Map Page

1. Login to Omeka to access the dashboard.
   

2. Locate and select your individual site from the Manage Sites list.
   

3. Select Pages along the left-hand menu
   

4. You can include maps and historical map overlays on any page in Omeka S. For this guide, though, select the Timeline & Map from the listed pages.
   

Adding a Historical Map Overlay
 

From the Timeline & Map page, you can edit a number of settings and add new content blocks along the righthand side. For the purposes of this guide, we’ll look specifically at how to add a historical map overlay to the Timeline-Map.

1. Identify the Map by query content block already set up. This is the block that controls the timeline and map.

   

2. Select WMS Overlay within the Map by query box to open overlay settings and add a historical map overlay.

   

3. Give your overlay a Label that will help you identify it.

   

4. Copy and paste the WMS Capabilities URL you got from the Map Warper database (see page 5) in the Base URL field.

   

5. Select Add overlay.
   

6. When the new overlay appears as a WMS Overlay option, select the check box next to it in order to make it the default overlay.
   

7. It you need to change anything about this overlay, you can select the pencil to edit the overlay information, the trashcan to delete the overlay, and repeat the process to add label, base URL, and select Add Overlay to add another overlay.
    

8. Click Save at the top of the page to save your changes.
   

9. Select View in order to make sure the map looks the way you expect it to.
   

10. Go through the timeline to make sure the map looks correct. Due to differences between the map content, projection, and control point quality, it is common that not all locations between historical map and underlying GIS will line up exactly. You want to make sure that it works for your purposes, though.

    

As you go through the timeline, consider if the map is zooming in too far or not far enough; the size and content of the map might change how far you want the map to zoom in as you progress through the timeline. You can change zoom minimum and maximum by going back to the Timeline & Map page and selecting Default View right above the WMS Overlay section.