by Nils Ulfers
- con4gis.io tariff: free
- con4gis brick: con4gis/maps (MapsBundle)
1. Create an overlay map for the base map
This tutorial is based on the manual of con4gis-Maps. If you haven't created a con4gis map yet, first check out docs.con4gis.org.
To create an overlay map for your existing base map, click on the overlay symbol on your base map in the menu item Base maps. Now you create a "New Overlay map". The name of the overlay is also the name in the Starboard. In the provider field, select "Georeferenced image". Next, select your overlay image from the file manager.
To provide the image with a georeference, please use the tool "viglino.github.io/Map-georeferencer".
With this tool you can easily place your image on the map. Open the link above and click on "Select file" and select your image from your files. Then click on "Georef" and your picture will be placed on the left half of the website, on the right half there is an OSM map.
In our example we provide a historical map of Emden with geo references.
For best results you should use the OSM overlay to see clear structures on the map. For this you can use the overlay settings from our example.
To place your picture exactly on the map click on prominent points that are visible on both sides. You need at least two points that match on the map and on your picture. For example, these could be buildings that exist in both maps. In the map of Emden we used the town hall as one of these points.
If you have chosen two points on both sides, your picture will be placed on the map. The more points you place on both sides, the more exactly the image will be referenced. To make it easier to place the points, you can reduce the opacity of your image in the overlay settings. If your image is well placed you can display the geo data by clicking on the "Georef-Button" on the left side of the screen.
Then you can copy the data and paste it into the field "JSON for georeferencing" in the backend.
In the field "Initial visibility" you can specify the visibility of the overlays in percent. The visibility can be adjusted individually by the user in the Starboard.
Under "Custom Attribution" you can for example specify copyright information of the map overlays.
Tip: For small overlays like in our example, you should adjust the map center and zoom level in the map structure.
Have fun with your new overlay map !