Commons:Geocoding/Overlay

This Help page explains how to map old city plans and regional maps onto current satellite imagery in GoogleEarth and make these Overlays available to other users. This allows to visualize progress in time such as city growth or damage caused by wars and natural desasters.

Usage

A collection of maps prepared as overlays is available here:

• Category:Geographic_Overlays

All maps in this category contain a box on their image description page with a link labled Open this map in Google Earth. Clicking this link should launch Google Earth (if installed). After some load time the maps will appear approximated to their geographic position and, and structures and objects can be compared to the Google Earth satellite imagery. Comparison is facilitated by Google Earth transparency sliding control. The checkbox next to the map in the side bar, the overlay visibility can be toggled. Concerning the agreement with today's satellite photographs please note that historic maps which may be over a hundred years old were not created using the exact surveying techniques available today. Furthermore Google Earth limits the overlay adjustments for position, rotation, and scaling, hence the overlay quality can vary considerably.

How to create a map-overlay

1. Find a map image to geo tag on Commons or upload it to Commons yourself.
2. Launch GoogleEarth and zoom into the area where the map-overlay will be positioned
3. Open the Add menu and select Image Overlay...'
   1. A detailed tutorial on image overlays can be found here
4. Assign an appropriate name. Under Link enter the upload path of the image, for example http://upload.wikimedia.org/wikipedia/commons/2/2a/Erfurt-1650-Merian.jpg and wait till the image is loaded. This way we are accessing the full resolution image, even if it gets scaled down for display purposes in Google Earth.
5. Now adjust the image to the map as good as possible by dragging and rotating it with the mouse. Hint: Adjust the rotation by matching a river or road first, then using points on opposite corners adjust scale and position, double-check the rotation, repeat if necessary... This part is a little tricky and might take up to 15mins.
6. If you are satisfied with your overlay proceed to the next steps. Please note that due to mismatching map-projections or inaccuracies of the overlay image you might have to abort at this step.
7. Edit the image description page on Commons and insert the {{Overlay}} template. The template creates a new infobox which contains a source link. Follow it and choose edit. For cosmetic purposes enter the following tags in the first line <source lang="xml"> and the last line </source> of the edit box.
8. Now insert the KML-source from Google Earth between the tags: select the overlay image in the Google Earth side bar and press Ctrl+C, switch back to the webbrowser and paste the KML-source between the two tags by pressing Ctrl+V, then save. Should this method not work in you OS then right click on the overlay in Google Earth, select Save As KML, and open the file in a text editor to do the copy/paste routine.
9. The final step would be testing the overlay on commons (remove the maps from your Google Earth beforehand!)

I hope it worked and it was fun.

Further remarks

• The reason for using Google Earth as a tool are its performanca, ease of use, free (as in beer) availibility across several platforms, and use of the free and standardized data format KML.