The Rumsey Historical Maps Layer offers a rich collection of historical maps from around the world that one can overlay onto the regular satellite and aerial imagery provided by Google Earth. The maps have been modified in order to be compatible with the virtual globe and viewed in pseudo-3D. The layer is available on Google Earth 4.0 onwards. The link to the maps gives us both the location as well as the date in addition to further historical context and esthetic information by clicking on the “Read more about this Map” thumbnail.
The David Rumsey Collection contains over 26, 000 maps that range from maps of the world throughout the ages to maps of major cities and even maps of parks such as Yellowstone. Over 120 historical maps have been selected from the collection to be displayed on the virtual globe which can easily be enabled by clicking on the box in the gallery bar.
from left to right, 1) various sites of historical maps over western Europe, 2) 18th century map of the world in pseudo-3D, 3) current 3D building over an ancient map of Berlin
Some of the maps fit and are closer to reality than others. In order for them to be made compatible with the software they have to undergo a series of transformation and editing without having to alter their content or appearance. Moreover, certain maps can be overlaid onto current 3D buildings, this is often the case for historical maps of current cities.
Process of making the historical maps compatible with Google Earth:
The original historical maps are first made into digital images by scanning them with high resolution digital cameras. Then these digital images are transformed in a process called georeferencing, which makes them display in their correct geographical spaces in Google Maps and Earth. Georeferencing is done using a GIS program, which takes points on the old maps (cities, coast lines, rivers, streets) and connects them to the same points on a modern satellite map image or a modern street map or a modern map showing boundaries of countries and states. The GIS program then takes all these points (as many as 200 are made for very large maps) and uses them to recreate the digital image so it will fit into its modern geographical space. Often the image has to be curved a bit for this to be accomplished.
retrieved from the David Rumsey Map Collection website (www.davidrumsey.com/view/google-maps)