Have you ever used the “Image Overlay” feature of Google Earth? Perhaps, as one of my professors did a few years ago, you want to add an old German military map of Stalingrad over the current city, or display a favorite old marked up map you have scanned up that has some key locations and annotations.
Unless you have a GIS background and some experience working with powerful mapping software like ArcGIS or QGIS, you’ll quickly discover that this is not really as easy as it seems. The issues with manipulating and using historical maps are manifold, ranging from the mathematical to the metaphysical. Let’s say you don’t want to heed the warnings of the map historians and cartographic conjurers, and you just want to get the darn thing out there both to demonstrate during a lecture and perhaps to share the file with students for them to browse around later. Once you add the image, Google Earth offers you some wonderfully simple tools for moving around and stretching images you add to help them fit better. They are lots of fun, but getting good results can often range from frustrating to impossible. It just wasn’t designed to be a full map warping utility.
Assuming your map image is somewhat internally to scale and fits some kind of coordinate system (you are probably not going to have much luck with your 14th century map of the Åland Islands), using an online map warping tool is a fast and easy way to get a map that mostly fits the underlying satellite imagery. There are several of them out there, and some tools you can download that does the same, but I have recently been using the Map Warper at Harvard’s Center for Geographic Analysis. This seems to be identical to the software that runs the NYPL Map Warper but recently I have had some trouble loading maps on the latter.
To use the Map Warper at either of the above sites, create a free account with the website. After activating the account and logging in, upload the map you wish to warp. At this point the site works like most map warpers you will find out there: to “rectify” the map you need to create “control points,” that is, points which indicate what pieces on your map connect to what geographic locations. The more control points you create around the map image, if I understand it correctly, the more closely your map will fit Google Earth. You select a point on your map, and then the corresponding point on the OpenStreetMap image and “Add Control Point.” I find that adding only three control points will give you a general fit, but leave lots of problems, so I suggest add many more from around the map. Finally, you can also crop the image, which is useful if you want to only make use of part of a map image. When you are done, “Export” the image as a KML file, which you can then easily open up in Google Earth. When combined with other placemarks, maps, or recorded as a Google Earth tour and incorporated into a lecture or class reference materials, the effect can be to add a powerful geographic dimension to the student experience.
