This black-and-white image of Bullfrog Lake, IL, consists entirely of thousands of small black dots arranged on a strict grid. The darker clusters signify lower elevations, while areas without any dots signify higher ones. Produced with the aid of a dot-matrix printer in 1977, the map was one of dozens of test prints created as part of a remote sensing project led by Waldo Tobler, a geographer who made significant contributions to the field of computer cartography.
Making this image was the product of computational experimentation that involved translating aerial photographs into digital terrain models, where each pixel on the grid represented a numerical elevation value. The fact that this test print still exists is unusual, and because these kinds of samples were often discarded in favor of more “final” products, map libraries tend to have an incomplete archival record from this experimental period in cartographic history.