The 1748 Pianta Grande of Rome (Fig. 1) by Giambattista Nolli (1701-1756) is a milestone in the history of cartography. Its copious size (1760 x 2085 mm), intricate detail, and accuracy have made it an essential document for studying the architecture, landscape architecture and urban structure of Rome for over 250 years. The Nolli Map has served as the basis for a series of interactive digital projects that present historic maps and images of Rome as key resources for understanding the spatial history of that city.[1] The first of these projects, The Interactive Nolli Map Website, which digitally remasters this classic as an interactive tool, making it widely available to students, scholars and the general public, was launched in 2005. The next stage of the project involved the vedute, or urban landscape views, of Rome from Delle Magnificenze di Roma Antica and Moderna by Nolli’s contemporary and collaborator, Giuseppe Vasi (1710-1782). These 237 images were geo-referenced to the cartography of Nolli and linked to modern views, resulting in Imago Urbis, Giuseppe Vasi’s Grand Tour of Rome, which was published in 2008. Our current ongoing project, Open Rome: A Spatial History, digitally derived from the Nolli map and other cartographic sources, develops a layered cartographic system that extends from antiquity to the present. This array of historic and contemporary maps of Rome, provisionally displayed at Mapping Rome, is geo-rectified and brought into real geographic space, providing a scientific dimension for further research and discovery—especially for ongoing archeological studies of the Eternal City. Central to this investigation is the Forma Urbis Romae by Rodolfo Lanciani (1845-1929). This huge map consists of 46 plates with an overall dimension of 4560 x 6960 mm and is inspired by the early third-century marble map, Forma Urbis, from which it takes its name. It is significant that Nolli based his plan view or ichnographic mode on the precedent of the Forma Urbis.