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I. INTRODUCTION
In his Descriptio urbis Romae, Leon Battista Alberti (1404, Genoa-1472, Rome) provides step-by-step written instructions for drawing an accurate map of the city of Rome.1 As scholars, especially Mario Carpo, have noted, Alberti's Descriptio urbis Romae is highly innovative for two main reasons: first and most surprisingly, Alberti provides only instructions, but not an actual illustration of his map of Rome, and second, the instructions allow the map to to be drawn accurately on any scale.2 Examination of Alberti's sources shows that both Ptolemy's Geography and his Almagest, the standard text for astronomy at that time, are key to understanding this work. The Descriptio urbis Romae is the result of fruitful interplay between the interrelated fields of geography and astronomy and, respectively, their neighboring fields, chorography and astrology.3 An appreciation of that interplay sheds new light on the image of Rome that it conveys.
Alberti's treatise consists of two parts: a short text in which Alberti explains how to craft and use one's drawing tools (called radius and horizon, see figures 1 and 2) and 16 tables with coordinates that are listed at the back of the treatise. He gives the coordinates of a total of 175 dots designating locations. These apparently roundabout steps prevent errors that could be introduced in the process of copying the images by hand.4 When these dots are plotted into a circular grid-with the central point symbolizing the Capitoline Hill-and correctly connected with one another, they form the city walls with the gates and the Tiber with its island, and they indicate the position of 35 churches and public buildings. Alberti duly starts from St. Peter's Basilica and ends with the Capitoline Hill, indicating the position of the church of Santa Maria in Aracoeli. To obtain the exact data of the points' coordinates for his map, Alberti developed a measuring technique that he does not share with the reader of the Descriptio, but which he describes in his Ludi matematici5 (before 1452), a treatise on amusing ways to use mathematical notions to practical ends, such as land surveying for Meliaduse d'Este.6 In the Descriptio he settles for painstaking instructions on how to depict and correctly fill the dots into a circular shaped coordinate system, which has been identified...