Xavier’s Descriptio Urbis Romae

Synopsis

In Descriptio Urbis Romae, Alberti describes a very simple way to record information from his survey of Rome, thus allowing anybody to redraw the resulting map of the city, based on his observations. The original point of observation of Alterti’s survey– that is point (0,0) in the tables – is the summit of the Capitol. The process used by Alberti is very simple. Strings would be stretched in the air (!), between the survey’s point of origin and the point of interest1. The distance and angle of measurement was then recorded in tables we still have nowadays in Alberti’s book. This surveying method was quite efficient for the time since the steep city of Rome made very difficult a direct measurement on foot (according to Pr. Bruno Queysanne, Queysanne, Bruno, Alberti et Raphaël, Descriptio Urbis Romae ou comment faire le portrait de Rome, Plan Fixe Édition, École d’architecture de Grenoble, Lyon, 87p, p.14.)

Alberti transcribed his measurements in a form that is basically polar coordinates (r,θ). The survey’s map can then be transcribed by anyone, in any chosen format. First, the “horizon” is divided into 48 equal parts (which are called degrees) around a circle (any chosen radius). The north is thus given by the 48th division, south by its 24th, west is number 36 and east number 12. Each of these 48 parts is then divided again in 4 equal parts, which are called “minutes”. This numeric value corresponds to the angle (θ) of the polar coordinates in Alberti’s survey.

The radius coordinate (r) is given by the “spoke”, which is basically a ruler that spins around the center of the map – that is the center of the horizon’s circle. The length of the ruler is the radius of the previously traced circle. It is again divided into 50 equal parts called degrees. These 50 parts are subdivided into 4 “minutes”.

To map of Rome can then be traced point by point by finding the corresponding points in the given tables of Alberti’s book; angle on the horizon scale, then radius on the “spoke” scale. There should be a certain “magic” feeling to see this map, surveyed by Alberti centuries ago, appear before your eyes, points after points. The final step is to connect these points.

It should be noted that there’s two different kind of point in Alberti’s tables. “Corners” point must be connected with a straight line. “Apex” points should be connected with a curved line. It should be the summits of an arc on the final map (see figure 1).

Construction of the instrument

The instrument was constructed in thick white cardboard (Peterborough wall mounting cardboard). The “spoke” will be attached to the center of the map with a pin so it can be rotated around the horizon. For now we consider a 20 cm long “spoke”. This should give a bigger map of around 40 cm tall, this allowing us to see better details.

Legend

The letters on the map correspond to the differents gates surveyed by Alberti.

• A: Porta del Popolo
• B: Porta Pinciana
• C: Porta Salaria
• D: Porta La Donna
• E: Porta San Lorenzo
• F: Morta Magiore
• G: Porta del Laterno
• H: Porta Latina
• I: Porta Appia
• J: Porta San Paolo
• K: Porta Portuense
• L: Porta San Pancrazio
• M: Porta Gianicolense
• O: Porta Santo Spirito Gianicolo
• P: Rear Gate over the valley
• Q: Gate on the hill
• R: Porta Palatina
• S: Porta Castello