The Horizon: Alberti’s Delineation of Rome

Alberti’s survey of Rome records the passage and lineamenta (outline) of certain features in the city of Rome including: walls, rivers, streets, hills, and buildings, as well as the locations of temples, public works, gates, and monuments.
The device is made up of two main components: the Horizon and the Spoke.
The Horizon is the circle within which the depiction of the city is enclosed. The Horizon is divided into equal degrees (up to 48) and then subdivided into four parts called minutes. The north will be marked at degree 0, the eastern equinox will be at 12, south at 24, and the west equinox at 36.
The Spoke is a straight rod much like an hour hand that is set at the centre of the horizon and extends to its outer circumference. The spoke is divided into 50 equal parts, similarly called degrees.
Once the device is made, the process of tracing the map involves identifying the degrees and minutes in the accompanying table that correspond with a title. These numbers will be used to guide the horizon and the spoke simultaneously to find the titles coordinates.
For example, if we start in the table labeled “Walls of Litium”, under “Horizon” we read: “43 degrees, 2 minutes”. Therefore we position the spoke at this number on the horizon. Next, under “Spoke” we read: “31 degrees, ½ minute”. Therefore, we locate this number on the spoke and we draw a dot at this coordinate. At the end we connect all the coordinates with a straight line, giving us the passage or lineamenta of the wall of Litium.

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