In the text called Delineation of the City of Rome, Alberti describes a tool and its instructions to follow in order to replicate a precise map of Rome. Using a circle divided into 48 degrees and 192 minutes called the “horizon” and a “spoke” the length of the radius of that circle divided into 50 degrees and 200 minutes, he explains how to plot a series of points within the horizon, similarly to a cartesian grid. He then lists multiple tables of points corresponding to various monuments, roads, river, walls and buildings according to their position in the city Rome, either as “corners” or “apexes”, meaning the meeting of two lines or two curves. Once all the corners and apexes are connected together, the result should be an accurate map of Rome proportionally sized to the dimension of the circle originally drawn.
In order to gather those tables of points that represent the positions of different elements of Rome, Alberti created a tool with which he could determine the position of a monument in degrees on the horizon. Standing still at a single point (which corresponds to the center of the circle on the map), Alberti looked through an instrument that allowed him to tell where on the circumference of the circle the various elements were positioned. Assuming that he was standing next to a tall monument with a determined height (in this case, the Capitol), Alberti could determine the distance of those elements from his viewpoint with a simple astrolabe instrument.