In the 15th century, Alberti was commissioned to carry out a topographical survey of the city of Rome. He invented a mathematical instrument to do so accurately, a technical drawing instrument, which would essentially locate the given sites. The instrument consisted of a circular disk he named, the ‘horizon.’ The circumference of this circle would be equally divided into 48 parts or ‘degrees.’ The distance between these degrees was further divided into 4 parts deemed ‘minutes.’ The completion of the first part of the instrument is now complete.
The ‘spoke’ will act as the rule of this circle and will rotate around it’s centre. Its length is equal to the radius of the circle. It is divided into 50 equal segments (degrees) and then further divided into 4 minutes. Once the spoke is in place, we are able to start depicting points and measurements from Alberti’s tables. For instance, if given ‘coordinates’ for the Porta Portuense (Horizon: 27degrees, 3minutes and Spoke:26degrees, 2minutes) then we locate them on the horizon and spoke appropriately. Finally, he differentiates the technique between apexes and corners. Points of an apex should be curved and not simply connected in a direct straight path.