Above are two versions of the Horizon tool invented and described by Leon Battista Alberti to allow a man of average intellect to accurately draw the map of Rome.
The tools have been cut to a 15″ diameter to allow for a decently scaled map. A second Spokeless version of the toll was created for more accurate plotting (as opposed to the previous version being interrupted with spokes).
After having read Alberti’s description of the construction the “Horizon” tool it becomes clear that this tool is made only to replicate his initial surveying studies. There is some complexity when he begins to differentiated a corner from an apex, consequentially suggesting different drawing rules when constructing the lines through the co-ordinates.
I would imagine that because the grid used to initially survey City Walls was a radial one as opposed to an orthogonal grid, that it would have been quite difficult to locate the co-ordinates (possibly having to return to the Capitol to count the number of steps for every important corner, apex or gate).