The wind rose designed by Fra Mauro (active ca. 1430-ca. 1459-1464) along the circumference of the world map leads us to believe that his cartographic method was derived from seafaring tradition. The almost perfect correspondence between the Vatican Library’s marine chart and the same area portrayed in the world map would confirm such a hypothesis, allowing us to transfer both the marine chart’s wind rose and its scale of miles to the world map, thus measuring a diameter of 9,200 miles, circa 12,000 kilometers. This diameter corresponds to the latitude of Jerusalem, with the 12,000 kilometer measurement fairly equal to the distance between Portugal and the eastern coast of China, which is the ecumenical space represented by Fra Mauro. This geographic area corresponds to a spherical cap of approximately 130 degrees, which in medieval cartographic tradition represents a quarter of the habitable regions of the Earth.
In terms of projection, the world map is a plane representation of the terrestrial globe. In the absence of explicit indications as to the method adopted, we can hypothesize that to achieve this geometrical transformation, Fra Mauro must have imagined a plane tangent to the center of the inhabited world, thought to be close to Jerusalem. Onto that plane he would have transferred the distances between parallels, drawing them as straight lines equivalent in length to the parallel arches of the spherical cap comprising the landmass. Onto this simple diagram of latitudes he would have then marked out the positions of the cities by employing only the latitudes and the known distances from one location to another.
A different hypothesis, more in accordance with the tradition of marine charts, is that he may have imagined the spherical surface of the globe transformed into the plane development of a cylinder, with meridians and parallels forming an orthogonal grid on which he traced a circle centered on the parallel of Jerusalem, with a radius equal to the distance from Baghdad to the Canary Islands. In a plane transformation of this type, each location is determined in relation to the others according to the direction of the winds, analogous to what is found on the terrestrial globe.