Marine charts were a typically Mediterranean product, the fruit of centuries of experience of navigation along the coasts of the inner sea. Disseminated from the second half of the 13th century onward, they reflected the scholarly world of medieval geography; at the same time, they were designed, like portolans, as navigational aids.
One factor that certainly contributed to their diffusion was the invention of the magnetic compass between the 12th and 13th centuries. The compass was the instrument that ensured a more accurate plotting of routes and reciprocal positions of coastlines. Marine charts were solely designed to represent coastal outlines, ports, reefs, and shallows, all arranged on a geometric grid formed by wind directions. The reference grid was not the orthogonal net of meridians and parallels used in universal maps, but the array of spokes of the wind rose surrounding the magnetic needles of compasses.
Wind roses typically had sixteen points, surrounded by an equal number of peripheral roses, arranged in a circle and often decorated in a variety of patterns. Every rose had radiating rhumb lines, that is, lines of different colors showing wind directions. The main winds were shown in black, half-winds in green, and quarter-winds in red. The geometric structure on which sea-chart designs were based did not involve any projection concept. It consisted of a transformation of the terrestrial globe into a flat surface, on which meridians and parallels formed an orthogonal grid with equal intervals.
Today, this is called an equirectangular projection, and it was first applied by Marinus of Tyre (second half of the 1st century CE - first half of the second century CE) in the second century CE. The geographic coordinates, however, were defined not by meridians and parallels, but by wind directions and the distances from an initial location. These charts were used to prepare navigation routes, following the rhumb lines for winds and measuring distances sailed on a daily basis. This was known as the “flat-earth” navigation method, so called because it did not take into account the Earth’s spherical shape.