Portolan charts and the sailing directions behind them
The oldest surviving portolan chart is usually called the Carta Pisana. It is kept in Paris and dated to the late thirteenth century. It shows the Mediterranean and the Black Sea with a coastline more accurate than anything earlier in Europe, and it has no graticule of latitude and longitude. The sheet is covered instead with a web of straight lines.
Rhumb lines
The lines radiate from points arranged around a circle, and each one stands for a compass bearing. A pilot who wanted to sail from one port to another laid a straightedge between them, found the nearest line in the web and read off the bearing. Distances came from a scale bar, usually graduated in miles.
Made to be used
The name comes from the Italian portolano, a book of written sailing directions listing harbours, headlands, hazards and distances along a coast. The chart seems to have grown out of those books. Place names are written at right angles to the coast, on the land side, so that the sea stays clear for the lines. Important ports are in red ink and minor ones in black. The charts were drawn on vellum, a single animal skin, and the outline of the skin, with the neck at one edge, is often visible on surviving examples.
Limits
A portolan chart treats the sea as flat. That works well within the Mediterranean and poorly over long distances, where the curvature of the earth matters and a straight line on the chart is no longer a constant bearing. The problem was one reason mathematicians looked for a different kind of projection, which is the subject of our article on the Mercator projection.
Chart-makers kept the portolan style for a long time. The Catalan workshops of the fourteenth century extended it to the Atlantic coasts of Europe and Africa, and the Catalan Atlas of 1375, attributed to Abraham Cresques of Majorca, joins a portolan coastline to a world map in the older tradition.