Others were disguised as such, notably the Periplus of Pseudo-Scylax from the 4th century BCE, which described the harbors and landmarks along the north African coast west of the Nile delta.
The reliance on the magnetic compass (an instrument that only really began being used for navigation in the 13th century,[3]) distinguishes the Medieval portolano from the earlier Classical periplus.
[4] The wealth of detail contained in portolano handbooks is reflected in the portolan charts, stunningly accurate even by modern standards.
For instance, they frequently had detailed physical descriptions of shorelines, harbors, islands, channels, notes about tides, landmarks, reefs, shoals and difficult entries, instructions on how to use navigational instruments to determine position and plot routes, calendars, astronomical tables, mathematical tables and calculation rules (notably the rule of marteloio), lists of customs regulations at different ports, medical recipes, instructions on ship repair, etc.
Among notable rutters is the Grand Routier, written by the French pilot Pierre Garcie, c. 1483 and published in 1502–03, which focused on the shores of the Bay of Biscay and the English Channel, and its peculiarities.
[6] Another frequently used rutter was the work Portolano by Pietro Coppo, published in Venice in 1528, which included a collection of sea charts and the description of Christopher Columbus's discovery of America.