The portolano first appears in full form in the early 14th century as a sailor's guide for navigating the Mediterranean. Although there are theories of a long period of development of these types of maps, the first hard evidence dates only to the work Lo Compasso da Navigare of 1296.
The usage of the portolano is extremely similar to the type of navigation documented among various Austronesian peoples. To understand this we will give a brief overview of the types of navigation discussed.
In early times, most peoples navigated by sailing within sight of the coastline. If they crossed bodies of water, there had to be some visible landmark, like a mountaintop, to use as a target.
However, some early peoples developed the ability to sail to destinations that were not visible.
Among such peoples who have been studied we find they often remarkable ability to orient themselves. That is they can point out the compass directions or the direction of their home without any reference aids. How they do this is not well-understood.
Using one's home port as a reference along with such orientation capability, one can confidently sail beyond the sight of land. This can be done because one can always point back to one's home and sail in that direction if needed.
When learning navigation one thinks of other destinations in terms of their compass direction from the home port/reference.
In the graphic above, we can think of Island A as the home reference. The navigator will then memorize the directions for Island B as a line AB, and for Island C as a line AC.
When sailing to these points, the navigator uses a type of dead reckoning by always looking back at the home reference -- Island A. There is a widespread proverb among Austronesian speakers that deals with many subjects including preserving the past and can also be generalized to navigation. In essence it says:
To know where you are going and where you are, you must first know where you came from
Thus, the navigator judges his deviation from the true course by referring back to the home reference. But what if the navigator having reached Island B from his home port now wants to travel directly to Island C?
In such a case, the direction from his home reference is no long valid except as a new reference. We will call this by the Micronesian name etak. Island B now becomes the new home reference and Island A is the etak. To obtain the course to Island C, the navigator completes the triangle with the line BC. Generally this is done all within the mind of the navigator. Of course it requires an ability to visualize geometric relationships.
The etak island is used to complete the triangle and the bearing stars provide the right wind direction
In practical terms, the navigator applied the course to a wind compass. The wind compass was simply a linking of prevailing winds with certain stars (a star compass).
Now moving to the portolano map, it displayed a number of circles, some resembling compasses with rhumb lines radiating outward like spokes from a wheel.
Portolano map of Piri Reis, 3.3 mb
From the image above of 16th century Turkish Admiral Piri Ibn Haji Mehmed, we can see the circles with the projecting rhumb lines.
These circles basically represented wind compasses! Unlike the coastal navigation of previous times, the navigator now sailed on courses using the wind compass linked with the newly-invented magnetic compass. In Austronesia, the wind compass was used with the star compass rather than a magnetic one.
When a navigator wished to travel to a certain destination, a line was drawn from the point of departure, the home reference, to the destination. The rhumb line that ran most closely parallel with this penciled line was selected. The navigator refers back to the circle for the compass direction and then uses this for guidance with the magnetic compass.
The different circles simply represent different etak that the navigator must choose from.
In the Austronesian method, when one sails past known areas, new etak are created. Thus, when Tupaia sailed to Batavia with Captain Cook's he probably created new etak at selected stops as he ventured thousands of miles into unknown territory where it was documented he could always point accurately to his home island/reference.
More than one commentator has noted that the portalano appeared suddenly in the Venetian and Genoese sphere. Did this represent a sudden innovation, or possibly a transfer of information? The charts arise during the same period of map evolution witnessed far to the east in China.
In China, new information appears to be incorporated into a grid system that had been developing for centuries. In Europe, new navigating charts appear using a previously unknown wind compass system.
Paul Kekai Manansala