As we have discussed in a previous post the 1516 Carta Marina by Martin Waldseemuller has many of the characteristics of a Portolan Chart including the rotation of the axis of the Mediterrenean Sea that runs from Gibralter to Antioch.
The fact that our rotation studies have shown that the axis of the Mediterrenean Sea on the 1516 map is between 7.9 and 8.3 degrees leads us to believe that portolan charts where used as sources for the information it contains. This is further brought out through the comparison of our rotation calculations with the rotation on other charts, paleomagnetic declination studies, and a comparison with Waldseemüler's 1507 World Map. The figure above (double click on figure to enlarge) shows a graph of the calculated rotation of various portolan charts (red line) compared with the historical magnetic declination studies of Tanguy, Bucur and Thompson published in the journal Nature in 1985. This group studied lava flows from Mount Etna in Sicily from 1301 to 1901 to determine the change in magnetic declination. Their results are shown in the blue curve on our graph. It can be readily seen in the figure that the rotation values of the Portolan charts match very closely the paleomagnetic data. Further, the calculated rotation of the Carta Marina sits on the graph in the location one would expect if we were dealing with a Portolan Chart. The 1507 map on the other hand has a rotation value that is much less and does not reflect the use of a Portolan source. A good study on the history of secular geomagnetic variation is that of Jackson et. al. based on a large scale compilation of historical magnetic field data, http://earth.leeds.ac.uk/~earccf/animations/Jacksonetal2000.pdf. The time-dependent field model that they construct is based on a dataset that is parametrized spatially in terms of spherical harmonics and B-splines.
Because the paleomagnetic values that we use here are from Mount Etna, near the center of the region that we are interested in for our rotation models, the magnetic declination differences are comparable with the rotation of portolan charts for the Mediterranean Basin. Our results here make us fairly confident that the Carta Marina not only resembles a Portolan Chart but is in fact directly derived from one.
For further reading see:
Malin, S.R. 1985. "On the unpredictability of geomagnetic secular variation", Physics of the Earth and Planetary Interiors 39: 293-296
Tanguy, J.C., Bucur, I. and Thompson, J. 1985. "Geomagnetic Secular Variation in Sicily and revised ages of historical flows from Mt. Etna" Nature 318: 453-455