Delete comment from: GeoGarage blog
This is surprising. As shown recently in a fascinating PhD (FROM THE PORTOLAN CHART OF THE MEDITERRANEAN TO THE
LATITUDE CHART OF THE ATLANTIC , Joaquim Gaspar, http://www.ciuhct.com/online/docs/thesis_joaquim_gaspar_2010-v2.pdf)
early portolan charts (more accurately 'charts based on portolans') are skewed by an amount that tallies with the magnetic declination at the time they were drawn. This strongly suggest that their dating is accurate and based on contemporary data (portolan s. s.), based on course sailed/rowed and distance (possibly the number of days of sailing). I suggest that experienced pilots/masters after decades of sailing could accurately estimate the time necessary to sail between ports (even taking weather into account).
The fact that portolan charts were drawn from a collection of smaller charlets has been suggested before (see the great ressource by Tony Campbell at http://www.maphistory.info/portolanchapter.html)
Mercator projection use (centuries before Mercator time) is hard to demonstrate as the latitude extent of the mediterranean sea is much smaller that its longitude breadth.
But who knows... Specialists will certainly take the matter up....
Mar 12, 2014, 5:42:04 AM