The question of when the grid was placed on the map is an important one, critical to visualizing the way the map was made and used.
If the grid was added after the chart was drawn, the rhumb lines could be used for navigation, but would not have any function in drafting the portolan chart itself.
If the grid was put on the map first, then it could be used in copying one chart to the next, as well as in navigation.
In a book about portolan charts published in 1911, Edward Luther
Stevenson wrote that the grid of rhumb lines was added to the
portolan charts after the maps were drawn (Stevenson 1911, 22).
This has been a matter of debate and since that time much progress
has been made in understanding how portolan charts were made.
When Stevenson wrote his commentary, he knew of only sixteen portolan
charts made before 1500. More recent scholarship using magnifying
glasses and microscopes and an experiment with four portolan charts
at the British Library, has confirmed that the rhumb networks
were drawn on the portolan charts first (Campbell 1987, pp. 390-91).
See the rhumbs on the Canepa.
See the rhumbs on the 1424 Nautical.
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