Unsolved Mysteries of the Portolans
The Grid: Rhumbs first or last?
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).