Rhumbline network

Since the invention of the Mercator projection c. 1600, the term rhumb line (or loxodrome) has been redefined to mean a mathematically precise curve of constant bearing on the Earth's surface.

The grid can be easily spotted (as parchment is quite translucent) by observing a chart from its rear face, with a light source illuminating the other side.

As Leo Bagrow states: The word ("Rhumbline") is wrongly applied to the sea-charts of this period (Middle Ages), since a loxodrome gives an accurate course only when the chart is drawn on a suitable projection.

The intersection of this set of "rhumblines" determine on the portolans a varied pattern of symmetrical squares, parallelograms, trapezoids and triangles.

In the Cresques planisphere one is able to read the names of those lines which were winds: tramontana, levante, ponente, mezzogiorno, greco, sirocco, and lebegio.

But on big oceans they do not follow either of them, due to the imprecision of the map making of that time, corresponding more or less accurately to rhumb lines only in the Mediterranean portolan charts and deviating greatly in the Texeira planisfere (among others).