Rhumbline network

A rhumbline network (or windrose network) is a navigational aid consisting in lines drawn from multiple vertices in different directions forming a web-like mesh. They were featured on portolan charts and other early nautical charts used in the medieval age and age of exploration in marine navigation.

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. To avoid confusion, the lines on earlier sailing charts can be unambiguously called windrose lines (after wind roses), since they are not true rhumb lines by the modern definition. A rhumb line in the modern sense is only straight on a chart drawn with the Mercator projection, but not on charts from the 13th–16th centuries. Older windrose lines were a close approximation on charts of the Mediterranean Sea and surrounding areas, but the rhumb lines on small-scale maps such as the Teixeira planisphere were highly inaccurate.

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. The hole in the center of the circle, origin of the whole network, is also clearly visible from the rear.

To calculate on a portolan chart the course to follow from a point of origin to a point of destination, one should transfer—using a parallel rule—the "line of course" drawn from the point of origin to the point of destination, on top of the windrose line on the compass rose closest to the ship's position, obtaining on it the theoretical course to be followed when sailing towards the destination. This theoretical course may have to be modified (as many times as needed) when tacking if the wind is right ahead of you, or to correct the effects of leeway, currents, etc. that a sailor with experience should be able to calculate empirically.

Before modern accurate surveying, there was no method for measuring longitude at sea so maps used to have many distortions, especially in the east west direction. There was also distortion due to the curvature of the Earth's surface. The multitude of compass roses with straight lines extending outwards across the map derived from how the maps were then made by compiling empirical observations from navigators who attempted to follow a constant bearing at sea.

All portolan maps share these characteristic "windrose networks", which emanate from compass roses located at various points on the map (or mapamundi). These better called "windrose lines" are generated "by observation and the compass", and are designated today as "lines of course" or "lines of rhumb" ("rhumb lines" in the fourteenth century, traced on portolan's particular projection, though not to be confused with modern rhumb lines, meridians or isoazimuthals).

To understand that those lines should be better called "windrose lines", one has to know that portolan maps are characterized by the lack of map projection, for cartometric investigation has revealed that no projection was used in portolans, and those straight lines could be loxodromes only if the chart was drawn on a suitable projection.

As Leo Bagrow states: