Rhumbline network

Not to be confused with Rhumb line or Compass rose.
Windrose network -16 vertex on each Portolan.
Image of Petrus Vesconte

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.[1] 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.[2] 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.[3]

  1. ^ Leo Bagrow (2010). History of Cartography. Transaction Publishers. pp. 65–. ISBN 978-1-4128-2518-4.
  2. ^ Cite error: The named reference Bagrow2010 was invoked but never defined (see the help page).
  3. ^ P. D. A. Harvey (1991). Medieval maps. British Library. ISBN 978-0-7123-0232-6.