page_title: Rhumbline network
section_title: Network design
hierarchical_section_title: Rhumbline network / Network design
caption_attribution_description: English: Map of the Atlantic, including Spain and the British Isles, comprising the first sheet of the atlas attributed to Genoese cartographer Pietro Vesconte, and tentatively dated 1325. Contained in a manuscript of Marino Sanuto Liber Secretorum Fidelium Crucis (c.1325) held by the British Library in London, UK.
context_page_description: A rhumbline network, more properly called, a windrose network, or sometimes also called harbour-finding chart, compass chart, or rhumb chart, is a navigational aid drawn on early portolan charts dating from the medieval to early modern period. This network is like a web forming a grid on the map.
  Before accurate surveying there was no method for measuring longitude at sea so maps possessed 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.
  To calculate a course to follow from a ships position to a point of desired destination, one would identify the winrose thought to be closest to the ships position. Then using a parallel rule — the "line of course" taken from the rose to the point of destination, would be transferred to the ships known position.
context_section_description: Pujades in his book "Les cartes portolanes" has a chapter with all known theories and, with the aim to clarify the controversial arguments, he shows an image of Petrus Vesconte drawing a portolan in one of his maps in which it is visible how he started by drawing first the rhumbline grid. Some authors call it "winds network" instead of using the term "rhumbline network" or "network of  rhumblines".
  The circle is divided into sixteen equal parts defining a hexadecagon, then the network of sailing directions "for a 16 winds rose" is represented by groups of 16 "straight lines" called rhumblines. From each vertex, 7 rhumblines are projected towards the hexadecagon's interior connecting "in an alternated pace" to 7 of the vertex opposite to it, but without routing any line to connect that vertex to the other 8 intermixed vertex. The remaining 9 rhumblines (to complete the 16 winds) are projected from each vertex towards the exterior of the hexadecagon, although in some portolans those 9 lines do not appear.
  The lines of the courses for the eight main directions (or winds) are drawn with black ink (or sometimes gold); the eight intermediate directions (half-winds) are drawn in green; and in the case of a 32 winds rose,  the sixteen remaining (quarter-winds) are drawn in red. The intersection of this set of "rhumblines"  determine on the portolans a varied pattern of symmetrical squares, parallelograms, trapezoids and triangles.
caption: Portolan by Petrus Vescomte
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