Rhumbline network

A rhumbline network, or more properly a windrose network, is a navigational aid drawn on portolan charts. This network is like a web (see picture) forming a grid on the map. The grid can be easily spotted (as parchment is quite translucent) by observing the map 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.

The lines are not true rhumb lines in the modern sense, since these can only be drawn on modern map projections and not on 13th century charts. They were close to true rhumb lines in the Mediterranean area but highly inaccurate in the Teixeira planisphere and the other planispheres drawn in any pre-Mercator projection.

All portolan maps share this characteristic "windrose networks", which emanate out from compass roses located at various points on the map. These better called "windrose lines" are generated by observation and the compass, and designated lines of course (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 they could be loxodromes only if the chart was drawn on a suitable projection.

As leo Bagrow states:"..the word ("Rhumbline") is wrongly applied to the sea-charts of this period, since a loxodrome gives an accurate course only when the chart is drawn on a suitable projection. Cartometric investigation has revealed that no projection was used in the early charts, for which we therefore retain the name 'portolan'."

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Wikipedia