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The Rhumbline network or properly defined Windrose network (for its lines shouldn't be confused with the false friend: rhumbline) A better term to be used for those lines would be "windrose lines" (or "lines of course") for they do not follow the outline of the modern rhumb lines (loxodromes), due to the non-existence of map projection in s.XIII,〔 being the rhumb line "almost true" only in the Mediterranean portolan charts and a complete mess (for its in-exactitude) in the Teixeira planisphere and the other planispheres drawn in any pre-mercator projection. It is a common characteristic of all portolan charts. This network is like a web (see picture), forming a grid on the map, which is a basic element of all portolan charts. 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. == Rhumblines vs. windrose lines == 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 rhumblines, 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'."''〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Rhumbline network」の詳細全文を読む スポンサード リンク
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