The Compass Rose - Map of China: Decoding the Secrets of a Vanished Cartographer - page 107

lenge a portolan chart-maker faced was how to move from section to section and
maintain a consistency of scale and orientation. Compass roses were the answer.
These were arranged in a large circle, ideally sixteen of them, to serve as nodes
of reference for every point on the map. If all points could be calibrated by their
magnetic angle to every rose, then the map that resulted would be perfectly cor-
rect. The lines radiating from the compass roses are known as rhumb lines: that is,
lines that maintain a constant magnetic direction. In the old portolan charts these
lines crisscross the surface of the map in a dense, Lilliputian web. Conventionally
the lines at the eight winds were drawn in brown or black ink, at the eight half-
winds in green, and at the sixteen quarter-winds in red. These rhumb lines are all
anchoredatsomepointtoacompassrose,whetheritisactuallyshownonthechart
or implied somewhere off it. There could be up to sixteen of these nodes (like the
sixteen half-winds), and therefore potentially sixteen compass roses to a full chart,
although most portolan charts show only part of the circle of nodes used to survey
the area within the chart, and therefore only a handful of roses.
Andthengraduallytheroseswereremoved,ascartographersshiftedawayfrom
portolanchartsconcentratingsolelyoncoaststocomprehensivemapsthatstroveto
incorporateothergeophysicaldata.Thisshiftoccursintheseventeenthcentury.We
canseeithappening,infact,inthetwomapsinJohnSelden's The Closed Sea .The
first map shows Great Britain's position in relation to the seas that surround it. It
names countries but does not draw their political boundaries (Fig. 14). The second
mapismeanttoillustratetheKing'sChambers(Fig.15).Thesewerecoastalwaters
across which foreign ships could take innocent passage but could not in any way
interfere with ships of other nations. Ships that came within the King's Chambers
were deemed to be in British territorial water and therefore subject to British law.
James I introduced the concept in 1604 in response to Dutch attacks on Spanish
ships within sight of the English coast. It was a unique innovation, the first time a
state had ever explicitly defined a zone of maritime sovereignty.
In The Closed Sea Selden explains that a committee of twelve men 'very well
skilled in Maritim affairs' was convened to determine the boundaries of the King's
Chambers. 'These twelv men beginning at the Holy Island, fetch'd a compass
roundfromtheNorthbytheEastandSouthtotheWest.'TheHolyIslandisLindis-
farne, the site of an ancient monastic community just off the coast in the North
Sea, close to the border between England and Scotland. This was the northernmost
point of designation. From there the committee drew a series of straight lines from
pointtopointdowntheeastsideofEnglandtoNorthForland(nowMargate, atthe