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such a problem, as the wayfinder can adjust his line constantly in relation to visual
reference points. This discrepancy can be fudged over open water and the pilot has
to work from what are known as dead ('deduced') reckonings, which he deduces
onthebasisofcompassangle,winddirection,current,durationoftravelandwhere
he was at his last dead reckoning. Miscalculate one dead reckoning and the next
must be at least as inaccurate, which only increases the overall degree of error.
Sailors in the Mediterranean tried to achieve geometrical control over maritime
space by using portolan charts. By deploying a dense web of magnetically fixed
lines (rhumb lines) radiating from a grand circle of compass roses, the pilot had a
potentialinfinityoflinesagainstwhichtosethiscourseonpaper.Butcurvaturede-
feated the assumption that magnetic bearings produced consistency. North, south,
east and west never varied, but every other position on the compass produced tan-
gents that went off direction as the earth curved. This didn't pose problems over
short distances, especially when the ready availability of coastal bearings allowed
for tiny corrections all along the way. What killed the portolan tradition was long-
distance voyaging over blue water, when mariners discovered that sailing on a
fixed magnetic bearing didn't take them where they thought they were heading.
This is where Gerard Mercator enters the story of European cartography. Born
Gerard Kremer, he latinised his surname when he started publishing his work.
(Kremer is the Dutch equivalent of 'merchant' or, in Latin, mercator.) He began
hisworkinglifeasaninstrument-maker ratherthanatailor,yetheanticipated John
Speed as a mapmaker by starting out with a six-sheet map of Palestine, the Holy
Land of the Bible. It was a predictable point of origin, understanding the truth
of the Bible being the first task of the antiquarian. He quickly grasped that the
secret of producing an accurate flat map for a spherical earth was to work from the
sea and not from the land. The sea was where the problem of constant direction
was more acute; it also provided the cartographic innovator with a large unmarked
space in which to work out a solution to what was a purely mathematical problem.
Mercator started to develop that solution in 1541 by drawing rhumb lines on a ter-
restrial globeandconsideringtheconsistencies inhowthoselinescurved-foritis
in the nature of a sphere that every line drawn on a constant compass bearing turns
out to be a spiral, ending at either the North or the South Pole.
ThechallengeforMercatorwashowtodrawthisspirallinglinesothatitlooked
as straight on a flat map as it felt to a mariner when he was keeping on a constant
compass bearing. His solution was ingenious. Rather than bend the line to accom-
modate the actual shape of the land, he chose to distort the land. This distortion
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