Secrets of the Selden Map - Map of China: Decoding the Secrets of a Vanished Cartographer - page 157

gicalSurvey,6°tothewestisroughlyconsistent fortheeasternedgeofAsiainthe
early decades of the seventeenth century.
The compass rose is not the only thing that tilts. So too do the routes. While
she was working on the map, my research assistant, Martha Lee, noticed that the
routes did not line up perfectly with the top of the map. In a world without mag-
netic declination these routes wouldbedrawninconformity with the Selden map's
north-south axis. In fact, they are not. Martha decided to compare the angles at
which the lines were drawn with the tilt of the rose. To give the Selden cartograph-
er reasonable room for error, she allowed him a margin of 2½° in either direction,
in keeping with the fact that the seventy-two points on the Chinese compass are
5° distant from each other around the circle. The easiest routes to test were those
having compass bearings of 180° ( wu ) or 0°/360° ( zi ). Allowing for the margin of
error that Martha assigned, they are drawn such that they deviate from the vertical
at a rate of roughly 6° to the left. The same pattern is true of the routes on other
bearings.Thevariationfromtruenorthisnotuniformly6°totheleftineverycase,
but it is close enough over all to conclude that the route information on the Selden
map has been magnetically coded to reflect the position of magnetic north at the
time it was drawn. The map, in other words, has a magnetic signature.
Now that we know this secret, there is something else on this map we can de-
cipher, and again I have Martha to thank for the discovery. As she measured the
angles of the route lines, she found that the route drawn most accurately in relation
to magnetic declination was the coastal freeway - minus the final run to Hirado,
which is skewed to the west by a further 5½°. Five other routes were drawn with
almost an equal degree of accuracy: the two routes connecting Moon Harbour and
Macao to Manila; the route running from Manila down the north-west coast of
Borneo, the one that connects the Gulf of Tonkin to Java; and another that runs
around Johor to Malacca and up the west side of the Malay Peninsula. The care
with which the Selden cartographer drew these routes suggests that he regarded
themasthemainlinksthatChineseshipsusedintheSouthChinaSeanetwork.All
the rest drift off their compass bearings. The route from Johor east across the bot-
tom of Borneo to Ternate starts well but soon wanders 10½° off its bearings. The
routedowntheeastsideofSumatratoBataviadoesthesame,althoughoncewithin
range of Batavia it gets back on course. The worst has to be the line running from
Moon Harbour out to the Ryukyus and up to Osaka. This route just gets wonkier
thefurtheritgoes,hitting adegreeoferrorof16½°offwhatitshouldbe.Butthen,
all of Japan is wonky: the Selden cartographer had never gone to Japan.