Secrets of the Selden Map - Map of China: Decoding the Secrets of a Vanished Cartographer - page 155

The reason for the startling accuracy of the Selden map is now clear. It looks
geographically correct not because the mapmaker traced the outline of a European
map, although he could well have seen one. It looks as well as it does because it
wasdrawnfromthesea.Thequality oftheresult isduetothequality ofthedata in
his rutters. Complete accuracy escaped the Selden cartographer, given his inability
to convert time uniformly into distance, but near accuracy was possible. The fact
that the Selden and Speed maps resemble each other is hardly surprising, for both
patchedtogetherthispartoftheworldbyworkingfromthewater.Itcouldnothave
been any other way. Both were engaged in roughly the same endeavour: producing
charts that would display what Selden called 'the Publique Right of Mutual Com-
merce' in order to facilitate the movement of cargo vessels through the China seas.
Now for one more startling discovery; at least it startled me. We speculated
earlier that the ruler beneath the compass rose may have had something to do
with how the map was drawn. Knowing now that the map is scaled at a ratio of
1:4,750,000, can we make the ruler fit with that scale? Let us do the math. If one
inch on the Chinese ruler (which as drawn measures 3.75 cm) equals a day's sail
at a speed of 6¼ knots, which calculates as 150 nautical miles (240 km), then the
distance represented by 1 cm on the ruler is 64 km. This tells us that the ruler was
drawn on a ratio of 1:6,400,000. This is too small for the Selden map, although
parts of Vietnam were drawn to that scale. But suppose we change the value of an
inch on the ruler? One way of getting the scale up to 1:4,750,000 is by slowing
down the ships. A slower speed would produce a smaller distance, which would
in turn push the scale up to the scale prevailing over most of the map. What speed
would do that? Four knots.
This is the startling bit, for 4 knots is the speed that the late Xiang Da came up
with when he patiently tried to work out actual distances against reports of time
travelled in the rutter. Xiang was the scholar who annotated the Laud rutter for
publication in 1959, and so he knew that text better than anyone. If the speed he
came upwithworksouttobethespeedthat worksbestthedistances ontheSelden
map,thisishardlycoincidental.ItcanonlybebecauseboththeLaudrutterandthe
Selden cartographer were working from roughly the same navigational data.
This was the last discovery I made while writing this topic, and it pleased me
no end. I have a fondness for Xiang. Xiang was born in 1900, the same year as
my mentor in this field, the great Cambridge historian of Chinese science Joseph
Needham. In 1935 Xiang travelled from China to Oxford to catalogue the Chinese
collection in the Bodleian, just as Michael Shen had done. Recognising the value