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This distortion doesn't mean that the cartographer made a hash of what he was
doing. Rather, it signals that, try as he might, he had no hope of ever getting all his
routestolineupproperly.Ifwepausetothinkwhythismightbe,wealreadyknow
the reason: curvature. Over this great a distance, a course on a straight bearing in
reality hastocurve;andifyouhavenowaytocompensate forthecurve-whichis
what the Mercator projection allows you to do - you end up sailing off at a tangent
to the course you intended to travel. Our cartographer didn't know this, nor did he
have a projection that could compensate for the three-dimensionality of the globe
whenhedrewhismap.Hehadonlyonecourseofaction: hehadtocheat. Tomake
sure that the most important lines stayed as closely aligned as possible to their true
magnetic direction, he had to jimmy the others so that they all linked up plausibly
to where they were supposed to. He was probably stumped by his discovery - and
he would have had to make it in the course of drawing the map - that uniform ac-
curacy was impossible. The route data were correct, and yet he couldn't combine
them visually. Nor did he have a way to account for this difficulty theoretically.
The best he could do was control the distortion by cheating here and there on the
secondary routes. He did it well, to judge from the end product.
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Fromthisrevelationflowsanevenmorefascinatingdiscoveryleadingtothemap's
fourth secret: there is a pattern to this cheating. This only became apparent when
Martha aligned the Selden map to GIS, the Geographic Information System. Using
a technique called geo-referencing, she took identifiable points on the Selden map
and matched them to their GIS positions. If you think of the Selden map as drawn
on rubber, then what she did was stretch it onto a map of the world as we know it
today.Theprocess iscalled 'rubber-sheeting'. Ahistorical mapdrawnwith careful
attention to distance and direction will require little stretching when it is rubber-
sheetedontotheworld.Wherethestretchisgreatestrevealswherethedistortionin
the original is greatest.
Given the reasonably high accuracy of the map, Martha didn't have to stretch
themapmuchtolayitontoitsGIScoordinates.Themainthingsheendedupdoing
was to break the original map into three large chunks plus a few smaller pieces
(Fig. 26). When she did that, a gap opened in the centre of the map. To put this in
terms of what the Selden cartographer did, the only way he could make his map
work was to push the land masses around the South China Sea closer to each oth-
er than they actually are. The South China Sea was an easy place for him to cheat
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