Tintin and location by geographic coordinates

Thierry Joliveau, Professor of Geomatics at the University Jean Monnet of Saint-Etienne (France) is describing in his interesting blog :

"A systematic inventory of maps in The Adventures of Tintin has still to be be done, but, surprising though it might seem, the reporter and globetrotter does not use maps very often. Take just one example : the Red Rackham’s Treasure album, that Steven Spielberg is adapting for the cinema.
No map is shown in the book, if we don’t count the globe at the very end of the album (see below). The story is, nevertheless, fully geographical.
When Haddock and Tintin ship out in search of Red Rackham’s treasure, they have two clues: a pair of geographic coordinates and a mysterious reference to “The Cross of the Eagle”.
And their journey on the Sirius begins with a localization error.

Tintin finally understands that the coordinates provided by the ancestor of the Captain, the Knight of Hadoque, were calculated in reference to the Paris and not the Greenwich Meridian, which became the official international reference in October 1884, after a long French resistance for imposing the Paris Meridian.
Knowing that the Paris Meridian is located 2° 20‘ 16” further east than the Greenwich Meridian, we see that the Island is located at 20° 37′ 42″ North latitude, 68° 31‘ 59” West longitude.
Actually to be exact 2° 20' 16.0058" is the offset with ED50 European Datum 1950. The right value is 2° 20' 11.4874" with RGF93 French Geodesic Network (very closed to WGS84).
This episode is often used by teachers as a geometry exercise (example).

Nowadays we simply enter these coordinates into Marine GeoGarage (20.628333/-68.534444, longitude as a negative digital number, since we are located west of the Greenwich Meridian) and we observe that the wreck of the Unicorn is located near Saint-Domingue’s coast.
If you have Google Earth on your computer, you can also use this kmz file (spiced up with a little surprise picture).
Note that Hergé placed the island not far from the shoals of Navidad Bank and Silver Bank, famous places for the number of ancient shipwrecks.

This episode confirms the well-known fact that Hergé’s creation was a well-researched and a very documented process, at last since The Blue Lotus."

But despite this realistic dimension, in a website written by another Tintin's fan, Nicolas Saborin pointed some approximation relative to geographic coordinates and globe representation.

Actually, at page 61, this is a curious globe which stands at the feet of the statue of St. John the Evangelist.
If the meridian of Greenwich (or Paris) is indeed well represented, we can also see the meridian 70° W (longitude of the wreck) and the meridian 50° E (eastern point of Africa).
Some would rather have drawn meridians every 60°.
A parallel is represented at the height of the middle of Spain, 40° N.
So finally, the point representing the wreckage is placed too high.
Its latitude (20° N) should be halfway between the equator and the 40° N parallel. Of course !