You can see that the outlines of the island are not very accurate.
The general coast line is not very accurate and the islands
are drawn with quite some fantasy. There are no city names shown,
nor any indication where harbors were. The scale is interesting
though since the on the map nautical miles are mentioned, there
were 20 nautical miles in a degree, which makes a nautical mile
(Lieu Marine) 5600 meter. The south point's latitude is accurate,
though the longitude mentioned is about 250 km too much west.
However when one looks at the meridian of Paris (which is probably
used by La Pérouse, since no self respecting 17th century
French cartographer would use the Greenwich meridian), then
again it's pretty accurate. The island of Kapado is recognizable
but it is drawn west of the peninsula Songaksan, instead of
south of it. Even on a distance of 10 km, one can see those
details, so one can start to wonder if they really saw the island
or made the map on the oral account from somebody else.
A team of De la Pérouse's men created the first map
of Cheju Island, using scientific land surveying equipment,
and described it as a place of superb beauty.
He wrote "We sighted it on May 21 in the finest weather
imaginable and in most favorable conditions for observations.
I coasted along the south-east shore at a distance of leagues
and we surveyed with the utmost care a length of 12 leagues.
One would be hard put to find a more pleasing prospect. ...
The various crops which presented a wide range of colors made
the appearance of this island even more pleasing,"
However, he did not anchor on the island where Hamel and his
unfortunate companions were shipwrecked in 1653, worrying for
the safety of his crew members.
"Unhappily, it belongs to people who are forbidden to
communicate with strangers and who currently enslave those unfortunate
enough to be shipwrecked on their coast. This story, of which
we had an account before us was not of a nature to encourage
us to send a boat ashore," adding that their appearance
caused some alarm among the locals, who began to light signal
fires on all the headlands along the coasts.
The ship logs of De la Pérouse were translated into
English in 1994.
Take a look at the following comparison: the map is drawn over
a modern map of Cheju-do