----------------------------Original message----------------------------
Sheila,
The longitude in that instance is measured only east of the prime meridian,
from 0 to 360. As for the prime meridian used for that map, I would expect
it to be "Ile de Fer" or, variously, "Ferro," or "Hierro" which was
originally defined as 20 degrees west of Paris, and is about 17:40 west
of Greenwich. To determine the Greenwich "western" equivalent the
following simple computation is used
360 - 280 = 80 (degrees west of PM used)
80 + 17:40 = 97:40W (degs west of Greenwich, assuming Ferro)
Remember that in 1748 longitude values were approximate.
Chris Baruth
AGS Collection