----------------------------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