Art'noon LM,
Negative longitude eh?
That's a new one on me............
It's a concept I got used to it in early NASA satellite tracking programmes that were typically Americentric and would only accept West Longitudes, so you either put in values over 180 (which some progs spat out), or a negative value for Eastern longitudes!
Now it's mostly used behind the scenes in a lot of map-making programs. To quote Wikipedia:
For calculations, the West/East suffix is replaced by a negative sign in the western hemisphere. Confusingly, the convention of negative for East is also sometimes seen. The preferred convention—that East be positive—is consistent with a right-handed Cartesian coordinate system with the North Pole up. A specific longitude may then be combined with a specific latitude (usually positive in the northern hemisphere) to give a precise position on the Earth's surface.
Lunch break over now, so back to the grind 'til coffee time.