Bem, para citar Memória Alfa :
According to Star Trek Maps, the XYZ spatial coordinates 0,0,0 used by the Federation are those of a central navigation beacon located near the core worlds of the Federation. The galactic XYZ values are in a grid in relation to this central beacon. Units in the grid are parsecs. The first value X is the distance towards (positive) or away from (negative) the center of the galaxy. The second value Y is the distance towards the left "Alpha Quadrant" direction (positive) or the right "Beta Quadrant" direction (negative). The third value Z is the distance in the galactic north direction upwards (positive) or galactic south downwards (negative) away from the galactic plane.