A reference system used to locate positions on the earth's surface. Distances eastwest are measured with lines of longitude (also called meridians), which run northsouth and converge at the north and south poles. Distance measurements begin at the prime meridian and are measured positively 180 degrees to the east and negatively 180 degrees to the west. Distances northsouth are measured with lines of latitude (also called parallels), which run eastwest. Distance measurements begin at the equator and are measured positively 90 degrees to the north and negatively 90 degrees to the south.