Longitude on a map is like vertical mapping lines on Earth which are well known as "meridians". One way to visualize this would be to think of splitting hula-hoops in half, then vertically, up and down, assembling them, with one positioned at the North Pole and one the South. Longitude lines show a numerical way to measure or show how far a location is east or west of the vertical line known as the Prime Meridian. The Prime Meridian runs vertically, north and south, over the British Royal Observatory, located in Greenwich England, from the North Pole to the South Pole.