Giza is the third largest city in Egypt. It is located on the West Bank of the Nile River, some 20 km southwest of central Cairo. Along with Shubra El Kheima, Cairo and Helwan, the four cities form the Province of Greater Cairo metropolis. Giza is most famous as the location of the Giza Plateau: the site of some of the most impressive ancient monuments in the world, including a complex of ancient Egyptian royal mortuary and sacred structures, including the Great Sphinx, the Great Pyramid of Giza, and a number of other large pyramids and temples. The Great Pyramid of Giza at one time was advocated (1884) as the location for the Prime Meridian, a reference point used for determining a base longitude.