Principal meridian
A meridian is an imaginary line on the Earth's surface from the North Pole to the South Pole that connects all locations with a given longitude. Each is half of a great circle on the Earth's surface.
In the United States Public Land Survey System, a principal meridian is the principal north-south line used for survey control in a large region, and which divides townships between east and west. The meridian meets its corresponding baseline at the point of origin, or initial point, for the land survey. For example, the Mount Diablo Meridian, used for surveys in California and Nevada, runs through the summit of Mount Diablo.
Often, meridians are marked with roads, such as the Meridian Avenue in San Jose, California and Meridian Road in Vacaville, California, both on the Mount Diablo Meridian.