U.S. Naval Observatory and the Washington Meridian

The meridian passing through the Old Naval Observatory in northwest Washington served as the de facto U.S. prime meridian on many American maps and charts in the 19th century, creating a national mapping standard different from the Greenwich Meridian. The observatory complex housed instruments and reference markers used for this purpose; the U.S. officially adopted the Greenwich Meridian for most purposes in 1912, but the Washington meridian remains a documented historical geodetic reference. The site therefore represents a historical cartographic anomaly where national mapping used a local meridian.