Modern geographers measure degrees of longitude from the Prime Meridian running through Greenwich, England, but things didn’t always work this way. In fact, America, like many other countries, long maintained its own “Prime Meridian” for domestic and international cartographic measurements and surveys. This meridian was the basis for determining state borders and other domestic surveys, and formed a key part of demarcating a rapidly growing United States.
The map above, from the Bureau of Land Management, U.S. Department of the Interior, shows the major survey lines used to demarcate the boundaries of most modern U.S. states. The borders of the first 17 states are not included on this map because they predate the establishment of the survey system. Texas is also exempt as its boundaries were established by treaty. Many of the secondary baseline meridians on this map were full degrees of longitude from one of several American meridians. Click here or on the picture above to enlarge the map.
America actually had four separate prime meridians, all of which ran through Washington, DC. The first was established in 1791 by Pierre Charles L’Enfant, the planner who designed the new capital city. L’Enfant established America’s first prime meridian running through the center of the U.S. Capitol Building, and used this as the basis for his original plans for the city.
L’Enfant designed his city around a giant right triangle, with vertices at the Capitol, the White House, and the Washington Monument. The leg of the triangle between the White House and the Washington Monument became America’s second prime meridian in 1793, as surveyed by then Secretary of State Thomas Jefferson. Stones marking this meridian can still be found on the National Mall today.
The second meridian persisted until 1850, when Congress established a new, third meridian explicitly for domestic surveys while adopting the British Prime Meridian for all nautical calculations. The new American meridian would run through the old U.S. Naval Observatory, now the U.S. Navy Bureau of Medicine and Surgery, in DC’s Foggy Bottom neighborhood. This meridian persisted until 1897, when the U.S. Naval Observatory moved to its current location on Massachusetts Avenue. Visitors to the area can find a small monument showing this meridian on the grounds of the George Washington University.
The third meridian was, by law, used for all domestic surveys from 1850 to 1897–the years of some of America’s greatest territorial expansion. As such, many of the Western states have borders based on this third meridian. Both the eastern and western borders of Colorado and Wyoming, for example, are demarcated at fixed, full degrees from the 1850 meridian. The table below shows all state boundaries based on this meridian:
25° W Kansas (29 January 1861) as a state
E Colorado (28 February 1861), NE not dependent on Kansas
27° E Montana (3 March 1863) as Idaho Territory
E Wyoming (3 March 1863) as Idaho Territory
W North Dakota as Dakota Territory
W South Dakota as Dakota Territory
32° W Colorado (28 February 1861)
E Arizona (24 February 1863)
W New Mexico
34° SW Montana (26 May 1864)
W Wyoming (25 July 1868), SW not dependent on Montana
37° E Nevada (5 May 1866) as a state (39° → 38° → 37°)
39° NW Montana (26 May 1864)
America’s fourth meridian, running through the new U.S. Naval Observatory, was used for several decades. It wasn’t until the International Meridian Conference in 1884 that most countries, including the United States, agreed to standardize their meridians on the British line.