Washington meridians

Pierre (Peter) Charles L'Enfant specified the first meridian in his 1791 "Plan of the city intended for the permanent seat of the government of the United States .

He ran all the lines by a transit instrument, and determined the acute angles by actual measurement, and left nothing to the uncertainty of the compass.

[2]The longitude of the center of the Capitol's dome (completed in 1863 during the Civil War) is now given by the National Geodetic Survey as 77°00′32.6″W (NAD 83).

The west side of L'Enfant's triangle forms a natural prime meridian passing through the center of the President's house.

"[16] The observatory decided that this meridian passed through the center of the original (small) dome atop the main building of the Old Naval Observatory, now abandoned southwest of the corner of E and 23rd Streets in Foggy Bottom (north of the Lincoln Memorial and west of the White House).

The clock room is a small building at the exact center of the 1,000 ft (300 m) radius observatory grounds, whose northern entrance is at 34th Street and Massachusetts Avenue.

West side of Jefferson Pier in April 2011 with Washington Monument in background.
Zero Milestone, 1923, looking north toward the White House.
Tablet facing 16th Street, NW, on west wall of Meridian Hill Park in 2006. The tablet states that a Washington Meridian marker stone was formerly located 52 feet 9 inches (16.1 m) west of the tablet.
Entrance marker stone near Silver Spring in traffic circle (Blair Circle) in December 2011
Meridian four blocks north of Old Naval Observatory, looking south in August 2005. [ 15 ]
Four Corners Monument 32° west of Washington Meridian, after 2010 reconstruction. [ 18 ]