Washington Meridian Marker, 16th Street, NW, Washington, DC. Courtesy of Ghosts of DC.Architect and planner Pierre L’Enfant (1754-1825) had originally planned the City of Washington around a right triangle, with the eastern portion at the Capitol, the northern portion at the White House and the 90 degree angle close to where the Washington Monument sits today. Thomas Jefferson marked this spot in 1793 with a wooden post, which was replaced in 1804 with the Jefferson Pier.
The north-south line from this point to the White House would form a prime meridian for the United States, dubbed the Washington Meridian. Full story on Ghosts of DC.
More postings about Meridian Hill Park on Ghosts of DC.
Thanks to Rev. John DeTaeye of Councilmember Jim Graham's office for the tip.