Washington meridians

The Washington meridians are four meridians that were used as prime meridians in the United States which pass through Washington, D.C. The four that have been specified are:

  1. through the Capitol
  2. through the White House
  3. through the old Naval Observatory
  4. through the new Naval Observatory.

Their longitudes may be reported in three ways:

  1. relative to the local vertical used by astronomic observations
  2. relative to NAD 27 (North American Datum 1927), an ellipsoid of revolution that is at mean sea level beneath triangulation station Meades Ranch, Kansas (not Earth-centered);
  3. relative to NAD 83, an Earth-centered ellipsoid of revolution with dimensions chosen to best fit the undulating (±100 m) geoid (world-wide mean sea level).

NAD83 longitude of the Capitol is about 1.1 arc seconds less than its NAD27 longitude; astronomic longitude there is about 4 arc seconds less than NAD83.

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