As there is no reference that designates one specific shoreline as the “legal” shoreline, numbers for the length of the U.S. shoreline can vary depending on how the shoreline is defined.
The NOAA figure was determined by hand in 1939-40 with a recording instrument on the largest-scale charts and maps available at that time. Shorelines of outer coast, offshore islands, sounds, bays, rivers, and creeks were included to the head of the tidewater or to a point where tidal waters narrow to a width of 100 feet. For the Great Lakes, the shoreline lengths were measured in 1970 by the International Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.
The total length of tidal shoreline includes measurements of the coastal states as well as the outlying U.S. territories and possessions.