The 1915 Mileage Tabulation

The first calcuation of shoreline mileage was in 1915. It consisted of tabulations for a general coastline, tidal shoreline with a unit measure of 3 statute miles, and a tidal shoreline with a unit measure of 1 statute mile. Although measurements used a set of measurement dividers and opisometer, mileage varied between these three measurements due to the scale of the charts used and the rules regarding shoreline extent.

Mileage tabulations between 1939-1959

Tabulations continued to be updated as technology improved. Dividers were replaced with an opisometer and used in the 1948 and 1959 shoreline remeasurements. This measuring instrument consists of a small wheel that rotates as it moves along a path. As the wheel traces the shoreline, a counting mechanism uses the number of rotations to record the distance traveled. Another major addition to the mileage summations occurred in 1959 when mileage was calculated for Alaska, Johnston Island, Navassa Island and Swan islands.

The 1961 tabulation

The last published tabulation for general coastline and tidal shoreline (detail) was in 1961. This tabulation used the largest scale nautical charts and included shoreline of outer coast, offshore islands, sounds, bay, rivers, and creeks to include the head of tidewater or to a point where water narrows to a width of 100 feet in the measurement. (Shallowitz, 1964) It was determined that the United States and its terrorities had 95,491 statute miles. The Coastline of the United States provides a breakdown of shoreline mileage for each state and territory.

The 2021 tabulation

Unlike past tabulations that used shoreline from the largest scale nautical charts, this modern tabulation used NOAA’s Continually Updated Shoreline Product (CUSP). This shoreline is a continuous vector that is updated frequently using both NOAA (predominantly National Shoreline) and non-NOAA contemporary sources. CUSP is a multi-scale shoreline referenced to mean high water, where applicable and is derived from a number of technologies, such as lidar, mono and stereoscopic compilation and automatic feature extraction . Shorelines, breakwaters, jetties, groins and rocks were included in the 2023 remeasurement process.

The CUSP shoreline was clipped to a 500 meter buffer generated from shoreline found on the largest scale electronic navigational charts. The clipped CUSP vector was then generalized using a simplification algorithm and modified to eliminate rivers and creeks that were less than 12 meters wide.

Mileage for each state and territory was then tabulation using geographic information system (GIS) software packages.