A U.S. National Gravity Base Net, to be published soon by the Coast and Geodetic Survey, was established on the mainland of the United States in 1966–1967 through the cooperative efforts of the U.S. Air Force 1st Geodetic Survey Squadron, U.S. Army Topographic Command (USATOPOCOM), the Coast and Geodetic Survey, the University of Hawaii, and the Ohio State University. The net consists of bases located in 59 cities throughout the conterminous United States. Survey operations were divided into two phases. Phase I consisted of measurements made in ladder sequence (ABCDEEDCBA) between major airports in 59 cities with four small geodetic model LaCoste-Romberg gravimeters (43, 47, 48, and 93). In phase II one-way measurements were made between cities, and excenter bases in and around each city were tied to the primary airport base in that city. LaCoste-Romberg gravimeters 43, 47, 93, and 115 were used in phase II.