Test results of first-order class III leveling

The National Geodetic Survey has programmed for a partial releveling of the firstorder, National vertical control net and for readjustment of the entire first- and second-order net during 1977 through 1985. In the past, first-order, class I or II, double-run leveling was used exclusively to establish and relevel the first-order net. Programmed funds permit releveling and replacing destroyed marks on approximately half of the National first-order net by double-run leveling, or on essentially the entire net by new first-order, class III, single-run leveling. The new specifications, based on an analysis of leveling errors, were field-tested on a level line from Waldorf to Baltimore, Md. between December 1975 and April 1976. Test results indicate that first-order, class III leveling can provide a viable alternative to first-order, class I leveling for releveling the first-order, National vertical control net.