NJDEP - NJGS - Bulletin 66, Mapping Digest for New Jersey, 1965

VERTICAL CONTROL DATUMSFirst-order control leveling by the United States Coast and GeodeticSurvey was begun in 1878 to provide elevation control for trigonometricleveling which accompanied the Transcontinental Arc of Triangulation.As additional level lines became available and connections to tidal stationsincreased in number, various adjustments of the level net were required.Since instrument and rod accuracies improved over a period of years,the weights assigned to level lines in each adjustment were based uponinstruments and methods used in each leveling operation. Beginningabout 1910, an orthometric correction was applied to level circuits. Becausethe earth is an oblate spheroid, level surfaces are not parallel butconverge toward the poles. As altitude above sea level increases, eachsucceeding level surface has a greater proportlonal flattening than thesea surface. Hence level surfaces with elevations of 500 and 1000 metersabove the sea at the equator rise 497.5 and 995 meters, respectively,above the sea at the poles. The orthometrie correction is maximum fornorth-south lines at high elevations, zero for east-west lines, and probablynegligible for a line run along a coastal plain. When applied toleveling in New Jersey, this correction never exceeded three-tenths of amillimeter between two successive bench marks, and the total effect onentire level lines was always less than two millimeters?The first level net adjustment occurred in 1899 and was followed bypartial adjustments in 1903, 1907 and 1912. In 1929 the entire levelnet was completely readjusted. In addition, Special Adjustments wereperformed in 1927 and 1929 for the express purpose of testing the variationof mean sea level from a level surface. The 1927 Special wasbased upon the first-order level net of the United States. The 1929Special extended this study by combining the level nets of the UnitedStates and Canada. The results of these Special Adjustments indicatethat mean sea level slopes downward to the east on the Gulf Coast andupward to .the north along both the Atlantic and Pacific Coasts; furthermore,mean sea level on the Pacific Coast is measurably higher thansea level along the Atlantic Coast. Both Special Adjustments basedelevations upon sea level at Galveston, Texas, and derived sea levelplanes for tide stations on .the Gulf, Atlantic and Pacific Coasts withrespect to the Galveston datum. The 1929 General Adjustment wascomputed by holding sea level fixed as observed at twenty-one tide stationsin the United States and five in Canada. The resulting datum fromwhich all elevations in the adjustment are based is referred to as theSea Level Datum of z929.Preceding the 1929 General Adjustment, mean sea level at the Sand),Hook, New Jersey, tide station was the datum for bench mark elevations.References to the "Sandy Hook Datum" still persist, even thoughthe term has been technically incorrect since the 1929 General Adjustmerit,z17NEW JERSEY GEOLOGICAL SURVEY