Home » Web Exclusive: Establishing Sea Level Datum
THE 1857 ANNUAL REPORT OF THE COAST AND GEODETIC SURVEY, APPENDIX 35, REPORT OF SUB-ASSISTANT H. MITCHELL ON OBSERVATIONS OF TIDES AND CURRENTS IN NANTUCKET AND MARTHA’S VINEYARD SOUNDS AND IN THE EAST RIVER AT HELL GATE, WITH REMARKS ON THE REVISION OF LEVELLINGS ON HUDSON RIVER, marks the first mention of leveling of more than just a local nature. Mr. G. B. Vose carried out the double series of leveling from New York City to Albany connecting the tide stations. Assistant Mitchell writes, "In order to place our results beyond all possible doubt, I directed Mr. Vose, to whom the leveling was assigned, to proceed slowly and with great care from station to station between New York and Albany. As you directed, a double series of levelings was made throughout the whole route and every doubtful step was retraced." Mr. Vose states, "From a hasty computation which I made, it appears that the probable error for the entire distance from New York to Greenbush does not exceed two-tenths of a foot. In some parts of the route—as, for instance, across the long bridge near Tivoli—it was necessary to run over the work five times." In Greenbush bench mark "Gristmill" was established at an elevation of 14.73 feet (which much later was determined to be about one foot too high, although this may be a result of differing tidal datums). No details as to the instruments, methods or results have been published and no record of the datum or origin has been found. The benchmark "Gristmill" was used by the United States Lake Survey as the basis of elevations in the great lake area as per a letter from the Superintendent of the United States Coast Survey dated July 9, 1880.
THE 1871 ANNUAL REPORT OF THE COAST AND GEODETIC SURVEY, APPENDIX 12, REPORT ON THE LEVELING OPERATIONS BETWEEN KEYPORT, ON THE RARITAN BAY, AND GLOUCESTER, ON THE DELAWARE RIVER, TO DETERMINE THE HEIGHT ABOVE MEAN TIDE OF THE PRIMARY STATIONS BEACON HILL, DISBOROUGH, STONY HILL, MOUNT HOLLY, AND PINE HILL, BY RICHARD D. CUTTS, ASSISTANT, COAST SURVEY, IN CHARGE OF SECONDARY TRIANGULATION. Written by Charles Ferguson, sub-assistant, published the results of the run from Keyport to Gloucester, a total length of 71 miles with 13 miles of offsets. As well as establishing elevations on the primary triangulation stations it established bench marks along its route and established the elevation of a mark, described as "Tuttle’s gate, Matteawan" and later known as "VI" in the report of 1882, as 16.780 meters. This mark was established as a "triangle upon a flagstone walk" and is one of a few ever recovered and releveled. In later reports the elevation for 1871 is listed as 16.7885 meters, although this may be a result of corrections applied to the levels. The elevations used in this report were established by tide observations consisting of ½ lunation at each end of the run. The leveling appeared to reveal that the ½ tides at Gloucester were 1.04 meters above those at Keyport. The leveling of 1871 is interesting in that it connects to a series of "barometer cisterns" used in 1870 to establish elevations at the listed primary stations and finds the cisterns to average 0.75 meter higher than the leveling with a range of +2.26 meters to -1.14 meters.
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