HEIGHT DIFFERENCES BETWEEN NAVD 88 AND NGVD 29 IN CONTERMINOUS UNITED STATES UNITS = CM (NOAA)Īccording to the NGS website, surveyors used a method called Differential or Spirit Leveling, where a rod is held on a mark with a known elevation. The data that surveyors gathered were extremely accurate given the tools they had and the conditions they endured.” “This enormous leveling undertaking supplemented the transcontinental arc of triangulation that followed the 39th parallel from Delaware across the country to California from 1877 to 1900. “Using the Hagerstown benchmark as a starting point, benchmarks were set westward to assist in leveling the wild terrain,” wrote Aria Remondi, in one of a series of articles on NOAA’s website. The first benchmark in this surveying effort was placed at Hagerstown, Maryland, in 1877. Coast and Geodetic Survey (renamed in 1878) to lead a national geodetic project. Berry described Vose’s confidence in his work: “From a hasty computation which I have made,” the surveyor claimed, “it appears that the probable error for the entire distance from New York to Greenbush does not exceed two-tenths of a foot.” Vose’s benchmark was used in 1875 to set the elevation of the Great Lakes.Ĭongress eventually authorized the U.S. Vose along the Hudson River between New York City and Greenbush (now Rensselaer), New York, across from Albany (“History of Geodetic Leveling in the United States,” Surveying and Mapping, June 1976, Vol. Coast Survey from 1856 to 1857, when a series of levels was run by G.B. (For) anything that deals with height that is more than just local, you need to have consistency.”Īccording to a 1976 article by Ralph Moore Berry, then assistant to the director of the NGS, the first real geodetic leveling was conducted by the U.S. “You need a national datum for national projects. if you are using a topographic map in North Carolina and then you cross into Tennessee and the maps are using different vertical reference frames, how would they match?” says Zilkoski. So why do we need them? “Because when you’re going to build a road like an interstate. This accounts for sea-level changes caused by the sun, the moon, and local variations due to storms, he says. But you can model it by measuring data over 18.6 years to calculate a mean value. What sea are you leveling? Where? And how do you account for the fact that the sea, unlike the floor, is in constant motion? You cannot measure it. Of course, measuring the sea is a lot trickier than measuring the floor in your house to check a person’s height. The vertical datum used by surveyors across the country is mean sea level, obtained through a tide gauge. You start somewhere and you say, ‘I know this number.’ In the case of measuring your height, you’re calling the floor zero.” “If you didn’t know that you’re measuring from where their feet touch the floor, you don’t have a good reference. You determine this by measuring from a person’s feet to the top of his or her head - the reference surface is the floor. He uses the example of measuring someone’s height. Zilkoski, the former director of the National Geodetic Survey at the National Oceanic and Atmospheric Administration and the man who helped supervise the monumental 14-year NAVD 88 project. “A vertical datum is just a reference surface,” says David B. To appreciate the ways in which the vertical datum ties the country together, it is helpful to know what it is. Zilkoski, the former director of the National Geodetic Survey at the National Oceanic and Atmospheric Administration, helped supervise the monumental 14-year NAVD 88 project. Army Corps of Engineers - will be replaced by a new, satellite-based system.ĭavid B. In a few years the key datum of the last 30 years, the North American Vertical Datum of 1988 - the datum used by federal agencies such as the Federal Emergency Management Agency and the U.S. In the late 19th century and through the early decades of the 20th, multiple vertical datums - the standard for accurate measurement of elevation across large distances - existed across the country.īut over time these datums became consolidated. In 19th century America, the expansion of the railroad famously led to the creation of our modern time zones, imposing temporal order on a nation that to that point set time by dozens of local “high noons.” In bridging east and west and transforming America into a continental nation, the railroads also shaped the development of geodetic surveying. Surveyors use a Fischer level at Glen Canyon in 1921.
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