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The Earth and Its Coordinate System
Published in Terry A. Slocum, Robert B. McMaster, Fritz C. Kessler, Hugh H. Howard, Thematic Cartography and Geovisualization, 2022
Terry A. Slocum, Robert B. McMaster, Fritz C. Kessler, Hugh H. Howard
In a similar manner, vertical datums used in the United States have also experienced updates as new techniques to measure gravity across Earth's surface, for example, have been developed. An early vertical datum used in the United States was the National Geodetic Vertical Datum of 1929 (NGVD29). This datum was developed from observing mean sea level at 26 tidal gauges (21 in the United States and 5 in Canada) located in harbors along the coastal waters. Observations of water heights at these gauge stations were recorded over an approximate 19-year period (235 lunar months) which corresponds to a Metonic cycle. It is known that tides along coastal waters rise and fall in a temporal manner due to gravitational interactions between the sun, moon, and Earth as each orbits the other. The Metonic cycle allows these interactions to reach a maximum and minimum from which an adjusted tidal height observations is computed. As a result, observations at all tidal stations throughout the United States had a common reference or zero elevation. However, the assertion that local mean sea level was constant at any gauge stations was fundamentally flawed as mean sea level is not constant between any two stations. Later analysis confirmed that fixing a zero elevation at these tidal gauge stations introduced considerable error in elevation values across the United States.13
Force-System Resultants and Equilibrium
Published in Richard C. Dorf, The Engineering Handbook, 2018
The NGVD 29, a long-used reference for mean sea level in the U.S. was the product of a 1929 general adjustment of the U.S. and Canadian vertical control networks. The 1929 adjustment was based, in part, upon the assumption that the local mean sea level at the tide stations used in the adjustment was equal (same equipotential surface). This is not a valid assumption since the elevation of mean sea level varies from the Atlantic coast to the Pacific coast of the U.S. This distortion of the vertical datum caused the official name of the 1929 datum to change from “Sea Level Datum of 1929 ” to “National Geodetic Vertical Datum of 1929” in 1976. Other distortions, including those from upheaval and subsidence of the Earth’s crust, are present in the NGVD 29; however, it remains a datum of reference for the U.S.
Effects of water level and climate on the hydrodynamics and water quality of Anvil Lake, Wisconsin, a shallow seepage lake
Published in Lake and Reservoir Management, 2018
Dale M. Robertson, Paul F. Juckem, Eric D. Dantoin, Luke A. Winslow
Anvil Lake's stage has been measured by observers since 1936 using staff gages with established nearby datums. In addition, stage was measured almost continuously (15 min) with a stage recorder from June 2011 to 2016 by the USGS (Fig. 2). To verify the consistency of the various datums used throughout this period, historical photographs with the water's edge identified were obtained (Gillum S, Anvil Lake Assoc., Sep 2014, pers. comm.). Historical datums, identified water's edges, and coinciding water level on 25 September 2014 were then tied to sea level (NGVD 29) using a Real Time Kinematic (RTK) satellite GPS and standard surveying (dumpy) equipment and used to validate historical time series. The surveyed elevations of the historical photos were consistent with those measured, resulting in almost a continuous elevation record since 1936, with only a few small breaks.