![]() NGS makes a gravitational geoid model based on gravity data collected from a variety of sources and a “Hybrid” model that builds on the gravitational model using GNSS on bench marks to enable a fit to NAVD 88. Accuracy of GPS-derived orthometric heights depends on both the accuracy of the geoid model and the accuracy of the NAVD 88 control. It is possible to obtain accurate heights through GNSS, and NGS has published guidelines that define equipment requirements, field procedures and data collection parameters, basic control requirements, and processing and analysis procedures. Conventional geodetic leveling is precise but labor intensive, so it is not feasible or practical to level across the country today as has been done in the past additionally, leveling yields cross-country error build-up. The current Height Modernization program uses orthometric heights from traditional leveling and ellipsoid heights from GNSS to create a geoid height model referenced to the NAVD 88. Science of Height Modernizationīecause the surface of the Earth is so complex, geodesists use simplified, mathematical models of the Earth for many applications. Additionally, regional coordination and communication efforts are increasing neighboring states with similar needs and challenges are collaborating more, leveraging their resources to create stronger regional networks. Currently the Height Modernization Program continues its state-by-state implementation approach focusing on individual user or state needs, and NGS continues to provide technical expertise and support. Since the 1998 Report, the Height Modernization Program has been funded at varying levels with most appropriations received by academic institutions or state and local governments. Congress directed NGS to determine the effectiveness of using GPS techniques to establish accurate heights in California and North Carolina, and the ensuing National Height Modernization Study officially documented the needs and benefits of accurate heights. The new datum, called the North American Vertical Datum of 1988 (NAVD 88), was defined through the leveling network and is the official vertical datum for civilian surveying and mapping activities in the United States performed or financed by the Federal Government.ĭuring the 1980’s and 1990’s, there were considerable indications that the utilization of new advances in GNSS technology could achieve significant efficiencies and cost savings. In 1988, the vertical control network, including several thousand kilometers of new leveling, was again mathematicallyadjusted to remove inaccuracies and to correct distortions. Since then, movements of the Earth’s crust have changed the elevations of many bench marks, and other bench marks have been disturbed or destroyed as communities expand or roadways widen. In 1929, NGS compiled all of the existing vertical bench marks and created the National Geodetic Vertical Datum of 1929 (NGVD 29). Since the first general adjustment of the geodetic leveling network was completed in 1900, NGS has been responsible for defining the official, legal vertical reference frame in the United States. History of NHMPĮarly morning GPS observations at the water gap at Harper's Ferry, West Virginia. The goals of the National Height Modernization Program (NHMP) are to provide access to accurate, reliable heights nationally to develop standards that are consistent across the nation to provide data, technology, and tools that yield consistent results regardless of terrain and circumstances and to establish a system or process that is maintainable over time. The traditional method for determining the elevation of these vertical bench marks is differential leveling, but the advanced technology of Global Navigation Satellite System (GNSS) has begun to replace this classical technique of vertical measurement in many situations. Round brass disks (bench marks) set in concrete or bedrock or stainless steel rods driven to refusal are most often used to mark positions in the vertical datum. The vertical component of the NSRS is the vertical datum, which is a collection of specific points on the Earth with known heights or elevations above or below a reference surface approximating mean sea level. The NGS mission is to define, maintain, and provide access to the National Spatial Reference System (NSRS) to meet our nation’s economic, social, and environmental needs. Brass monuments permanently affixed in concrete or surrounding bedrock indicate accurate geodetic reference positions within the NGS horizontal and/or vertical datums.
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