Published: Jan 2003
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Global Positioning System (GPS) surveying is helping to expand the sparse coverage of geodetic stations referenced to the North American Vertical Datum of 1988 (NAVD 88) in California. Accurate and precise vertical control is an important aspect of solutions to various hydrologic problems such as floodplain mapping and aqueduct gradient determinations. The National Geodetic Survey (NGS) has developed guidelines for GPS procedures that will result in local accuracies for ellipsoid heights of 2 cm at 95% confidence. The NAVD 88 datum in California is being expanded in segments of GPS networks that are observed by multiple-agency partnerships. Networks between San Francisco and Sacramento have been completed, and expansion in the Central Valley is being pursued. Determination and maintenance of accurate vertical heights in the Central Valley is particularly difficult because of large historical and unknown current effects of land subsidence resulting from massive quantities of groundwater withdrawal for several decades prior to the 1980's. Other NGS goals are to establish more geodetic and tidal datum connections, and to provide real-time water level information to harbor pilots for safer and more efficient vessel loading and navigation.
geodetic surveying, vertical control, Global Positioning System
Geodetic Advisor for California, National Geodetic Survey, Sacramento, CA