Geodesy, geoids, and vertical datums : a perspective from the U.S. National Geodetic Survey
Roman, Daniel et al.
NOAAAs National Geodetic Survey is responsible for maintaining the U.S. National Spatial Reference System, which includes positions in both ellipsoidal and geopotential or orthometric frameworks. As part of this, geoid height models developed by NGS exist in one arc-minute (2 km) grids and provide ready transformation between ellipsoidal and orthometric datums. In the United States, NAD 83 serves as the ellipsoidal datum suitable for use with GPS surveys while NAVD 88 is the orthometric datum suitable for use in leveling surveys (for the Conterminous U.S.A. and Alaska). Thus NGS provides the means for transforming coordinates easily and accurately derived through GPS into orthometric heights more suitable to applications involving waterflow (e.g., flood plain determination). Recently released models include the USGG2009 and GEOID09 models for all regions of the United States and its territories. These models are being uploaded and employed in GPS software as well as used in post-processing routines such as NGSSs Online Positioning User Service (OPUS). USGG2009 is built upon the EGM2008 model, which is, in turn, based on GRACE gravity satellite mission data. USGG2009 also incorporates millions of surface gravity observations over the entire region of North America and the oceans around it. GEOID09 was developed starting from USGG2009 and combining it with nearly 20,000 points where GPS-derived NAD 83 ellipsoidal heights are known on NAVD 88 leveled bench marks (GPSBMMs). These GPSBMMs act as control points for determining a conversion surface between the geopotential datum determined by USGG2009 and that of NAVD 88.
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