and LiDAR (Light Detection and Ranging) and make extensive use of the geodetic infrastructure for determining flight paths to centimeter accuracy after the data are collected.

Improved Floodplain and Inundation Maps

Floodplain maps are used to predict how water will flow on the Earth’s surface and are crucial to assessing the risk of floods. The creation of floodplain maps is an important part of the National Flood Insurance Program because these maps are used for setting flood insurance rates, regulating floodplain development, and communicating the one percent annual chance of flood hazard. The Federal Emergency Management Agency (FEMA) is undertaking an ambitious five-year program to update and make digital the floodplain maps of the nation (NRC, 2007b). These maps are derived from a combination of topographic data (elevations to an accuracy of 10 centimeters or better) and map of the geoid (refer to Figure 1.2), because water flows downhill relative to the undulating geoid surface. The North American Vertical Datum of 1988 (NAVD 88) is the official reference surface against which elevation measurements are made in the United States. NAVD 88, however, has an average bias of 1 meter and erroneous tilt amounting to an additional 1 meter error across the coterminous United States; it also has a 1–2 meter bias in Alaska (Childers et al., 2009a). Improving the accuracy of floodplain maps, therefore, will require improving the vertical datum, which in turn will require the use of either denser and more accurate geodetic leveling observations or Global Positioning Systems (GPS) measurements and a high-accuracy geoid model (NRC, 2007b). The National Geodetic Survey (NGS) has embarked on the GRAV-D Project (Gravity for the Redefinition of the American Vertical Datum), an airborne gravity mission to measure gravity and its changes more accurately than was previously achievable (NOAA, 2010; see Box 5.2). The goal of GRAV-D, therefore, is to model and monitor the Earth’s geoid, which serves as the reference surface for zero elevation. The new gravity-based vertical datum resulting from this project will be accurate at the 2 centimeter level for much of the country. The benefit of GRAV-D to society has been estimated at $4.8 billion over 15 years (Leveson, 2009).

Uses of Real-Time Geodetic Positions

Accurate real-time locations are used in a wide range of commercial applications and services. Accurate positions of Global Navigation Satellite System (GNSS)/GPS satellites in their orbits and a terrestrial reference frame are used to determine the location of an object on the surface of the Earth accurately. The NGS Continuously Operating Reference Station (CORS) Network, which enables precise real-time positioning for applications, including precision agriculture, surveying, and even GPS-guided snowplows, makes extensive use of the global geodetic infrastructure. The CORS Network, in turn, is a fundamental component of the National Spatial Reference System (NSRS),1 which provides a highly accurate and consistent geographic reference framework throughout the United States, allowing various layers of data to be spatially registered and integrated within geographic and land information systems (GIS/LIS). The NSRS has been estimated to provide benefits equivalent to $2.4 billion annually (Leveson, 2009). The NSRS, in turn, is the backbone of the National Spatial Data Infrastructure (NSDI), which was recognized in a 2004 report by the Federal Geographic Data Committee as the “primary mechanism for assuring (national) access to reliable geospatial data” (NSDI Future Directions Planning Team, 2004).

Real-time positioning data are often used by commercial augmentation services that provide corrections to standard GPS positioning to a global set of customers requiring sub-meter and deci-

1

The NSRS, defined and managed by the NGS, is a consistent national coordinate system that specifies latitude, longitude, height, scale, gravity, and orientation throughout the United States, as well as how these values change with time.



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