BOX 5.2
Gravity for the Redefinition of the American Vertical Datum (GRAV-D)a
NOAA’s National Geodetic Survey (NGS) has a federal mandate to provide accurate positioning, including heights, to all federal, non-military mapping activities in the United States. Accurate heights are critical to many scientific endeavors but are particularly important to understanding and protecting low-lying coastal areas, which are subject to flood hazards. In 2007, NGS embarked on the GRAV-D project to determine the gravity-based vertical datum (elevation) with two-centimeter accuracy for much of the country. Because of the fundamental connection between the Earth’s gravity field and the definition of height above mean sea level (see Figure 1.2), complete gravity coverage of the continent is needed to connect the geometric height system measured by GNSS/GPS to the physical height system referred to as the geoid. The goal of GRAV-D is therefore to measure the Earth’s geoid. “The geoid is theoretical only. You can’t see it, touch it or even dig down to find it. Simply put, the geoid is the natural extension of the mean sea level surface under the landmass. We could illustrate this idea by digging an imaginary trench across the country linking the Atlantic and Pacific oceans. If we allowed the trench to fill with seawater, the surface of the water in the trench would represent the geoid. Not a bad way to imagine the geoid, but in reality not something we could easily do” (Natural Resources Canada).b
The GRAV-D project consists of three major campaigns:
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A high-resolution “snapshot” of gravity in the United States will be obtained by a predominantly airborne campaign. The highest-priority targets are the coastline of the continental United States and the American island holdings.
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A low-resolution “movie” of gravity changes determined by episodic re-visits of absolute gravity sites in an attempt to monitor changes to gravity over time at selected points. Space-gravity missions like GRACE are essential to monitoring changes in the geoid at regional scales.
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The third component depends on regional surveys where NGS collaborates with local governmental, commercial, and academic partners that are willing to support airborne or terrestrial surveys or to monitor local variations in the gravity field.
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