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Observing Gravity to Protect Lives, Property

A flooded road in the Southern U.S.

Very precise height measurements make a big difference, especially when it comes to predicting and preparing for flood conditions. Just 2 to 5 centimeters (one or two inches) of error in the height of this road, for instance, might make all the difference in predicting if it will flood.

The National Geodetic Survey (NGS) is moving forward with an effort to measure and monitor variations in the gravity field of the earth, part of an ambitious program to provide a better, faster, and less expensive way to acquire accurate elevations for the nation.

On Oct. 20, NGS began collecting airborne gravity and other observations along the Gulf Coast of Louisiana.

The eventual goal of the project, called the Gravity for the Redefinition of the American Vertical Datum (GRAV-D), is to compile observations throughout the entire U.S. to form a more accurate model for defining heights.

Accurate height measurements are critical for determining water flow. Accurate water flow information, in turn, is critical for monitoring and mapping storm surge, tsunamis, flood plains, irrigation, and navigation.

A flooded road in the Southern U.S.

NGS plans to map the coasts first. In addition to the recent Louisiana flights, staff have already gathered observations over Mississippi, Alabama, and parts of Florida. The Survey hopes to complete observing the coast of Texas later this year, wrapping up observations for the flood- and hurricane-prone Gulf Coast.

While the Global Positioning System (GPS) revolutionized the measurement of latitude and longitude—providing pinpoint coordinates anywhere on earth—there has not been a comparative leap in technology for height measurement. The NGS program is designed to bridge this gap for the U.S.

GPS height measurements today are not very accurate because the system uses an oversimplified model of sea level to calculate height.

Once completed, the new NGS model will permit fast and accurate height measurements within an accuracy range of two centimeters (.8 inches) in the U.S. The new model will be incorporated into future GPS receivers.

Project managers expect that the project, while costly upfront, will be cheaper in the long term since it will virtually eliminate the need for people to gauge heights by hand using land-based survey technology.