An ambitious project to survey the nation's gravity field will soon allow us to calculate heights to within about an inch for most locations in the United States.

Mapping out hurricane evacuation routes and floodplains are just two cases in which it is critical to know accurate land heights. That's because heights tell us which way water will flow. How we calculate heights based on mean sea level today is good, but it's complicated, expensive, and time-consuming. But National Geodetic Survey experts are working to make it better, faster, and cheaper.

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