Henry Grabar is a staff writer for Slate’s Moneybox and a former fellow at CityLab. He lives in New York.
The survey will kickoff the most revolutionary American mapping project in a century.
Ten months after Superstorm Sandy made landfall on the U.S. East Coast, causing over $65 billion in damage and bringing New York and New Jersey to a standstill, one consequence of the storm is just now coming into focus: its impact on the shape of the coastline, underwater and above.
The storm's devastating effect on human settlements—flooded subways, submerged houses, and hundreds of fatalities—has been well documented. But as the National Oceanic and Atmospheric Administration, the U.S. Geological Survey, and the U.S. Army Corps of Engineers begin a post-Sandy survey from Myrtle Beach to Montauk, its geological footprint remains largely unknown.
Revealing it, with the help of the laser surveying technology lidar, will be the first step of a program that will ultimately overhaul American cartography. By 2023, the USGS aims to have have the entire contiguous United States mapped with a margin of error of just a few centimeters.
It begins with the coastline of the Mid-Atlantic, as fickle a landscape as there is. The sandy shorelines that make the beaches of New Jersey and Long Island so popular with sunbathers ensures that their shape is, from a geological perspective, remarkably dynamic. Before human settlement shored up the form of Fire Island, for example, the 26-mile barrier off the coast of Long Island moved west at a rate of 150 feet per year.
The physics of sand is not rocket science; it may be even harder. Albert Einstein supposedly warned his son against pursuing it professionally. Accordingly, Jeff Ferguson, chief of the NOAA Hydrographics Survey Division, was reluctant to speculate about what the new maps will show. Underwater sand deposits may have accumulated, or receded. Changes may be limited to shallow depths, or not.
Large storms can produce deep shifts in the shape of inlets, bays and sand bars, though because measurements aren't taken every year, it can be hard to tell exactly what happens when. "Longer-term changes and big storm changes are piled on top of each other," Ferguson explains. "Some of the changes we're seeing now, the prior charts were made decades ago."
If preliminary reports are any guide, the effect of Sandy will be enormous. A USGS study released last week, which used lidar to chart changes to Fire Island, revealed that the beach and dunes had lost more than half their volume during the storm. "Hurricane Sandy fundamentally changed the morphologic configuration of Fire Island," the authors wrote. The elevation of the beach sank as much as nine feet. Other studies of storm-ravaged areas have shown similarly dramatic changes on land.
Underwater, the contours of elevation matter a great deal to boats, of course, but also to landlubbers. The position of submarine sand shelves determines wave patterns and is a crucial component in modeling storm surges and erosion. It's also important to the Army Corps of Engineers, which replenishes hundreds of miles of ocean beaches with sand dredged off-shore. When sand grows scarce, as it has off the Atlantic coast of Florida, the outlook for dredged beaches dims.
The current mapping project will chart the coastline with unprecedented precision. Deeper seafloors will continue to be measured by boats with "multi-beam echo sounders," which patrol the sea in regular patterns. (They call it "mowing the lawn.") But shallower depths and coastal settlements will for the first time be systematically surveyed with lidar, the airborne "light radar" that has recently revolutionized archaeology.
Lidar, says Mark DeMulder, director of the National Geospatial Program at the USGS, is "the biggest thing that's happened in mapping since a person first put a camera on an airplane. It gives you, in essence, a virtual world; not just the surface of the Earth, but everything that's on it: trees, power lines, houses."
The results of the post-Sandy survey will eventually be folded into the 3D Elevation Program, a billion-dollar survey projected to generate $13 billion in benefits each year. The transition from the familiar topographic maps, with their winding 10-meter elevation lines, to the digital reality of lidar will be the mapping equivalent of replacing a sundial with a Timex.
"Historically we've measured accuracy in meters. With lidar, we measure accuracy in centimeters," says Larry Sugarbaker, a senior advisor to the USGS. Most of the U.S. still relies on surveys performed by hand between the 1930s and the 1970s, so the new lidar maps will be a crucial revision for fields in which a few meters difference in ground elevation matters enormously: aviation, wind energy, water supply, and agriculture, to name a few.
Nowhere will that new level of precision be appreciated as much as in coastal communities threatened by storm surges and rising seas. FEMA will wait for the lidar data to revise its flood plain maps, which have a tremendous effect on insurance costs. Insurance companies, in turn, will be able to make smarter coverage decisions. Seaside towns and cities will be able to draft more effective disaster plans.
"It allows communities to understand their risk of flood — just imagine the difference in knowing your elevation down to nine centimeters as opposed to several meters," DeMulder says. "Are you going to be underwater or not?"
Top image: Homes severely damaged last October by Superstorm Sandy are seen along the beach in Mantoloking, New Jersey in April 2013. (Mel Evans/Associated Press)