LIDAR Header Imagery
February 2, 2004, written by Holli Riebeek, design by Goran Halusa
 
 

Three days after Hurricane Isabel thrashed the North Carolina coast, Wayne Wright took his twin-engine Cessna 310 to the skies. Flying just 300 meters above the islands of the Outer Banks, he and his team bounced a green laser across the landscape, tracing the contours of the beaches to build a picture of how the land had changed. Sand dunes were missing and a new inlet was gouged across Hatteras Island. They took millions of measurements and over 30,000 pictures that will help geologists understand how extreme events, such as hurricanes, shape our coast.
 

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North Carolina's Outer Banks after hurricane Isabel, September 21, 2003
 

 

Water washes through a newly formed breach on Hatteras Island, North Carolina, a few days after Hurricane Isabel swept through the area.

Image courtesy of Asbury Sallenger and Amar Nayegandhi, USGS Center for Coastal and Watershed Studies.

  Wright is the lead investigator for NASA’s Experimental Advanced Airborne Research Lidar (EAARL) system at Wallops Flight Facility on Virginia’s eastern shore. He is collaborating with Asbury Sallenger, Jr., a research oceanographer at the US Geological Survey in St. Petersburg, Florida, to determine how much damage Hurricane Isabel inflicted on US beaches. At other times, Wright has used EAARL to map shallow coral reefs, chart the invasion of foreign plants, and map shallow Midwest river bottoms.
 

 
 
 

Their primary tool is a lidar, which is short for Light Detection and Ranging. Lidar works like radar, but uses light waves instead of radio waves to measure the distance to objects. Typical mapping lidar systems bounce a laser light pulse off a surface and record the time it takes for the light to return to determine the distance it traveled. Plants, water, bare exposed earth, and the bottom of a shallow sea each absorb and reflect different colors (wavelengths) of light in different ways. Most lidars are optimized to map one or two types of surface terrain because vastly different lasers and laser receivers are required for, say, bare earth compared to plant cover. EAARL is distinct in that it can trace out the topography of bare ground under vegetation, map out the vegetation itself, and, where the water is clear enough, measure the depth and clarity of the water as well as the shape of underwater surfaces.

“EAARL is uniquely able to make measurements over ground that varies tremendously in reflectivity and complexity,” says Wright. “We could fly over water, and one pulse could fall in the water and the next on a sandy beach.” Water, Wright explains, tends to absorb the light and return a much weaker signal, while exposed sand is highly reflective and vegetation falls somewhere in between. “Other [lidars] tune for either weak or strong signals,” Wright says. But EAARL can capture them all because it makes frequent measurements – about four billion per second using multiple detectors. This trait makes it perfect for tracking hurricane damage in coastal areas, which are made up of water, sand and plant life.

EAARL’s flexibility gives it another advantage for charting storm damage. Because it is already designed to measure all sorts of terrain, it can be kept on a dedicated airplane, ready to go at almost any time. It doesn’t have to be re-calibrated for every project and every sort of terrain. “EAARL was on standby. It works on short notice,” says Sallenger, and for his beach mapping project, time was in short supply.

  EARRL laser points scanned across Wayne Wright's hand

Green points of light from EAARL’s laser track across Wayne Wright’s hand.

Image courtesy of the U.S. Geological Survey web site.

   

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Capricious Coastlines

“If you fly along the barrier island coast, you’ll see cuts through the islands. Almost all of them are formed the same way,” Sallenger explains—by hurricanes. To understand how severe storms shape the coast, Sallenger needed accurate before and after maps. But beaches shift daily with the tides, waves, and winds, so no map would ever provide a perfectly accurate before-after comparison. He needed to make a map of the beach immediately before a major event and compare it to a map made immediately after to know what changes were caused by the storm and what occurred as a result of other natural processes. Having EAARL on standby to map out the beach just before the hurricane hit was essential to the success of the project.

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Astronaut photo of North Carolina's Outer Banks
 

 

Astronauts onboard the International Space Station snapped this photo of Cape Hatteras on July 30, 2002, as they orbited the Earth. A line of islands, the largest of which is Hatteras Island in the center, form the Cape. The islands hem in the sediment-rich waters of Pamlico Sound. The narrow strips of water separating the islands were probably formed by hurricanes.

Image ISS004-E-7784 courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center. Web site: The Gateway to Astronaut Photography of Earth

 

Two days before Hurricane Isabel came ashore, Sallenger and Wright collaborated to survey the areas where the storm was predicted to make landfall. “We were able to survey 250-300 kilometers of shoreline in just a couple of days,” Sallenger reports. They repeated the survey five days later to track the changes Hurricane Isabel made. Among other things, they mapped the new inlet that was cut across Hatteras Island, giving scientists the most detailed data set on storm inlets to date, says Sallenger. Those data will be used to build models of beach erosion to predict how and where inlets may form in the future.

   
 

 
Hatteras lidar comparison 09/16/2003 - 09/21/2003
 

 

Images of Hatteras Island before Isabel show a long line of high sand dunes along the outer coast with buildings behind them. The highest sand dunes are peaked with dark orange and red to indicate elevation. The dunes in the center of the line are smaller, as is evident by their lighter tops in the image. It is no surprise then that, in the post-Isabel image taken on September 21, 2003, the low dunes have given way to the ocean. The powerful surf cut across the lowest point on the island. Below the breach, the dunes were washed away, as were a couple of the buildings behind them. Above the breach, the high dunes were worn into smaller dunes. The straight, dark stripe behind the dunes in the pre-storm image is Highway 12. Sections of the highway have clearly been washed away or covered with sand in the post-storm image.

The elevation measurements in the images are amazingly accurate. EAARL measures elevation to within 10 centimeters and horizontal distance to within 40 centimeters. Its accuracy depends mostly on how well its operator knows where it is and how it’s oriented in relation to the Earth’s surface. Global Positioning System (GPS) instruments on the plane give a precise location for the system, usually to within a few centimeters—a typical thumb length. Wright then accounts for the tilt and heading of the plane—if the plane is flying level, a light shining out of the bottom will hit a different place than if the plane is banking left or right. Once he knows the precise location of the laser, Wright records the laser’s echo strength once every billionth of a second to get an accurate measurement of how far the light traveled between the plane and the ground and back again. “It’s all a matter of time,” says Wright. “Time is the critical measurement in everything that we do.”

 

NASA’s Experimental Advanced Airborne Research Lidar shows where sand and buildings on Hatteras Island disappeared in the wake of Hurricane Isabel.

Images courtesy Asbury Sallenger and Amar Nayegandhi, USGS Center for Coastal and Watershed Studies, and Wayne Wright, NASA Wallops Flight Facility.

   

next Reefs, Reeds, and Riverbeds


 
 

 

Reefs, Reeds, and Riverbeds
 

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Johnson's Coral Reef

 

Johnson’s Reef on the northwest shore of St. John Island in the U.S. Virgin Islands forms colorful peaks in this lidar image recorded by NASA’s Experimental Advanced Airborne Research Lidar (EAARL) in June 2003. The coral reef is part of the Virgin Islands National Park. Reef managers could use such a complete picture of the shape of the reef as a standard when tracking reef health, growth, or damage. This EAARL image provides a much-needed overview. Each grid in the image covers 100 square meters of the reef. The red peaks are sections of the reef just one meter below the surface of the water. The dark blue troughs are 10 meters below the surface. Read more on Johnson's Reef.

 

Despite the accuracy and importance of the information gleaned from the North Carolina coastline, Wright almost seems to regret he couldn’t track damage in more complicated coastal regions. If Isabel had passed through the Florida Keys, he notes, EAARL could have seen through the clear water to chart damage to coral reefs. Indeed, the team has already imaged coral reefs in the Florida Keys. EAARL could also show damage to vegetation along the South Florida coast. Determining the status of mangroves—the tropical trees that filter water for coral reefs and provide an important habitat for fish and wildlife in tidal waters—could have given significant insight to the overall impact of a storm. But along the North Carolina coastline, EAARL was used primarily to chart sand transport. The water in the mid-Atlantic is generally too murky for EAARL to assess underwater damage like the depth of the new inlet on Hatteras Island, and there is little vegetation on the sandy beaches for EAARL to map.
 

   
 

Photo of phragmites
 

 

Stands of invasive Phragmites, a common reed, grow in Central Delaware. Photo courtesy Wayne Wright, NASA Wallops Flight Facility.

 

Though it didn’t track the changes Hurricane Isabel wrought on plants, EAARL has already come in handy for mapping coastal vegetation. Common reeds, called phragmites, are invading eastern shores and marshes. Crowding out native plants that support wildlife such as ducks and geese, stands of phragmites grow up to six feet tall and pose a significant fire hazard. It is unclear how these European reeds came to take over US shores, but it is possible that they were stowed on boats in the late 1800s as shipping material. Now, like generals preparing for a battle, scientists want a clear picture of where these invasive plants are and how extensively they cover the coast. Wright has been flying reconnaissance with the EAARL instrument to map out stands in central Delaware. The data he collects will be used to estimate the current state of phragmite infestation and to show how effective efforts to control them have been. Such information will help the National Fish and Wildlife Service reclaim coastal marshes for native plants and animals.

EAARL’s capabilities aren’t limited to what’s visible above the surface, however. To illustrate how EAARL might be used to look for storm related damage in shallow waters, Wright describes a water management project along the Platte River in Nebraska. This tributary of the Missouri is typically less than a meter in depth but several hundred meters wide and filled with meandering sandbars. Hydrologists want to understand how these sand bars migrate along the river bed. “The river is far too shallow for a boat and most lidars. You need a real short laser pulse to measure the river bottom,” says Wright. EAARL has the short light pulse needed to read the shifting sands both above and below the water. Traditionally, hydrologists waded into the river with a GPS unit, manually recording variations in depth. It took six to eight people an entire day to cover a single kilometer of the river, making about 2000 measurements. At up to 5,000 measurements per second, the EAARL instrument can do the same in less than a second.
 

   
 

Platte river

 

A color infrared photo (top) of the Cottonwood Ranch section of the Platte River gives an aerial view of the area shown in the lidar image (bottom). In the photo, higher elevations are white. The Overton Bridge forms a straight line over the river, and Interstate 80 runs along the north side of the river (top). The infrared photo is especially helpful in seeing where the river flows in deep channels. The photo shows channels of water in blue, while the lidar image shows the shape and depth of the channels in strips of color. Read more about the Platte River Lidar project.

The photo, taken in 1998, was provided by Paul Kinzel, USGS. The lidar image, taken in 2002, was provided by Paul Kinzel and Wayne Wright, NASA Wallops Flight Facility.

 

The result is breathtaking. Swirling ribbons of color resembling fire more than water mark out deeper channels in the river. Sandbars are wider, more solid areas. Since the west end of this section of the river is about 20 meters higher than the east end, a simple color bar can’t show all the subtle changes in elevation while also showing the east-west elevation change. To solve the problem, the color scale “wraps around,” that is, starting on the east (right), blue is the lowest elevation going up to red, 3 meters above that. Then, blue begins to represent 3 meters and red, 6 meters, and so forth. The effect is a rainbow of color going from east to west. Similar maps could be made of the coastline in clear, shallow waters, though in coastal waters, the changes are more dramatic, so “wrapping” the colors isn’t necessary.

The purpose of the Platte River mapping project was to see if lidar data could be used to map the riverbottom at all. Paul Kinzel, a hydrologist with the United States Geological Survey and the principle investigator in the project, compared the data with handheld GPS measurements as a sort of “truth test.” He reports that the lidar measurements were within 15 centimeters of the more accurate GPS measurements, and better in shallower water. As the accuracy improves, he hopes to use lidar to measure changes to the river bottom before and after controlled releases from reservoirs. Measuring changes brought about by a man-made flood isn’t all that different than tracking storm damage along the coast. “We are interested in seeing how well the technology will perform,” Kinzel notes, referring to EAARL mapping inland rivers in general. “It’s proven that it has potential.”

   
   

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