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GCN : April 2014
The U.S. Geological Survey is spearheading an effort to collect high-resolution elevation data for the entire United States that will be used to create 3D maps for gauging ood risks, monitoring the health of crops and measuring the biomass of forests. The technology for gathering the data, called 3DEP for the 3D Elevation Program, is an advanced version of LiDAR (light detection and ranging) that is accurate for measuring elevation and offers greater resolution than earlier technologies. But while all of the continental United States will be included in 3DEP's LiDAR data set, USGS has to turn to a different set of tools to capture data on the rugged contours of Alaska. Because of its size, remoteness and nearly constant cloud cover, Alaska is being scanned with IfSAR, or interferometric synthetic aperture radar. Where LiDAR can't penetrate cloud cover, IfSAR can. And IfSAR can be used effectively in jets ying at higher altitude than is optimal for LiDAR scanning. That's critical in reaching remote areas that are far from refueling facilities. The trade-off? "The IfSAR is less accurate," said Mark DeMulder, chief of USGS's National Geospatial Program. "Generally you get one elevation value for every 5 meters. The vertical accuracy is about 1 meter rather than just over 9 centimeters. But even at that level, that accuracy is so much better than what is been available for Alaska in the past. It's a tremendous improvement." The current statewide base maps for Alaska were created around 1960 and at a lower resolution than the maps made over the continental United States. Improved data holdings for Alaska are required to meet current safety, planning, research and resource management standards, according to USGS. Currently, DeMulder said, all public agencies combined are collecting elevation data on about 5 percent of the country each year. "Our goal is to move that up to the 11 percent to 12 percent per year," he said. And then begins the job of refreshing the data. "The 3DEP initiative is designed to establish a national baseline of LiDAR data," said Larry Sugarbaker, a senior adviser at the USGS National Geospatial Program, which manages 3DEP. "For events such as Superstorm Sandy, having a baseline is really important to be able to do change analysis. In areas of coastal impact like that, every time there is an event we may need to acquire new data." Marie Peppler, ood and hazards program coordinator at the U.S. Geological Survey, agrees. "The coastal system of the highly variable and changing landscape needs to be updated as often as you can afford to," she said. "For environments that are not changing as much and are not as variable, you don't need to collect as often. For Iowa they are using 5-year-old and 8-year-old LiDAR data, and that is perfectly ne." -- Patrick Marshall Alaska puts USGS mapping to the test pens in the private sector, where it is driven by competition." The resulting data sets are available to oth- er agencies, and even to the general public. "Our data is in the public domain," DeMulder said. "Our philosophy is that if the federal government is producing it, the taxpayers al- ready paid for it. So access to the data is avail- able without additional charge and without a copyright or any kind of use restriction." 3D ELEVATION PROGRAM While individual agencies may undertake or contract out a LiDAR survey for a specific purpose, such as to assess storm damage, the USGS is coordinating a program to collect high-resolution elevation data for the entire United States. Known as 3DEP-- the 3D El- evation Program -- the effort is expected to take eight years. The LiDAR being used for 3DEP is not only highly accurate for measuring elevation, but it also offers greater resolution than earlier technologies, including previous versions of LiDAR. "Our specification requires two data points for every square meter of terrain," said Larry Sugarbaker, a senior adviser at the USGS National Geospatial Program, which man- ages 3DEP. "Historically, it s been about 0.7 points per square meter. So it s a little over twice as dense as what we have collected historically. We re going from measuring ac- curacy in meters to measuring accuracy in centimeters." Sugarbaker credits a number of factors for making LiDAR more accurate and less costly. Because the instruments are being improved, the scan rates are increasing and the optics are improving, so contractors can collect more data for less money. Moreover, the processing cost for LiDAR is going down. According to DeMulder, the resulting data set will provide much more than terrain el- evation data. "It also includes information about all of the features that sit on top of the terrain, such as buildings, power lines, roads -- any feature that would return a laser beam from the sensor on board the aircraft, " he said. • GCN APRIL 2014 • GCN.COM 29