A lidar data investigation that began with a muddy trek through South Carolina’s saltwater marsh has important implications for coastal professionals trying to produce accurate sea level rise scenarios. The take-home message: review your lidar data and check the findings against information gathered through ground survey methods.
Brian Hadley and Keil Schmid, two remote-sensing scientists with the NOAA Coastal Services Center, began the investigation in 2009. “Like lots of other lidar specialists,” Schmid says, “we initially thought that the ‘scrub/shrub’ land cover category would work for marsh areas in terms of the pulse energy striking and transmitting elevation information in a similar way.”
However, as the two researchers cross-checked the lidar findings against information gathered through traditional survey and GPS methods, the numbers did not add up.
“We’d walk into these wide marsh areas and could see and feel that the land was mostly very flat. But when we looked at the numbers processed through lidar software, the elevation readings were jumping up and down,” notes Hadley.
Marsh vegetation is often much denser, and closer to the “bare earth” surface, than is typical “scrub/shrub.” As a result, the lidar pulses were hitting vegetation but were registering as “bare earth.” These faulty lidar readings were off the mark by up to 9.8 inches.
“That sort of inaccuracy can cause big problems, particularly because the elevation errors go just one direction—upward,” says Schmid. “Let’s say you have projected one foot of sea level rise, but your data is skewed, upward of nine inches higher than the actual land elevation in a marsh. The result is that there will be flooded marsh areas you didn’t expect.”
“Any coastal lidar user is going to want a less biased, more accurate model,” says Hadley. “For coastal marsh, that may mean wading in and getting ground control points in addition to lidar data. When you’re planning for sea level rise, it’s better to be safe than sorry.”
To learn more about the Center’s lidar services, contact Keil.Schmid@noaa.gov. Additional findings of this lidar investigation were covered in a LiDAR Magazine article at www.lidarnews.com/PDF/LiDARMagazine_Fahey-ModelingSeaLevelRise_Vol2No4.pdf.
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