SUPERIOR SURVEY
NOAA’s remote-survey technology is making better charts, which means safer boating for all yachtsmen.
Maritime wisdom holds that there are two kinds of boaters: those who have found the bottom, and those who will. I joined the former club in 1988 while cruising the Chesapeake Bay with my dad and his buddies. We were flying our brightly colored spinnaker when Windancer — my parents’ C&C 37 — “discovered” an uncharted sandbar. Dousing the kite wasn’t problematic, but refloating our vessel involved the crew dangling from the starboard shroud and rocking her keel free. We eventually found deeper soundings, but not before I learned some creative expletives from my dad, who wasn’t thrilled with his brand-new-but-already-obsolete paper charts. As we learned that day, sandbars sometimes move faster than survey crews. ¶ The National Oceanic and Atmospheric Administration is using forward-leaning technology to close that knowledge gap. NOA A produces and maintains all official cartography for U.S. waters, including a staggering 95,000 miles of shoreline. The agency’s traditional methods for collecting bathymetric measurements work well for soundings deeper than 30 feet, but recent years have seen the rise of remote-sensing technologies, which allow NOAA to acquire precise, high-resolution bathymetric data for shorelines and nearshore waters.
“We maintain the shoreline on NOAA nautical charts, which is considered the nation’s legal shoreline,” says Gretchen Imahori from NOAA’s Remote Sensing Division, whose tools include light detection and ranging laser-pulse technology (lidar). “Aside from being able to derive our shoreline from our topographic-bathymetric lidar data, it also allows us to help update nearshore and back-bay areas on NOAA charts. It is more efficient for us to survey these areas, and safer than sending in launches with sonar.”
NOAA employs its remote-sensed data to update its nautical charts quickly and accurately, and to support emergency efforts after major storms (see sidebar). The agency’s charts are available to all mariners to download for free, and virtually all third-party cartography companies use NOAA data as the U.S. base maps underpinning their proprietary offerings. The sensing division, Imahori says, works closely with NOAA’s Hydrographic Surveys Division and Marine Chart Division to coordinate survey-collecting efforts. The sensing division uses two specially equipped planes to collect shoreline, nearshore and back-bay survey data, while the hydrographic division surveys waters deeper than 13 feet.
NOAA’s DHC-6 Twin Otter aircraft serves as the slow, low-flying operating platform for the agency’s Riegl VQ880-G airborne laser-scanning system. The Riegl lidar uses a green laser in an elliptical scanning pattern to measure the distance to a surface from survey heights of roughly 1,000 to 2,000 feet. While previous-generation lidar equipment could collect “a few” data points per square meter, Imahori says, the Riegl collects as many as nine points per meter in shallow water (down to 56 feet).
“It’s an immense amount of detail,” Imahori says, adding that lidar works in places where NOAA’s vessels previously found it difficult to operate. The technology works best, she says, with bright, light-reflective bottoms and calm, crystal-clear waters.
NOAA’s King Air, meanwhile, carries two 80-megapixel cameras. One collects “red-green-blue” color imagery, the other near-infrared imagery. Both are accurate to 35 centimeters from 10,500 feet. This ultra-high-resolution imagery is used for photogrammetry, the science of taking precise measurements using photography.
Given the speeds of these aircraft, when compared with a slower, ship-towed sonar array, the sensing division can cover ground quickly, a critical ability when collecting post-hurricane data. However, given the sheer volume of nearshore bathymetric data that must be compiled, Imahori says, the division also contracts work out to commercial survey companies and is researching the use of eBee drones. The drones would collect aerial photography to produce 3D models accurate to 3 centimeters.
While the sensing division is doing its work, the hydrographic division develops project instructions for in-house and third-party survey missions across 3.4 million square miles. It then takes that data — along with sensing-division data — and reviews it for quality-control before applying it to official NOAA products, including the bathymetry database or nautical charts. One of the hydrographic division’s most important resources is its Hydrographic Health Model, which factors the age, acquisition technique and seafloor type of all existing NOAA
bathymetric data and cartography, as well as AIS traffic data, to help create a nationwide priority list for survey work. The hydrographic division uses modern sonar for its survey work, with side-scanning sonar in relatively shallow waters (down to 330 feet) and a towed, multibeam sonar (broadcasting across chirp frequencies) to take deeper soundings. “With multibeam sonar, we can collect bathymetric data but also backscatter, and we can use science to estimate what the seafloor is made of,” says Rick Brennan, NOAA’s chief of the hydrographic division. “Sonar gives us a wide array of information, and it can measure things in the water column — it’s the gold standard — but its swath width is related to the depth.” Shallow waters, he adds, produce narrow-diameter sonar cones on the seafloor (read: time-consuming survey work), so the division instead uses side-scanning sonar or, if conditions allow, lidar.
Satellite imagery is another important asset. “We have the ability to take a picture of [a specific part of ] the Earth once per day,” says Brennan, whose division then uses the imagery to seek out and monitor areas with dynamic seafloors. “The accuracy may be coarse, but it’s the ability to see change that’s really valuable.”
Once bathymetric data is collected, “we can use statistical filters to look for places where the seafloor isn’t consistent,” explains Brennan. The tools help focus eyeballs on inconsistencies or potentially problematic data. “Modern multibeam [sonars] can produce anywhere from 10 to 100 depth measurements per square meter of the seafloor,” he adds. “The standard deviation of these measurements is typically about 8 to 10 inches. In high-traffic areas, we have the ability to cut this in half with modern surveying methods.”
The result of NOAA’s forward-leaning work is much more accurate nautical charts and higher-quality bathymetric data, both of which benefit all mariners. With luck, these technologies will yield newer generations of yachtsmen who never have to find the bottom the hard way, and help the rest of us avoid being reinaugurated into “the club.”