Power & Technology
Mapping the sea floor with lasers provides the most accurate picture of what lies beneath our hulls.
The next step in mapping, LiDAR utilizes high-tech lasers to give us the most accurate maps yet.
Ientered the tidal strait with apprehension. It was always busy and seemed to have no slack tide—it just ripped like hell in either direction. I handled both with confidence; it was what lie beneath the surface that unnerved me.
The wrecks on New Jersey’s Shrewsbury River are charted, some with the dreaded, slanted symbol of a half-hull that designates Position Approximate. Storms, high winds and strong tides would move the lost craft just about every season.
The wrecks keep salvage operators busy. I once watched as a man prepped a damaged convertible for towing. I waved hello and as I motored off, he pointed off his stern. “That wreck sent my kids to college,” I remember him saying.
We hope to avoid catastrophe by updating our paper charts and utilizing today’s advanced electronics suites—they’re light-years ahead of models from just a decade back, with a wealth of functions, crystal-clear displays and the ease of use of an iPhone. The next step in mapping promises to blow these out of the water.
LiDAR is an acronym for light detection and ranging and it doesn’t use sound (sonar) or radio (radar) waves to detect its surroundings, but light—coherent light in the form of a laser—to get an exact depiction of the sea floor’s characteristics. For bathymetric (what’s used to map waterways) LiDAR, four chief pieces of hardware are involved: A laser scanner, a specialized GPS receiver, an IMU that gives roll, pitch and yaw of the aircraft they’re deployed from and a sensor to read the returning signal.
NOAA’s website on LiDAR is encyclopedic and technical, so I reached out to Alexandra “Xan” Fredericks, cartographer for the United States Geological Survey (USGS). Xan tells me the laser’s wavelength is unique to bathymetry. “We use a 532 nanometer blue-green wavelength. In the coastal zone ... chlorophyll actually absorbs more of the blue light, so the blue-green has a better chance for penetration.”
In a nutshell, the laser is pointed at a targeted area and then reflected back by any surface it encounters. The laser ranges are then combined with GPS and a wealth of other precise calibration data to produce a point cloud. With baked-in latitude, longitude and height and colors to differentiate bottom characteristics, the result is a richly detailed 3D rendering of what lies beneath your hull.
LiDAR is not a panacea, as water depth, sun angle and surface conditions can all weaken the laser’s effectiveness. Bottom reflectance and water clarity are two chief factors in its success: LiDAR works best in clear water with a bright, sandy bottom, and turbidity renders it ineffective. It can also be tricky to determine where the water surface actually is.
But it’s got the tide—via big government—on its side. Xan is also a LiDAR Coordinator for the USGS Coastal Marine Hazards and Resources Program, an interagency group made up of no less than eight agencies (including NOAA and the U.S. Army Corps of Engineers) with the common goal of facilitating efforts and improving bathymetric LiDAR methods. “We work really hard to coordinate our efforts and ensure collaboration where and when possible,” she explains.
Citizen science can also do its part. Earlier this year, NOAA announced the launch of its crowdsourced bathymetry database. Mariners can contribute bathymetric data via Rose Point Navigation software; it can determine if an area needs re-mapping or be used to ID navigational hazards, allowing NOAA to issue a Notice to Mariners often within 24 hours.
I’m reminded of a comment made to me by a custom boatbuilder, speaking of a lobsterman client. “That guy can see underwater,” he said, as the mariner could describe the bottom characteristics of dozens of locations. LiDAR could allow all of us to do just that.