• After NOAA’s Hydropalooza survey, Kachemak Bay’s sea floor more faithfully rendered in charts
By Naomi Klouda
Oyster farmer Ron Bader found an odd ocean floor formation recently while consulting his global positioning system.
Stretching east to west in Cook Inlet, a perfect 90-degree angle appeared to map a giant ridge or trench-like formation. The east-west leg, about 30 miles long, aligned with St. Augustine, while the other leg pointed toward magnetic north.
“After some 20 inquiries, there have been no viable answers to this vexing observation,” Bader said. “Some of these explanations have been an artist touch up on the photo or jet contrail, tide rips, or snow fence effect from cable or pipeline.”
Bader passed the GPS rendition of the trenching to various entities in his quest to find answers.
The mapping experts at NOAA provided some insight after Kasitsna Bay Director Kris Holderied forwarded it to Anchorage. Matthew Forney, the navigation manager for the Alaska Region with the NOAA Office of Coast Survey, examined it.
“It looks like what we call an ‘artifact,’ a difference in depths recorded in two actions,” Forney said. “Two different surveys likely were spliced together, with the ridge indicating its seams. The source of the track line survey is the USGS branch of Pacific Marine Geology conducted in June of 1981. Google Earth may have uploaded the mapping data since that was the source of Bader‘s map.”
The discrepancy between maps all across the globe comes from there being different data of differing periods uploaded onto the Web, Forney explains.
“If this was the USGS survey in 1981, it looks like the L shape is created by the track line of that boat. We didn’t have a NOAA study in that area at the time. Technology now is vastly different,” Forney said.
NOAA’s latest 2008-2009 project mapping Kachemak Bay, called “Hydropalooza,” used a multi-beam sonar rather than the single-beam sonar used in the past when the data for the feature Bader saw was created. The multi-beam sonar uses a swath out to each side to get full coverage of the ocean floor, with much greater detail than the lines of depth seen by a single-beam sonar. Single beam works like a typical fishing boat fathometer, while multibeam collects data like a lawnmower. But the Hydropalooza project boundaries didn’t include the portion of the bay with the apparent L shape formation. Nonetheless, it provides a good example of where mapping is at nowadays and provides a caution to GPS and navigation aide users of just how outdated some of the accessible data out there may be.
“So much of the data on Alaska is vintage – it really is the last frontier in the science world, even though Alaska accounts for more than 35 percent of all the coast line in the U.S,” Forney said.
The last time the St. Augustine area was mapped was by a crew aboard the NOAA Ship Rainier in 1983.
Forney gets many questions from people using chart plotters or other navigation aides. Whenever new information is available, he recommends buying the paper chart or updating navigation equipment with new disks. Free electronic chart downloads are also available at www.nauticalcharts.noaa.gov
There’s more work to be done on the Kachemak Bay Hydropalooza project, Holderied said. Mapping has been done for the sea floor, but shoreline-mapping is not yet complete. A NOAA aircraft mapped the coast on the north side of the bay from Fox River Flats to Bluff Point by Anchor River using a LiDAR (LIght Detection And Ranging) instrument.
The airborne LiDAR sensor provides very high resolution information on coastal land elevations.
“When you think about the shoreline marked on a nautical chart – not many people think how that line gets created. Just like you survey with ship sonars to get accurate information to chart water depths with confidence, you need to do the same on land,” Holderied said. “Only with LiDAR, that is done with an aircraft-based sensor that uses a laser to bounce light off land, whereas the ocean is mapped by bouncing sound off the bottom.”
Holderied said she is hoping that the south coast of the bay will be mapped if the NOAA plane is back up in Alaska for other missions. “Hurricanes and the Deepwater Horizon oil spill took priority for aircraft missions the past two summers, so there’s no guarantee, but we still hope to get it done,” she said.
When the NOAA cartographers put together maps, they correct sea floor data for tides and other smaller effects. NOAA uses extensive quality control to prevent “artifacts” from combining data from different sources, Holderied said. The primary correction is for the tide, which has to be calculated and subtracted from every sonar depth measurement.
“Apparent L-shaped bottom features can happen when two data sets are slightly mismatched along the edges where they meet, perhaps due to a difference in the tide correction. In NOAA charting, we look for things like this and take them out,” Holderied said.
Within Kachemak Bay, the 2008-2009 Hydropalooza project did not find big changes in ocean depths.
“We didn’t find any thing different in a gross way, as expected, but there were features that were missed, like in the Jackolof trench where we found a previously unmapped, five-story tall bottom feature,” Holderied said. “Having detailed information on sea floor shape will also significantly improve ocean circulation models used for oil spill response, fishery management and tidal energy studies.”
Some areas do see big depth changes, including Upper Cook Inlet, where big tides and storms shift sediments so much that NOAA sea floor surveys need to be done every three to four years. Holderied said navigation safety is key for determining how often surveys must be done.
“If you’re in 1,000 feet of water and the depth changes by 30 feet, that doesn’t matter to shipping traffic. But if you’re bringing tankers in the shipping channels of Cook Inlet where the water isn’t that deep, a 30-foot change is a big difference,” she explained.
All of this works to explain mapping challenges and why some data becomes incorrect, but there’s one more piece of information to consider before Bader has all his questions answered. He may have to ask Google Earth for the source of its data in the area of the apparent mapped trench. Google does not have to cross-check between different data sets and perform the quality control that goes into data used for NOAA nautical charts.
As Forney wrote in an e-mail, “Google simply brings in all data sets that are available on the Web, including NOAA’s sea floor mapping data.”
Comments are closed