Studying the use of satellite-derived bathymetry as a new survey tool

by Ensign Kaitlyn Seberger, onboard NOAA Ship Thomas Jefferson

Nautical charts are an important tool in navigating safely in coastal waters, and Coast Survey’s mission is to keep these charts up to date. However, maintaining accurate charts can be a challenge in locations where sandy shoals may shift seasonally and present a danger to navigation. These areas differ from the current nautical charts, and bottom contours change so rapidly that it may seem an impossible task to keep up using the traditional survey methods. Office of Coast Survey and NOAA Ship Thomas Jefferson are seeking a solution to this ongoing problem and may have an answer with satellite-derived bathymetry.

Satellite-derived bathymetry (SDB) begins with using multi-spectral satellite imagery, obtained by satellites such as Landsat and WorldView2, which compares green and blue color bands.

Multi-spectral satellite imagery of Mutton Shoal in Nantucket Sound, overlaid on the chart.
Multi-spectral satellite imagery of Mutton Shoal in Nantucket Sound, overlaid on the chart.

Green color bands are attenuated by the water faster than blue bands and help to infer relative depths of the water (blue areas being deeper than green). These images are then transformed into a color range scale applicable to the color scale used when surveying with a multibeam echo sounder. With the color range applied, reds on the image represent an area that may be shoal whereas blues and greens represent deeper water.

Satellite-derived bathymetry of Mutton Shoal with a color range scale that is correlated with the color scale used for multibeam processing
Satellite-derived bathymetry of Mutton Shoal with a color range scale that is correlated with the color scale used for multibeam processing.

Since the images are based on attenuation of color bands, depth can only be inferred, so survey equipment (such as vertical beam and multibeam sonars) is necessary to acquire true depth.

This fall, NOAA Ship Thomas Jefferson investigated the use of satellite-derived bathymetry imagery as a new survey tool. Survey technicians will calibrate the application of this imagery through bathymetry studies for Nantucket Sound and Chincoteague Island. NOAA Lt. Anthony Klemm, who is leading the studies, chose these project areas because they both had relatively clear shallow water and were in a highly changeable area. At these locations, he chose specific shoals for exploration based on vessel traffic density.

In October, Thomas Jefferson spent two days in Nantucket Sound researching shifting shoals using the satellite-derived imagery overlain on the most recent chart. Ensign Marybeth Head developed line plans to acquire data over the potential location of shoals as seen with the satellite images, as well as their charted locations. Survey launches acquired multibeam data in water deeper than six feet, and Z-Boats were sent in to acquire vertical beam data in areas too shoal for the launches to safely operate.

The video shows Z-boat surveying alongside the launch in shoals too shallow for the launches to operate safely. (Video credit: ENS Head)

Satellite-derived bathymetry of Mutton Shoal with multibeam data from the investigation overlaid. This picture demonstrates how accurate the location of the shifted shoal was compared to the SDB imagery.
Satellite-derived bathymetry of Mutton Shoal with multibeam data from the investigation overlaid. This picture demonstrates how accurate the location of the shifted shoal was compared to the SDB imagery.

During routine conductivity, temperature, and depth casts for sound speed velocity, Ensign Head and Ensign Kaitlyn Seberger used a Secchi disk to determine the attenuation coefficient at each cast location for later comparisons.

The satellite imagery was a vital tool in project planning, as well as determining safe navigation of the ship and the survey launches. Below is a picture of the chart location where Thomas Jefferson intended to anchor. The adjacent image is the satellite-derived bathymetry imagery indicating the anchorage would have been within a shoal area and unsafe for anchoring.

Side-by-side picture of the chart and SDB imagery for the intended anchorage location in Nantucket Sound. SDB imagery indicated a shoal that covered half of the anchorage safety circle. A Z-boat verified the indicated shoal was almost 30 ft shoaler than charted and without this useful imagery, the ship and launches could have run aground.
Side-by-side picture of the chart and SDB imagery for the intended anchorage location in Nantucket Sound. SDB imagery indicated a shoal that covered half of the anchorage safety circle. A Z-boat verified the indicated shoal was almost 30 ft shoaler than charted and without this useful imagery, the ship and launches could have run aground.

Ensign Head determined safe passage routes for the survey launches, using the satellite-derived bathymetry imagery overlaid on a chart of the area, as the charted soundings were not reliable. For example, a safe passage route between the study areas and the ship was located between two shoals that had shifted considerably from the chart of the area. Sections of the passage are currently charted at 20 feet or more of water, but the fathometer on the launch displayed depths of less than 10 feet.

Boat sheet for the launches indicating a potential safe passage route from the project area to the ship.
Boat sheet for the launches indicating a potential safe passage route from the project area to the ship.

After processing the multibeam data, Ensign Head determined that more than half of the charted shoals in the project area had shifted and the red zones depicted in the satellite-derived bathymetry imagery were significantly shoaler than charted depths for the surrounding area. Results from the investigation showed that the satellite-derived bathymetry for Nantucket Sound was exceptionally accurate and aided in the identification of current navigational dangers.

However, more research is needed regarding the use of satellite-derived bathymetry as a contemporary survey method. Limitations on use of the imagery can include variables such as cloud cover, turbidity, Chlorophyll a, and other water quality properties that may affect attenuation. Despite these challenges, satellite-derived bathymetry is a new tool that could support survey efforts by reducing the amount of time and area necessary to survey and by increasing the effectiveness of NOAA’s efforts to efficiently provide safe navigation to the local mariner.

New unmanned surface vehicles to deliver shoaler depth measurements for NOAA nautical charts

If you look closely at any U.S. coastal nautical chart, you’ll likely find that the areas closest to the shore, shoals, and rocks do not have updated depth measurements. In many areas, safety concerns prohibit the use of NOAA ships or launches to survey the shoalest depths. In many areas, the water is too murky to be mapped with the airborne lidar systems used in clear waters. Now, however, charting those shallow areas is about to get safer, thanks to recent purchases of small, commercial off-the-shelf, unmanned surface vehicles.

This summer, NOAA Ship Thomas Jefferson will deploy a “Z-Boat,” offered by Teledyne Oceanscience out of Carlsbad, California.

Lt. Carrier deploys Z-Boat
Lt. Joseph Carrier, operations officer on NOAA Ship Thomas Jefferson, deploys a Z-Boat from the ship.

The Z-Boat complements the ship’s existing hydrographic toolkit.

  • Thomas Jefferson uses its multibeam echo sounder to measure depths from 45 to 1000 feet.
  • For shallower and more constricted waters, the ship’s two hydrographic survey launches with multibeam echo sounders efficiently and safely survey areas from 12 to 200 feet deep.
  • With the new Z-Boat (using a single beam echo sounder), Thomas Jefferson can measure depths in areas as shallow as one foot, and get that data into processing almost immediately. The boats are highly maneuverable, turning in their own 5.5-foot length, meaning they can get much closer to piers, pilings, and the shoreline than a full-sized launch.

This new capability is important to improving charts for smaller vessels operating near the coast, and in the inlets, bays, and harbors so critical to many small coastal towns. In the 1930s, the Roosevelt Administration – through its massive Depression-era public works program – hired hundreds of men to survey shallow Intracoastal Waterway areas. However, NOAA has done very little survey work in shallow water in the 80 years since then. Not surprisingly, there is a backlog of reported shoals, rocks, wrecks, and obstructions in shallow water, leading to an increased risk of grounding for those smaller vessels. Knowing the depth in these inlets is also important to accurately predicting coastal inundation during storms.

Thomas Jefferson, with the support of NOAA’s Office of Marine and Aviation Operations’ innovative platform program, plans to use two Z-Boats this summer in Massachusetts to investigate shoals and rocks in Buzzard’s Bay and Vineyard Sound. This December, they will use them in a project near Chesapeake Bay.

Doug Wood deploys Z-Boat
Doug Wood, physical scientist on NOAA Ship Thomas Jefferson, deploys a Z-Boat from the ship’s fantail.

“Coast Survey has been exploring the use of autonomous underwater vehicles – AUVs – to support nautical charting for over a decade,” explains Rear Admiral Gerd Glang, Coast Survey director. “Unmanned surface vehicle – USV – technologies have advanced in recent years, and NOAA is now also exploring these for our hydrographic operations. The Z-Boat is one of several unmanned surface vehicles that we are experimenting with.”

Through a hydrographic survey contract with NOAA, TerraSond (Palmer, Alaska) is using a USV in addition to their traditional manned boats. (See this article in Marine Technology News.)

One of the benefits of using off-the-shelf vehicles like Z-Boats is that hydrographers are able to calibrate the boats and put them into use quickly, without the need for additional installation and integration of a survey system. Thomas Jefferson took delivery of the boats on August 13. They now have qualified the system for hydrographic use, developed first-generation deployment and retrieval systems, and trained a cadre of Z-Boat “pilots.”

“Two weeks from delivery to a calibrated system with trained operators is a significant achievement,” said Capt. Shepard Smith, Thomas Jefferson’s commanding officer. “We have already used them to conduct a small survey in Newport, Rhode Island, and we are thrilled with the new capability this boat will give us in our coastal projects.”

Thomas Jefferson will operate the boats from a control station on the ship or one of their launches. Depending on the circumstances, technicians have several options to control the boats, by using: 1) a handheld remote control; 2) a networked radio link with one-mile range; or 3) an onboard autonomy module. NOAA is working with Teledyne and with researchers at the University of New Hampshire-NOAA Joint Hydrography Center to develop improvements to the boat’s autonomy system that will permit it to gradually work more independently of the operator. With more Z-Boat autonomy, survey ships can operate a larger fleet of boats without adding additional operators.

ENS Head pilots Z-Boat
Ensign Marybeth Head pilots a Z-Boat in preparation for autonomous operations during training.

Capt. Richard T. Brennan, chief of the Coast Survey Development Laboratory, puts this move into a strategic technology context.

“NOAA envisions unmanned and autonomous systems working in conjunction with our manned systems, deployed and controlled from our hydrographic survey ships,” Brennan explained. “The Z-Boats are the first step towards unmanned surface vehicles. We are looking forward to the lessons learned to drive further innovation in communications and automation technology.”

Thomas Jefferson will be exploring other options for the boats. For instance, Z-Boats have an onboard streaming video camera, so the operator can see what the boat “sees” in real-time, raising the possibility of additional uses beyond depth measurements. And although these Z-Boats are fitted with single beam echo sounders appropriate to very shallow water, there is an option to fit them with side scan sonar, or a multibeam system, for other applications.

“Deploying the Z-Boat from the Thomas Jefferson is a significant milestone for the NOAA fleet,” said Rear Admiral David Score, director of the Office of Marine and Aviation Operations. “In the coming decade, these types of unmanned systems will become the norm. We will be able to build on Thomas Jefferson’s experience in unmanned systems as we expand these programs into the broad range of scientific observations that the NOAA fleet provides.”

The ship is selecting the nicknames of the two Z-Boats. Go to the NOAA Ship Thomas Jefferson Facebook page, and see what names they are considering!

NOAA Ship Thomas Jefferson celebrates survey success with maritime community

by Ensign Diane Perry, onboard NOAA Ship Thomas Jefferson

From 2005 through today, NOAA Ship Thomas Jefferson has been surveying Long Island Sound, one project area at a time. Some of the area was last surveyed between the late 1800s and 1939. For the 2014 field season, Thomas Jefferson was assigned her final Long Island Sound project, 89 square nautical miles of Eastern Long Island Sound, Fisher Island Sound, and Western Block Island Sound. When this project is complete, we will have resurveyed over 95% of Long Island Sound and all of Block Island Sound with modern survey technology that allows for a complete picture of the seafloor and highly accurate soundings.

This image depicts Thomas Jefferson's bathymetry from eastern Long Island Sound to Gardiner's Bay.
This image depicts Thomas Jefferson‘s bathymetry from eastern Long Island Sound to Gardiner’s Bay.

Data acquired by the Thomas Jefferson will update the region’s nautical charts and will serve other users within NOAA, the U.S. Geological Survey, and a New York and Connecticut Long Island Sound Seafloor mapping initiative. The mapping initiative creates products for habitat mapping and geological interpretation, and supports state planning and management of this vital resource.

Bringing the hydrography of this area into modern times has been a huge task, and we appreciate being welcomed as a part of the area’s maritime community! When Thomas Jefferson was asked to participate in the Connecticut Maritime Heritage Festival in New London this summer, the crew was excited for the opportunity to showcase the results of nearly a decade of surveying effort.

On September 12, Thomas Jefferson docked at City Pier, dressed in semaphore flags to welcome crowds lining the pier eager for guided tours. As the sun set, Thomas Jefferson hosted judges and the announcer during the festival’s lighted boat parade. The ship continued to provide tours the next day, and was the highlight of the event for many visitors. More than 500 visitors toured from fantail to bridge, learning about the ship’s mission and hydrographic survey operations, life at sea, and maritime heritage of NOAA and the Office of Coast Survey.

As the festival ended, Thomas Jefferson’s crew cast off from City Pier to return to their Long Island Sound working grounds and continue survey operations. We are excited to return to the survey area and complete the 2014 Long Island Sound mapping project.

NOAA Ship Thomas Jefferson
NOAA Ship Thomas Jefferson dressed in semaphore flags for Connecticut’s 2014 Maritime Heritage Festival. Photo by Lt. Cmdr. Abigail Winz.
LCDR Jim Crocker and Alex Ligon wrestle with wayward semaphore flags
Cmdr. James Crocker and hydrographic assistant survey technician Alex Ligon wrestle with wayward semaphore flags to keep NOAA Ship Thomas Jefferson looking her best for Connecticut’s 2014 Maritime Heritage Festival. Photo by Lt. Cmdr. Abigail Winz.
Lt. Guberski talks to tour group
Lt. Megan Guberski greets a tour group about to board NOAA Ship Thomas Jefferson during Connecticut’s 2014 Maritime Heritage Festival. American flags patriotically line New London train station in the background for the special event. Photo by Lt. Cmdr. Abigail Winz.
Photo of Guertin, Stone, Moulton, and Johnson
From left to right, “Teacher at Sea” Dr. Laura Guertin, hydrographic survey technician Allison Stone, Ensign Stephen Moulton, and general vessel assistant James Johnson dedicated their time to spreading the word about NOAA’s hydrographic mission.

NOAA Ship Thomas Jefferson prepared for 2013 survey season

By Ensign Brittany Anderson, Junior Officer, NOAA Ship Thomas Jefferson

After a quiet winter at home port, the NOAA Ship Thomas Jefferson completed her sea trials this week in preparation for the 2013 field season.

Each year, prior to departing for working grounds, the Thomas Jefferson transits to the Chesapeake Bay to perform tests on the ship’s and launches’ systems and hydrographic survey equipment. Crews conduct numerous tests to check the accuracy and precision of multibeam echosounders, side scan sonar, and the sophisticated suite of programs that process all the data. Additionally, this is an opportunity to ensure the safety of the vessel and her crew by performing numerous safety drills and readdressing safety standards and operating procedures.

Screen grab of sonar images
This is a screen capture of the simultaneous multibeam and side scan coverage of an obstruction used to verify the NOAA Ship Thomas Jefferson’s imaging and bathymetric sonars.

But it wasn’t all just tests and drills. During her transit, the Thomas Jefferson also deployed a GPS tide buoy to make real-time tides more accurate and efficient for the region.

Deploying GPS tide buoy from TJ
Jack Riley and Brian Murray from Coast Survey’s Hydrographic Systems and Technology Programs Branch assist with GPS tide buoy deployment.

Now that the vessels and equipment are ready for the season and the crew has their sea legs back, the NOAA Ship Thomas Jefferson will be returning to the Northeast this year to further update nautical charts for critical shipping and transportation regions.