The launch of the joint Landsat satellite program was certainly a landmark for Earth observation more than five decades ago. Despite the program’s focus on land surfaces, some scientists imagined using Landsat to reveal more about water, too—and even, improbably, the seafloor beneath.
The most famous researcher to explore this possibility was Jacques Cousteau, who, along with his diving team, partnered with government agencies and universities in 1975 for an elaborate test of whether Landsats 1 and 2 could determine the depth of clear, shallow ocean waters. (They could.)
Today, scientists at the USGS Earth Resources Observation and Science (EROS) Center work on techniques to enhance the usability and accuracy of satellite data such as Landsat for estimating water depth in areas close to shore. New research has found a method that works quickly and doesn’t even require calibration, or separate verification of the satellite sensor’s accuracy.
EROS Scientist’s ‘Novel’ Way to Calculate Depth
To help an international initiative, Seabed 2030, complete a map of the world’s ocean seabeds, USGS geographer Jeff Danielson, who leads the USGS Coastal National Elevation Database (CoNED) Applications Project from EROS, teamed up with Curt Storlazzi, a coastal modeler with the USGS Pacific Coastal and Marine Science Center. Their proposal to the Office of Naval Research led to research on techniques for using satellite imagery to map bathymetry, or water depth, across select coral reef areas. One promising method led by Minsu Kim, a contractor at EROS, used Landsat data in the Florida Keys, Puerto Rico and Guam.
Danielson calls this method, characterized in a recent research publication, “quite novel.” The algorithm is physics-based, meaning it calculates the depth of the water based on modeling the sunlight traveling to the seafloor and back up to the satellite sensor—in this case, Landsat. Many physics calculations are involved to ensure accurate measurements amid the extra radiance detected from atmospheric particles and the sky’s reflection off the water, the difference in reflectance from a sandy or grassy sea bottom and other factors.
Kim and another EROS contractor, Seonkyung Park, created software to perform this technique quickly on a satellite image that contains three to five bands, or wavelength ranges, that are visible (like blue, green and red). These bands are included in the most common satellite sensor type, such as the Landsat Operational Land Imager (OLI), which was used for this study.
“This approach and this paper really demonstrated the potential strength of Landsat to do this bathymetry water depth estimation,” Kim said.
“The Landsat optics and photo detector technology are state of the art,” he added. “It’s one of the best sensors in terms of radiometric optical quality. That’s why we were able to reveal its capability in the water.”
Fast Method Also Measures Deeper than Expected
Bathymetry is “essentially, the elevation under the water,” said Danielson. Bathymetry data provides crucial information for safe ship navigation; monitoring coastline changes including beach erosion, sediment transport, sea level rise and sinking land; modeling tide conditions and coastal flooding; and studying fish and coral habitats.
The new satellite-derived bathymetry technique from EROS “has improved atmospheric correction, and it’s very impressive what that piece of software can do so quickly compared to other methods,” Danielson said. “This is very impressive, what Minsu’s done using Landsat data and able to solve for these different optical properties to get bottom depth.”
One big advantage of this method is that it doesn’t require calibration, so the accuracy can be trusted for an area that has no verification via another bathymetry method such as lidar. However, calibration can improve the results further, Danielson said.
The publication points out that fine-tuning of the method with known data from one small area would help extrapolate depth measurements for a seamless map of the larger surrounding area.
One aspect of the study surprised Kim. “I initially thought that the maximum depth would be more limited, but the result I got here is much better,” he said. “We can penetrate clean water way beyond 20 meters. That was much deeper than I expected.”
The other satellite-derived bathymetry technique Danielson supported for the Office of Naval Research also involved the Key West and the southwestern corner of Puerto Rico. That study, led by Monica Palaseanu-Lovejoy of the USGS Minerals, Energy and Geophysics Science Center, used WorldView satellite stereo imagery and triangulated two images of the same nearshore location on the same day from different angles to determine depth.
Cabo Rojo, Puerto Rico
This is a view of part of the coastline of Cabo Rojo, which was a satellite-derived bathymetry study area in the southwest corner of Puerto Rico.
Usefulness of Satellites
While the earliest depth measurements were made simply by throwing a weighted rope over the side of a boat, all of today’s commonly used methods lean more high tech.
Sonar, the more traditional bathymetry method that typically relies on boats equipped with a sonar device, emits sound and measures depth by timing the sound’s returning “echo.” It can measure thousands of feet deep, but it also can be hazardous nearshore, Danielson said.
One better nearshore option for mitigating safety hazards is lidar. Lidar measures depth by reflected light using green laser pulses from above via sources such as airplanes or drones. However, it is limited in the depths it can measure to about 70 meters in clear water.
Satellite-derived bathymetry also has limitations in depth and resolution, but it is a far less expensive and time-consuming method that can help fill the critical gaps in global bathymetry where data is sparse or nonexistent.
Danielson anticipates Kim’s algorithm can help with a variety of scientific applications, including nearshore flood modeling. There’s also potential for satellite-derived bathymetry to help fill in gaps along U.S. coastal waters, which currently stand at only 55% mapped, he added.
Of course, ocean coastlines and rivers inland can change dramatically over time and require remapping, Danielson said. “There is definitely a need to do a time series of bathymetry, and that's where satellite data really could be very useful is the temporal aspect.”
The Landsat archive at EROS spans more than 50 years and covers the globe, holding a lot of potential for studying bathymetry changes over time.
Apra Harbor, Guam
A U.S. Navy submarine tender enters Apra Harbor on the west side of Guam. The Apra Harbor area was used for one site in the satellite-derived bathymetry study. (US Navy public domain photo)
Future of Landsat for Coastal Science
The two Landsat satellites in use now are Landsat 8 and Landsat 9, but the USGS and NASA are also planning for the joint program’s successor, Landsat Next, scheduled to launch in late 2030/early 2031. Landsat Next will have three satellites, significantly more bands and a higher resolution, going from 30 meters to 10 meters. Danielson and Kim are both looking forward to the improvements.
“I think Landsat Next is going to be a game changer for both the USGS and for satellite-derived bathymetry,” Danielson said. For example, it will enable agencies such as the National Oceanic and Atmospheric Administration that require a higher resolution to add Landsat data to their bathymetry mix.
They both also see great potential in the additional coastal zone bands. The current band limitations require water quality to be pretty even across an area for accurate measurements. “With more bands, we will have more freedom to evaluate differences of water quality on different spots, so we’ll have a more localized approach,” Kim said. “Hopefully that will improve the accuracy.”
In the meantime, research like Danielson’s and Kim’s will help us get a better view from above of the ocean floor below.
Learn more about the new Landsat satellite-derived bathymetry technique.
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