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The shape of water: bathymetry in action

November 10, 2020  - By

As the skipper of Galileo 4, a 50-foot sailboat on the Columbia River, I instruct my crew to alert me if the water under the keel drops below 10 feet and take immediate action if it drops below 5 feet, because I cannot constantly monitor my chart to avoid running aground. Yet, the huge cargo ships that navigate the river for 100 miles from its mouth at Astoria to the Port of Portland sometimes have as little as two feet of vertical clearance.

This feat of navigation is made possible by the knowledge, experience and electronic equipment used by the river pilots who steer the ships, the hydrographers who survey the river, and the dredge operators who perform the Sisyphean task of maintaining the required depth of the navigation channel. Each additional inch of draft they enable allows a ship to carry additional cargo worth up to several million dollars.

In similar ways, marine professionals around the world cooperate to chart ocean bottoms and to keep ports, harbors and navigable waterways safe for the more than 90% of trade that is carried by ships. Additionally, off-shore installations—such as fiber optic cables, pipelines, drilling platforms and wind turbines—all require accurate surveys of the ocean floor. Finally, population growth in coastal areas and sea level rise due to climate change are driving the need for bathymetric data for planning and emergency management.

Bathymetry

For centuries, mariners recorded water depth using nothing more than a lead line, a compass, a sextant and a rudimentary nautical chart. This was such a time-consuming process, however, that they could only perform it for a tiny percentage of the world’s oceans and coastlines. Today’s technology makes the process not only more accurate, but also vastly more efficient.

In deep waters, depth data is collected using huge multi-beam echo sounders (MBES) that operate at very low frequencies. As the depth decreases, smaller devices are used that operate at higher frequencies and, therefore, have higher resolution. However, close to shore, the efficiency of these devices drops dramatically, as the cone of their sound signal is cut off by the slope of the shelf. This is where airborne lidar sensors become a much more efficient means of collecting depth data.

In addition to data from the sounders, bathymetry requires data about the vessel’s location and attitude. The former, an obvious requirement for any kind of mapping, are collected by differential GNSS receivers. The latter, collected by an inertial measurement unit (IMU), are used to compensate for variations in the depth measurement depending on the vessel’s rotational movements (roll, pitch and yaw) and translational movements (heave, surge and sway). This is the same reason that aerial photogrammetrists use IMUs on aircraft.

Challenges

Traditionally, MBES systems have been large, complex and expensive. However, they are rapidly becoming smaller, cheaper, quicker to deploy, and easier to use thanks in part to the introduction of inertial systems that use microelectromechanical systems (MEMS), said Ludovic Bazin, technical support manager for SBG Systems, which specializes in MEMS technology. “You can see that the new systems are being increasingly deployed in smaller autonomous vehicles, on smaller autonomous surface vessels (ASV), and even smaller vessels. So, people can go quickly in operation,” he said. An additional advantage, he pointed out, is that they do not require an export license.

A key to accurate bathymetric surveys is reducing the error budget aboard the vessel, where the survey positions are tied back to a GNSS antenna. “You have errors all the way through the system,” said Richard Turner, vice president of global marine sales for Hexagon’s Autonomy & Positioning division, which caters mostly to the market for survey related to oil and gas. He attributes the largest improvements in recent years to the increase in accuracy using precise point positioning (PPP). “If you are out of range of real-time kinematic (RTK) and any other near-shore positioning, the accuracy of PPP is constantly improving,” he said. “It is getting down into the five-centimeter range horizontal or better than that.”

Turner also pointed to the tight integration of inertial navigation system (INS) technology with other systems. “Every time you improve the accuracy of your system the specs go up,” he said. Therefore, the challenge is to ensure that the equipment is installed properly, which requires very accurate offset measurements. “It is no good having two centimeters position accuracy if your heading or your offsets are wrong.” Generally, he points out, boats are not designed for this type of installation, due to such things as long cable runs.

Hexagon will send surveyors out with equipment from Leica, one of its divisions, to do the dimensional control and to calibrate the gyroscopes, which are another source of error. In 2014, Hexagon acquired Veripos. “Many of the people in the Veripos organization come from the offshore survey world or the dredging world, so it is very marine focused,” said Turner. “No other providers have the marine experience that we do.”

For bathymetric software companies, the main current challenge is “keeping up with all the modern and cheaper hardware,” including RTK receivers, echo sounders, and side scan sonars, said Leon Steijger, owner and programmer at Eye4Software B.V., which makes the Hydromagic software.

Requirements and capabilities

To get accurate data, all position and depth records must be timestamped with high precision so that the location of the echo sounder pings can be calculated during post-processing, Steijger noted. “The software needs to be able to generate elevation maps, depth contours, and 3D terrain views and must support volume calculations to calculate how much water there is in a basin, or to determine how much material has been removed during dredging operations.”

Hydromagic uses “plugins,” which are pieces of software that are loaded optionally to interface sensors with the software. “For some hardware we also offer a plugin containing a user interface that can be used to, for instance, upload a planned route to an automatic pilot or to control the signal processing parameters of an echo sounder.” Operators only need to specify the dimensions of their vessel and correct for the sound velocity and the static draft (the distance between the water surface and transducer). They see the vessel’s track in real time, but the rest of the data are post-processed.

Hexagon controls its own correction services and the network that delivers them. “We obviously build our own GPS receivers, so we can tightly integrate inertial systems,” said Turner. “We use third-party inertial systems. However, because we have access to the tracking loops on the GNSS boards, we can tightly integrate that inertial system so it gives a level of coupling that’s difficult unless you are actually building those boards yourself.” While near-shore operations can use RTK or post-processing, he pointed out, “the offshore guys often use real-time positioning to collect data for oil and gas. And that is really where we come to the party, because we have all those services too.”

SBG Systems designs, manufactures, and calibrates its own IMUs, then integrates them with GNSS boards, creating OEM products. “We also design and produce our own firmware algorithms to merge all those datasets,” said Bazin. “From the selection of the MEMS sensor to the final product, SBG will design, manufacture, develop, and produce the entire systems. We also provide tools for people to integrate our systems to develop their own libraries or to integrate our libraries into their systems and work with some integrators for APIs so they can control our systems from their own application.” The company’s post-processing system, Qinertia, integrates GNSS corrections with raw IMU data. “So, when we do post-processing, we reprocess an entire solution at the end for position, but also for stabilization for pitch, roll and heading,” Bazin explained. One of the benefits is the ability to remove many multipath effects.

For bathymetric surveys using an unmanned aerial vehicle (UAV), the control software must keep the platform at a constant altitude and speed over the surface of the water, because the echo sounder is dragged through the water at the end of a cable, explained Alexey Dobrovolsky, CTO of SPH Engineering, based in Riga, Latvia, which delivers UAV-related software. Therefore, he said, “missions should be executed in a fully automated mode.” His company’s software only requires the UAV’s operator to define the survey area, set the direction of the survey lines, and specify the distance between them. The software will handle everything else. “We automatically recalculate the depth measured from the echo sounder to the real depth in our data files using data from a radar altimeter,” he said. “Our software contains a high-end model of the echo sounder, which has a tilt sensor and a pitch sensor.”

Of course, dragging an echo sounder from a UAV only works for small areas, such as in open pit mines where the liquid can be very contaminated. “The flight time with an echo sounder of the most popular UAV will be around 20 minutes,” said Dobrovolsky. “That determines the maximum length of the survey lines that can be covered by a single flight.”

A couple of years ago, SPH began to provide some UAV-based bathymetry solutions that use low frequency ground-penetrating radar (GPR). There are two scenarios when GPR can be useful for bathymetry, Dobrovolsky explained. The first one is to do bathymetry through ice on the surface of lakes or rivers, which would require drilling holes to use an echo sounder. “With GPR, you can do bathymetry through the ice layer,” he said. The second scenario is mountain rivers with extremely strong currents, when it is not possible to use a standard manned or unmanned boat, because GPR works without contact with the water.

Bathymetric systems are now also deployed on autonomous underwater vehicles (AUVs) that are only one to three feet long. “MEMS INS are compact and can be integrated directly with MBES systems, which provide an all-in-one compact system that can be easily deployed and operated because they are lightweight and their mechanical alignments are known and fixed,” said Bazin. “Some of these systems can go 2,000 meters below the surface of the water.” In post-processing, he pointed out, some MEMS INS can have an angular accuracy as low as 0.07 degrees for the vessel’s pitch and roll and a heading accuracy of as little as 0.01 degrees.

Outputs

To integrate diverse sensors with a UAV, SPH developed an onboard computer, called UgCS SkyHub, that logs data from the sensors. In the case of the echo sounder, it can be an NMEA stream or just a stream of current depth measurements, said Dobrovolsky. The device is also connected to the UAV’s autopilot, so it logs the platform’s position and speed, and with the altimeter. UgCS SkyHub can record three types of data files: a CSV file containing the coordinates, depths, and a few additional parameters; a file in NMEA 0183 format, which is also standard for bathymetry; and a SEG-Y file containing the full echo sounder data, including, for example, sediments and objects in the water.

SBG Systems’ software has two kinds of outputs, Bazin explained. First, a proprietary binary format, as well as NMEA and ASCII formats, that are used to provide stabilization and navigation for the platform in real-time. Second, a standard as-built survey format for post-processing. “Then, we have very powerful tools to output ASCII files that are completely configurable from header to footer,” he added.

Eye4Software’s main outputs are volume reports or plot sheets for end customers containing a map with depth colors and depth contours, as well as cross section views or XYZ export files for further processing in, for instance, AutoDesk Civil 3D and AutoCAD.


Feature image: A UAV from SPH Engineering tows a bathymetric sonar just under the surface of a river. (Photo: SPH Engineering)

About the Author:


Matteo Luccio possesses 20 years of experience as a writer and editor for GNSS and geospatial technology magazines. He began his career in the industry in 2000, serving as managing editor of GPS World and Galileo’s World, then as editor of Earth Observation Magazine and GIS Monitor. His technical articles have been published in more than 20 professional magazines, including Professional Surveyor Magazine, Apogeo Spatial and xyHt. Luccio holds a master’s degree in political science from MIT. He can be reached at mluccio@gpsworld.com or 541-543-0525.

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