A hybrid system for capturing land and water promises dramatically improved efficiency over legacy aerial approaches.

Capturing the four distinct elements of shorelines and coastlines often entails using two, three, or more separate systems. Zones of offshore, nearshore, the shoreline, and uplands, captured with separate systems, are stitched together—not always seamlessly. The zones represent a single ecosystem, and the latest approach captures it with one system.

The concept is not entirely new, however, advances in sensor technologies have made the execution much more practical, with combined outputs rivalling the precision and accuracy of individual sensors. The key to this is advances in green laser technologies.

While hydrography and bathymetry professionals would be well aware of these advances, the goal of this article is to help you explain these new possibilities to others in the decision chain of your enterprise or organization.

Not long after the first practical laser was introduced in 1960, systems employing red lasers were tested for not only terrestrial mapping but also for bathymetry. While there were limited successes, red lasers (infrared wavelength of 1064 nm) were not as well suited for bathymetry as green lasers (which operate at 532nm).

The first green lidar for bathymetry was tested in the late 1960s to mid-1970s. However, the first (in the sense of today’s technology) green laser diode was not developed until 2009 (Sumitomo, Osram, and Michia). Utilization of these types of green lasers for bathymetry would soon follow.

There are a few, commercially available large-format green laser bathymetry sensors, typically deployed for wide-area mapping applications by manned aircraft. We’re beginning to see many small-format systems for drone deployment, with significantly less collection efficiencies well suited for smaller projects. To capture the entire shoreline ecosystem, these might need to be deployed in huge numbers as separate captures in phases, together with other sensors and conveyances. The fixed-wing platform with a high-efficiency system are more cost efficient.

Green laser sensors for bathymetry often employ a circular or elliptical sweep pattern, a method proven to deliver consistent results and maximize efficiency.

“Our sensors use circular or elliptical sweep pattern,” said Andy Waddington, vice president of Bathymetric Services at Hexagon’s Geosystems division. “You get a front and a back return as the mirror rotates. However, they don’t have to be circular or elliptical. One type of sensor uses a sweep type approach. However, we found in our work that the circular or elliptical pattern produces the best efficiency for a green pulse laser. One advantage of such patterns is that they are less sensitive for breaking waves, as the forward and backward scan passes at different times and the waves move between the two captures,” said Waddington.

Legacy Approaches

Offshore bathymetry is often acoustic (single-beam, split-beam, multi-beam, side-scan), mounted on a ship or boat. Dedicated green-laser systems may be deployed on aircraft as well. In the shallows, small, unmanned surface vessels are fitted with acoustic sensors. Again, green laser systems, drone or aircraft carried, can also be used. For the shore and uplands, red laser systems on drones, aircraft, or terrestrial systems might also be used.

It is not uncommon for surveying and mapping firms to have a boat, a USV, a drone, and airborne and/or terrestrial scanners. The firm might perform all tasks or contract out the airborne component. Then each mapping component output needs to be stitched together. Positioning approaches, such as ground control points (GCP), often include post-processed kinematic (PPK) using on-board GNSS and IMU data. The capabilities of each system could vary, bringing uncertainty and inconsistencies. A single positioning stack, on a single platform, plus red and green lasers—and imaging—could all but eliminate inconsistencies inherent in many merged datasets.

To derive depth, the water surface must also be captured. Multiple returns, such as those that capture canopy layers, as well as the ground, which terrestrial applications often provide, is one approach.

“A red laser, which is what most people associate with lidar, reflects off the water surface to allow it to very accurately measure the water surface location and apply corrections for difference between the speed of light in the air and the speed of light in the water,” said Waddington. “A green laser reflects on the water surface as well, but the reflection is blended with backscatter reflections from the water volume below which gives a higher uncertainty compared to the near infrared wavelength. If there’s something in the water column, light reflects off of that and it may not go down to the seabed. Full waveform algorithms are used on post processing to extract multiple points from the water surface, objects in the water column and from the seabed for each laser pulse.”

How Deep

Mention bathymetric lidar to anyone in geomatics or mapping and the first topic is “How deep can it go?” You will often see the capability of a particular sensor expressed as a factor of the Secchi Depth (how deep a small disk can be seen from the surface). You might see 1.5x, 2x, or 3x the Secchi Depth in the specs for smaller green laser systems. But Secchi Disk measurements are very subjective, there are different standards of Secchi disks, and there are many more factors to consider.

“Although the physics is pretty well established for how well light will penetrate different types of water, it depends primarily on the system itself. And the actual physics terminology that we use is the Kd value (Diffuse Attenuation Coefficient),” said Waddington. “The Kd value is calculated via an equation which, more or less, equates to the Secchi depth, which is what you can see in particular conditions. But also, what’s going on in the water column is really important, and that’s where the turbidity observation comes in.”

If you’ve got a lot of suspended sediment or microscopic life in the water, then the green energy is either absorbed by some of that sediment or reflected by it. The green energy is sent off in different directions so that the expected return pulse never actually makes it back to the aircraft.

“If the water’s quite turbid and has a low Secchi, you won’t be able to see through it. We can associate with that as human beings. But equally, the laser light, as it goes in, can get absorbed or scattered.”

The reflectivity of the object you’re trying to detect is another key factor.

“If you’re trying to detect the seabed, and it’s nice white sand under the water, there’s a very good chance that you’ll detect it deeper,” said Waddington. “And less for say, black rock, because it tends to absorb the green light more than the white sand reflects it. We try to use a standard measure. So, when we say, for example, it’s 3.5x Secchi depth, that’s assuming that the seabed is about 15 percent reflectance value.”

The power of the laser and flight height are also factors.

“Increasing the power and energy of the laser pulse tends to result in a wider beam,” said Waddington. “If you want to get very deep, you may only get one ping from the laser because you’re putting so much energy into it. But the height you’re flying is really key. Obviously the shorter the distance, the less water there is, or the less air if you’re flying at low altitude. Then the more of the pulse actually gets reflected from the seabed. It’s the strength of that reflected pulse that’s critical. You can put as much power as you like into the transmit, but if you aren’t able to pick up the sensitivity of the reflected pulse, it doesn’t really matter how much energy you put into it.”

It would be an oversimplification to say that flying lower is better, as some of today’s green laser systems can deliver quality at depth, even at altitudes that are common for red laser systems.

“Our latest generation of sensors, including the Leica CoastalMapper, are addressing this altitude issue,” said Waddington. “We’ve developed a new sensor specifically for this type of application, a new green laser that means you can fly higher and still get good returns from the seabed. Higher than previously possible with older generations of sensors.”

Green lasers on drones can be attractive for small area missions, and where lower flights could suffice. There are issues of endurance and payload limitations for drones as it stands at the moment. And while they might become better for these sorts of applications, there are trade-offs in endurance, and how much energy a sensor might draw. Aircraft do not have such limitations, nor the weather constraints for drone operations. Coastal and shoreline environments tend to be windy, another handicap.

Next, consider that hybrid sensors, with red and green lasers, and imaging, might not be practical for all but some very large drones. Fixed-wing aircraft can target wide-area survey work much more efficiently than patched-together data from discrete sensor approaches.

One Ecosystem

“I’m a hydrographer by background and my original work is all based around acoustics,” said Waddington. “Particularly multi-beam, but also synthetic aperture sonar, and side-scan. If you look at the workflows for each of those things individually, they are pretty well established, but quite challenging to bring that data together into a genuine data set that covers the whole ecosystem. From my perspective, and I’ve been talking about this for a while: the coastal environment is a single ecosystem. We look across and analyze the whole swath of a coastal or shoreline area. And then we start to get new insights into how important one bit of that environment is to another, which is why I refer to it as an ecosystem—and it should be captured as such.”

To this end, Lecia Geosystems has developed a series of both green and red laser systems and cameras for efficient, wide-area capture.

“We are now exclusively down a hybrid route. Our current generation of the green laser is the HawkEye and the Chiroptera sensors, which are aimed at what hydrographers will refer to as shallow water but in the lidar world, we refer to as deep water,” said Waddington. “Our Chiroptera sensor is primarily a green laser, but has a red laser, and a camera in it as well that targets the back of the beach. It also targets inland and around the 20- to 30-meter depth contour, conditions permitting. And, if you want to go a bit deeper, we have the HawkEye module, which is a bolt-on to the Chiroptera. This can get you into the 25-to-45-meter range, though in clear water conditions, we’ve been down more than 50 meters with both the Chiroptera and HawkEye.”

The recently announced Leica CoastalMapper was particularly intriguing–a single-sensor combo to capture a whole environment. This is much the same approach as CityMapper does for mapping cities (e.g., for creating and updating 3D digital twins) or other broad areas, combining a high-definition aerial camera and lidar in one system. The CoastalMapper now provides this type of functionality for coastal and shoreline areas.

“We’ve brought in a high-end red laser topographic sensor and a new green laser designed so that we do not need separate sensors for the shallow and the not-so-shallow,” said Waddington.  “And it is one unit, as opposed to a bolt-on. There is an improvement in efficiency because we’ve developed this new bathymetry laser, and we’ve worked on the receiver sensitivity. You can fly much higher as well. In our current generation of sensors, you would normally operate at around about 500 meters, whereas with the new sensor, we’re reckoning to operate between 600 and 900 meters.”

“The CoastalMapper is going to be the first system that uses the new Leica MFC 250 imaging sensor,” added Waddington. “It is a 250-megapixel, multiple format camera using the latest technology and we’ve got some fantastic images from tests.”

What about the vegetation and canopy performance of such a hybrid system?

“Both red and green lasers can have good penetration performance,” said Waddington. “You can set the parameters for multiple returns from different parts of the laser waveform yourself. Unlike a lot of green lasers, we do find that our green laser can be good at this as well: the top of the canopy, the middle of the vegetation, and the return from the water surface. Though of course, the red laser is often the go-to for this. In this case, the red laser is our Hyperion laser, which is the same one we use in our high-end terrain mapper sensors.”

The Hybrid Trend

We see less often now stand-alone sensors in geomatics, surveying, reality capture, and mapping. For example, in surveying you see scanning and imaging total stations. Newer GNSS rovers sometimes feature cameras and even small scanners.

Mobile mapping systems, backpack systems, drone payloads—hybrid sensor is the new normal. Miniaturization of certain types of sensors has made it practical to combine them on one platform. However, this might not be the case for all applications, yet.

Shoreline and coastal mapping present challenges to combining sensors for low-cost operations. The efficiency afforded by large-form hybrid systems like the CoastalMapper might make it out of reach for many small firms, but the same goes for terrestrial mobile mapping systems. The efficiencies will be such that firms may no longer seek to do all elements themselves, but where contracting and partnering with aerial mapping firms, or buying data from service bureaus would be the most practical option. Whichever approach is chosen, hybrid topobathy systems are a welcome development. 

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Legacy-Shoreline-Mapping

Legacy shoreline/coastal mapping was often undertaken with different sensors on different platforms. For example, sonar on boats for offshore, small format sonar sensors on USV for inshore, and aircraft, drone, or terrestrial scanners for onshore—then each dataset is stitched together. Now, the entire ecosystem can be captured with one airborne hybrid system that combines different types of sensors, such as green lasers and imagery. Credit: Gavin Schrock  

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