The majority of people working with lidar elevation data use airborne topographic lidar or perhaps mobile lidar. However, in the coastal zone, there is a high demand for nearshore bathymetric lidar, especially for shallow areas that can’t be cost effectively covered by survey ships. Bathymetric lidar has been around for many years now (not a new technology), so why does bathymetric lidar cost 5 to 10 times more than topographic? Today we’re going to look at the difference between topography and bathymetry from an airborne lidar sensor.
Seeing Through the Water
The first problem is water penetration.The majority of topographic lidar sensors use a wavelength in the infrared, typically 1064 nm in the U.S. and 1550 nm in Europe. Those wavelengths will only go a few centimeters in water before they’ve lost most of their power. You need to use a wavelength that isn’t absorbed rapidly by water. If you had pure water, that would be somewhere around 440 nm. Ocean water isn’t pure, especially in the coastal zone, and absorption by chlorophyll in the blue tends to push the wavelength of maximum penetration into the green region. If you frequency double a 1064 nm laser, you get 532 nm (green) output at the cost of some power.
The Need for Power
So, now you’re using a wavelength where you have a chance, but you’re still limited. The green wavelengths may be where you can get maximum water penetration, but that’s not the same as air. You’ll often see references to how many Secchi depths a system can penetrate. A Secchi depth is simply an approximation of water clarity. It is based on a broader spectrum than our laser in combination with the human eye to determine the depth at which a disk disappears from view. A better value might be the attenuation coefficient (k) at 532 nm. This describes the exponential decay of light with depth according to the equation Ed = Eoe-kz . In rough terms, this means that for every 1/k meters you go down, you’ve lost ⅔ of the laser light you had. Since our laser light has to go both down and up, you can double the number of meters traversed in figuring out your power loss.
Ok, so you’re losing a lot of power, just pump up the output on the laser, right? Not so fast. We’ve got a couple of problems here. The first is simply how much power you can pump out of a laser. You’re only going to get so much power out before you start to fry your lasing medium. On top of that, you have to divide up that power across your pulses, so higher power equals lower pulse rate. The second is eye safety. There are regulations to follow to make sure you don’t blind someone that happens to look up at your low flying plane. You’ll spread your laser beam to deal with that and probably end up illuminating something on the order of a square meter on the water surface. In that case, trying to get multiple points per meter is a bit silly.
Diagram showing the interactions for a bathymetric lidar laser pulse. Source – USACE.
If I put enough power through the system at an appropriate wavelength to get to the bottom and back, I should be good, right? Just calculate the range, all the angles to figure out where I was pointing, etc., and I should have my answer. Physics says “no.” For topography, the things in the way, like trees, just block some portion of the light and reduce your signal. Similarly, in the water, the absorption reduces your signal. Water does something else though. It bends the light and slows it down. It’s not a straightforward calculation anymore. We have to know where it bent, how much it bent, and how much it slowed down. If we can figure out where the water surface is, this isn’t too bad. After all, we know the index of refraction to get the speed of light in water. We know the angle at which the laser pulse hits a flat water surface (let’s not think about waves right now). If we have the water surface, we’ll know where it bent and can figure out the rest. Unfortunately, a lot of the time with just a green laser, you can’t tell where the water surface is. Many bathy systems will use an additional infrared pulse to determine the water surface. That’s yet more complexity in the system and the processing. The system also can’t use simple response thresholds to determine object returns the way a topographic lidar can. You’ll get response from scattering within the water column and due to that exponential signal decay, the response near the top of the water column is likely larger than the response from the bottom surface. This means you have to do waveform analysis.
Simply More Complex
So, what does this all boil down to? Bathymetric lidar systems are far more complex than topographic, they fly low and slow to get the power they need, and the processing is much more complex. All of these add up to systems that cost a lot more to run. On top of that, water clarity is a huge issue. If you can’t get enough signal to the bottom and back, you can’t see it. You can use a lot of time and money sitting on the ground waiting for clearer water. This can be worse than waiting for the clouds to clear in the tropics (been there, done that), especially in the surf zone. All told, it makes bathymetric lidar expensive relative to topographic lidar. Keep in mind though, it’s still cheaper than trying to get a ship in many places and sometimes a lot safer too.