Why Is It So Difficult to Map the Ocean? | Hacker News Reader

Why Is It So Difficult to Map the Ocean?

nautil.us

70 pointsBrajeshwar56 comments

I do research on oceanic lidar. The basic problem is that light is absorbed and scattered by water, to a far greater extent than it is by air. Thus, even though we have no problem seeing through hundreds of kilometres of air, even a kilometre of water is sufficient to filter out nearly all light [0].

In practical terms, this restricts the ability of lidar systems to detect features underwater. As a general rule of thumb, in the very clearest oceanic waters, a lidar has a maximal range resolution of roughly 70 metres. Naturally, this low range becomes further degraded in coastal waters, which are more turbid. Thus, lidar — the best method we have for mapping land — is basically impossible to use outside shallow coastal areas, and we have to rely on more difficult methods such as sonar (about which I don’t know very much at all).

If you want to learn more about this stuff, I can highly recommend [1] — a free and remarkably comprehensive reference on ocean optics (which turns out to be a surprisingly large and useful field).

[0] https://en.wikipedia.org/wiki/Aphotic_zone

[1] https://oceanopticsbook.info/

how are bathymetric maps made? if i go to google maps i can see the ocean floor in low res - is that done by satellites?

i remember trying to find data of a particular area (near azores) using NOAA maps and all i found was some sort of compendium maps made by different vessels across the years mashed together in a hodge podge of resolutions

life sounds rough for ocean floor researchers lmao

oh i read the article, its measurements of gravity wew

> "The Ocean," it says, "is big. Really big. You just won't believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to the ocean."

Somewhat related: Earth Gravity Map, GOCE, European Space Agency (ESA)

Visualized: https://www.youtube.com/watch?v=d4E5T9PSbo0

> ESA's Earth Explorer GOCE mission was dedicated to map Earth's gravity with unrivalled precision and provided the most accurate model of the 'geoid' ever produced to further our understanding of how Earth works. After more than four years in orbit, the mission came to a natural end on 11 November 2013, but the wealth of data from GOCE continues to be exploited to improve our understanding of ocean circulation, sea level, ice dynamics, Earth’s interior and climate change.

GOCE: https://www.esa.int/Applications/Observing_the_Earth/FutureE...

I bet the US Navy has classified ocean floor maps that are far beyond those available to the public.

Knowing the ocean floor better than your opponents is likely to be a big advantage when playing the various cat and mouse games with attack and ballistic missile subs that the US and the Soviet Union played for decades.

Still not good enough. US Navy subs have underwater collisions from time to time.

In 2015 USS San Francisco collided underwater sea mount at full speed. It was supposed to be deep blue sea.

In 2021 USS Connecticut collided with underwater mountain in South China Sea.

US Navy needs to map ocean floor only to 500 meters to keep it safe for submarines but they can't even do that.

To be honest; that is actually a pretty dramatically good operational excellence.

I wouldn't be surprised if the uss connecticut was a resolution issue with the mapping system. And also I think it might be hard for the USN to get accurate maps of the south china sea currently.

"San Francisco's captain Commander Kevin Mooney was reassigned to a shore unit in Guam during the investigation of the collision. The Navy concluded that "several critical navigational and voyage planning procedures" were not being implemented aboard San Francisco, despite Mooney's otherwise remarkably good record. Consequently, the Navy relieved Mooney of his command and issued him a letter of reprimand."

"The seamount that San Francisco struck did not appear on the chart in use at the time of the accident, but other charts available for use indicated an area of "discolored water", an indication of the probable presence of a seamount. The Navy determined that information regarding the seamount should have been transferred to the charts in use—particularly given the relatively uncharted nature of the ocean area that was being transited—and that the failure to do so represented a breach of proper procedures.

Nonetheless, a subsequent study by UMass Amherst indicated that the Navy's charts did not contain the latest data relevant to the crash site because the geographical area was not a priority for the Defense Mapping Agency"

source: https://en.wikipedia.org/wiki/USS_San_Francisco_(SSN-711)

So it looks like the USN charts are not exactly cheap to generate. And there is a large human element of knowing where the charts are mostly accurate and where there is information missing.

Also keep in mind both of these are attack submarines and not Ohio class which generally are routinely patrolling the sea very deep and remaining as operationally quiet as possible to act as nuclear deterrents.

I think their track record is more interesting because they have to patrol and be extremely quiet (zero sonar).

The SSN/fast attack subs usually are escorting other groups / actively following targets so they end up in situations they can't entirely control.

The reason current mapping data is difficult to find/distribute is because it is secret.

> In 2015 USS San Francisco collided underwater sea mount at full speed. It was supposed to be deep blue sea.

First, it was 2005, not 2015.

Second, the seamount they struck was marked on charts the crew had as a potential hazard, they failed to review those charts and transfer the notation to the chart used for navigation.

> In 2021 USS Connecticut collided with underwater mountain in South China Sea.

And not long before that, with a pier at Naval Base Point Loma.

Had the pier been mapped? ;)

The US Navy reports from the 2005 (not 2015) USS San Francisco and 2021 USS Connecticut groundings can be found here. In both cases primary blame was placed on the crew, although charts were a contributing factor.

https://www.cpf.navy.mil/FOIA-Reading-Room/

If you have accurately marked charts but fail to use them, charts were a contributing factor. However, it is the human error that was the blame.

Were the collisions due to mapping error or human error? Afaik, the USN hasn't said.

Human error in both cases, though both involved incomplete charts (the ridge the Connecticut hit was uncharted but there were all kinds of command/procedure errors that contributed to hitting it [and a number of significant command/procedure problems involving the ship in the months prior to the collision], the seamount the San Francisco hit in 2005 was marked as a possible hazard on charts the crew had but not the one used for navigation and the notation wasn't transferred to the charts used for navigation.)

https://en.wikipedia.org/wiki/Ehime_Maru_and_USS_Greeneville... There's other kinds of Sub collisions as well

Any ship using sonar to map the floor would not be able to do so in secret. The only hope it would have would be to take advantage of just how vast the ocean is. However, sound carries a long long long way, and it would be very difficult to have a ship doing this in secret. Any opfor ship could just tail behind the ship and passively listen to the same signal and generate their maps in secret.

Ocean floor is not mapped in secret. Most of the work is done by non-military government agencies.

US Navy has 7 Pathfinder-class survey ships under Naval Oceanographic Office scanning the ocean.

The ship will not go in secret but the resulting map can be kept in secret if military decides it makes sense.

Again, you miss the point that while they might want to make it secret, anyone following the mapping ships can secretly have a copy of those same maps.

Sounds harder than just making your own maps tbh.

not sure if ths is a daft question or not,

but could they just reduce the power of the sonar?

It's hard to appreciate just how sensitive the towed array sonars actually are. The line from Hunt For Red October about a boat off the coast of San Francisco hearing a boat "way the hell out by Pearl" is something that always comes to mind.

One could do some mapping by listening to the reverberations from underwater rock slides and other naturally occurring sources of sounds--if you just so happen to find yourself in the right spot at the right time.

If you can pick it up, so can they.

I doubt it.

Most of the ocean floor is very deep, so deep that even modern submarines can't reach it. If I were a non-human, intelligent species, that would be a perfect place to hide.

To add: the test depth for the Virginia-class submarines is 240 meters. The previous Seawolf-class had a test depth of 490 meters.

The average depth of the Pacific ocean is around 4000 meters. The average depth of the Atlantic is about 3600 meters.

Submarines still travel in what are basically shallow waters, there aren't that many of them, and mapping ocean floor isn't their primary mission. So, however detailed those maps are, I'd wager that they are still largely "here be dragons".

Because we have to use sound waves instead of radio waves. But the article is mostly about satellite based gravimetric mapping techniques which are fundamentally low resolution.

You can use laser, but that's only good for up to around 50m deep.

https://www.dst.defence.gov.au/innovation/laser-airborne-dep...

50m of clear blue Caribbean paradise water. Or 5cm of turbid Amazon river outflow.

that's not very useful for the task then is it? maybe along shorelines, but that's not what needs mapping since it's already been covered in details.

Yeah, that kinda sounds like the equivalent of "this tool maps land perfectly unless there's a tree or snow within sight".

Not useless, but far from a solution.

Right, not so much for purposes mentioned in the article, but still valuable for reefs and navigation.

No comment on the mapping specifics for this but I always found it fascinating and cool how sound waves move almost 5x faster through water than air.

If you have 100 ocean survey ships scanning the ocean floor 24/7 moving 6 knots with 1 km wide side scan sonar, it takes over 900 years to map all oceans.

10,000 autonomous ships to do it in 9yrs?

https://www.saildrone.com/solutions/ocean-mapping

with what funding for that many ships for what purpose of recovering the investment?

Sometimes purpose isn't about money. Though funding would be an issue.

Could I know your formula?

ocean : 361,000,000(km2)

6knots: 11.1km/h

361,000,000/(24*365) = 41,096(km2/day)

41,096/11.1 = 3,702

so, 100 ships take 37years?

Thank you, klaus23!

I'll throw my result into the ring. I get 37 years:

<https://www.wolframalpha.com/input?i=%284*pi*%2812733km%2F2%...>

Did it ever occur to you that you take a square kilometer as a linear kilometer in your calculation?

Don’t the ships have a kilometre wide sonar scan? Therefore traveling 1 kilometre counts as scanning 1 kilometre squared.

to what depth?

https://seabed2030.org/

Just heard about that on NPR last week: https://www.npr.org/2023/11/06/1198908501/mariana-trench-cha...

This is great, thanks for sharing.

Check out Kraken Robotics. https://krakenrobotics.com/

Synthetic aperture sonar is a massively underrated technology for undersea map making and remote sensing.

Because it gets a lot more expensive the deeper you want to go/look.

"Everything about the seafloor is bigger, bolder, and more extreme than the comparatively quiet terrain we know as land."

Well said :-)

Why is that, when it's just land covered in water

Land covered in miles of water. A distance which would crush you into jello many times over. A human body would be disintegrated. There's no light, the temperature is right around freezing, the pressure is around 500 atm.

Is it actually difficult or just not profitable? The ocean is inconceivably large.