visibility estimation

[tursiops]

The Secchi disk was not invented to measure horizontal visibility, but rather vertical clarity/transparency/turbidity, because the purpose of the measurement was to estimate light penetration into the volume for purposes of biological productivity. The same principle can be used in the horizontal, by divers: when does an object lose sufficient contrast that it can no longer be identified....this is less than the distance at which the object can no longer be seen. Some tests relating subjective "diver visibility" and actual transmissometer (turbidity meters) measurements were confounded because the Navy divers in the test were competing with each other to get the greatest visibility distances...

Today's best practices for visibility estimates use black objects, and loss of contrast. See, for example, this study. So divers dressed in black are a good target, not their neon yellow fins! The variability in estimates decreases if people all use "identifiable" distances rather than "can sort of see something" distances.

 

[DivemasterDennis]

Everyone estimates, including me. However, I will use a vertical measure, based on depth. If I can see the broken light from wave action at the surface, I figure the viz is at least equal to my depth. Distance measured in kick cycles, if your know your typical kick cycle, can also be used. But estimate work fine.
DivemasterDennis

 

[djtimmy77]

Take the report from the dive boat/dive shop and subtract 20-30'

haha

 

[alewar]

The people from project baseline use some sort of barcode and a 21w hid light to measure visibility. I couldn't find any scientific data on that method though.

 

[Wojo]

I love this question. I'm further challenged with determining distance. I'm legally blind in one eye, and determining distance even on land is a challenge.

 

[drbill]

Truly a subjective measure. I dive mostly at night during summer and fall so vis is the farthest object my video lights can illuminate.

 

[nolatom]

Yeah, it's subjective. I see a darker mass at say 60'. Then, I see it sort of has borders at 50'. And at 40' I recognize it as the corner of the sunken tug I'm looking for.

What's the vis? 60?


I estimate it based on slowly increasing experience, wreck dimensions when we're diving one, then ask the DM and others back on the boat before I write.

 

[tursiops]

Yeah, it's subjective. I see a darker mass at say 60'. Then, I see it sort of has borders at 50'. And at 40' I recognize it as the corner of the sunken tug I'm looking for.

What's the vis? 60?


I estimate it based on slowly increasing experience, wreck dimensions when we're diving one, then ask the DM and others back on the boat before I write.

The vis is 40. You need to be able to tell what you are looking at, and it needs be black....not just see a shapeless blur or someone's light. Go talk to an optical oceanographer rather than yourselves....this is a solved problem. No reason to start over. Drop an apple, it falls. Do you really want to start saying you don't know if it will fall or not, because you don't understand gravity? Yes, some subjectivity in whether you can tell what you are looking at, but the meaning and methods of vis estimation is not really an open issue. Except on SB, I guess. Sigh.

 

[nolatom]

The vis is 40. You need to be able to tell what you are looking at, and it needs be black....not just see a shapeless blur or someone's light. Go talk to an optical oceanographer rather than yourselves....this is a solved problem. No reason to start over. Drop an apple, it falls. Do you really want to start saying you don't know if it will fall or not, because you don't understand gravity? Yes, some subjectivity in whether you can tell what you are looking at, but the meaning and methods of vis estimation is not really an open issue. Except on SB, I guess. Sigh.

I'd' have said 50, maybe 40.

What's an optical oceanographer?

 

[tursiops]

An oceanographer who specializes in ocean optics. Lots of different kinds of oceanographers, like physical, chemical, biological, geological, fisheries, etc.

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