visibility estimation

[mathauck0814]

If you did your PADI AOW they teach you how to measure your kick cycle distance on the navigation dive. Count the number of kicks it takes for you to get to the horizon of your visibility and multiply that by the kick cycle distance you've established and you've got a rough idea of the visibility.

 

[Mike]

Best way I've found to get better at judging visibility is to start judging visibility vertically. You can hone your ability to judge distance this way since you have a much better idea what 40 feet is when you're diving in 50 ft of water. If you're guessing 40 feet and you're at 20 feet and it's 30 ft to the bottom you can start having something to correct your perceptions instantly with feed back, since you know it can't be more than 30 ft.

 

[dshorwich]

I've measured the vis many times in the course of doing environmental surveys with Reef Check California, because we need a certain minimum vis to conduct our surveys (3m, although sometimes we'll make do with a bit less). We use a measuring tape on a reel - the same sort of reel used to measure track and field events - to lay out our survey area, and before we lay out the tape we use it to check the vis, thus:

One person holds the reel, the other takes the loose end of the tape; the diver with the reel swims away from the one with the tape, who holds their hand out away from their body (black gloves against black suits being hard to see) with fingers spread. When the diver with the reel can no longer make out the other diver's fingers, they stop and note down the length of tape played out. This is not the absolute limit of visibility, but it's pretty much the limit of functional visibility, especially when your function is to identify fish species in a low-vis environment.

I think you will really have a hard time distinguishing 10 foot increments over 50 feet.

I'd agree that as the visibiilty increases it becomes more difficult to make fine distinctions - it's a lot easier to tell the difference between 10' and 30' than it is between 70' and 90'. Most of my diving is done in waters where 20' of vis is a nice day, and the vis will often vary in the course of a dive, so I might log the vis one day as 7-10', another as 15-20'. I went Maui this April and the vis was (of course) far better and far more consistent; I logged it as 100'+ and left it at that. (I didn't have my measuring reel with me, and besides it only has 100' of tape on it to begin with...)