Quiz - Skills & Environment - Determining Accurate Distances

[NothingClever]

Hello. I thought, I would respond based on your Signature. It can be extremely difficult to judge distance underwater. Especially, without a reference point, and it is not something that you can train for. (My personal thought.)
I think the "Blue Hole." in the Red Sea is a prime example. The ceiling of the arch is 170ft. deep, and with the clarity of the water it can be extremely deceptively, alluring, with no real depth reference. (You said, brain, and eyes.....so, I'm leaving the dive computer out of the equation.)
My opinion is that poor gas management is probably the leading cause of deaths there. (Which more times than not......judging distance underwater was a contributor.) Edit: I just noticed that your location says Red Sea. I used the Blue Hole, as an example before I saw that..Hmmmmm, interesting.
Cheers.

You raise valid and important points. My methods presume a diver has visible terrain to estimate distance horizontally and for short distances vertically.

If diving in the blue, then kick cycles it is but I assert the reliability of that method is contingent on the diver having spent some time on a known distance course in various conditions and counting their kick cycle until they know it under all conditions (day-no current, day-current, night-no current, night-current).

 

[Altamira]

You raise valid and important points. My methods presume a diver has visible terrain to estimate distance horizontally and for short distances vertically.

If diving in the blue, then kick cycles it is.

I am going to disagree about "kick cycles is it" in blue water. We were taught kick cycles for distance measuring in the navigation part of OW training. However, I have always found timing to be a better and easier method of determining distance travelled or computing time required to swim X distance. Of course, this assumes the diver has a timing device, and has done enough timing runs to know how long it typically takes to swim a known distance at usual finning speed. My perspective is based on the fact that I am very used to flying precise low level navigation runs using time and distance, and yes, I do realize there is a difference between navigation while flying vs diving. My main beef with kick cycles is it is too easy to lose track of kick cycles with even minor distractions, especially if you are diving with a less than stellar dive buddy. Assuming you remember to set your watch and annotate the start time, you don't have to count one-two-three, etc. Obviously, both timing and kick cycles can be affected by the current, but I think it is easier to mentally adjust the time to compensate for current, and avoids having to keep track of the kick cycles.

 

[loosenit2]

I suppose it makes sense to simply measure your arm span.

but if you measure your arm span is it calibrated?

 

[CT-Rich]

Personally, arm length seems useful only for things that I can see from end to end, or if I was clawing across the bottom. How good you are at judging distances underwater really comes down to experience. In low visibility (~10-15’) over open bottom, I rely on swimming time (I lack the attention span for counting kick cycles) And bottom topography (am I getting deeper and when am I at the depth I am expecting to reach my destination?), the direction of the sand ripples. My compass, it doesn’t matter how far you swim if it isn’t in a straight line. Every dive computer gives time and depth. And finally, you need to know where you are going, even if it is just straight out and back.

I would be curious to see the passage in the course that actually explains how to use arms’ length to measure distance.

 

[Pedro Burrito]

From the PADI Underwater Navigator manual:

For short distances and greater accuracy, use arm spans. Arm spans work well in poor visibility, or when you're dealing with currents that affect swimming. There are two methods of using arm spans: vertical and horizontal.

Vertical arm spans (also called arm reach) are quickest. Measure by reaching overhead and forward with one hand as far as you can. Swim or pull yourself forward, keeping the hand in place on the bottom as you pass it. When you've gone as far as you can, reach your other arm forward and swim/pull yourself to that hand. Repeat the process, counting arm spans.

For horizontal arm spans, reach forward with one arm and put your hand down. Then, pivot on that hand and reach out with the other, so you end up perpendicular to the travel direction with your arms wide. Your first arm should be pointing back the way you came and the second the way you're going. For the next arm span, pivot on the second hand and reach out with the first. Alternatively, keep your forward hand in place and move toward it until you put your rear hand in its place. Now keep the rear hand in place while you spread yor arms for the next span. Repeat arm spanning until you reach your destination. This is usually slower than vertical arm spans, but it is slightly more accurate.

To use arm span, it's best to measure yours in advance.

...

 

[tursiops]

From the PADI Underwater Navigator manual:

The OLD versions of the AOW manual in the Underwater Navigation chapter have a brief paragraph on arm spans for distance estimation, but the NEW version of the manual removes that. And, where mentioned, it is restricted to short distances, and a bottom/structure that it is OK to touch. So it seems PADI is evolving its material to downplay arm spans, and they were for use in limited situations anyway.

This exam question is in that category of not-written-very-well.

 

[CT-Rich]

The OLD versions of the AOW manual in the Underwater Navigation chapter have a brief paragraph on arm spans for distance estimation, but the NEW version of the manual removes that. And, where mentioned, it is restricted to short distances, and a bottom/structure that it is OK to touch. So it seems PADI is evolving its material to downplay arm spans, and they were for use in limited situations anyway.

This exam question is in that category of not-written-very-well.

A better phrasing of the question would be “what method of measurement is only useful in the most limited way possible and virtually is, in general practice completely worthless?”

I rereading the question, it does say a measuring device, which, l assume means a submerged whale carcass or freshly discovered ship’s cannon....

 

[tursiops]

I rereading the question, it does say a measuring device, which, l assume means a submerged whale carcass or freshly discovered ship’s cannon....

Sorry, I don't follow. How is a whale carcass or a cannon a measuring device?
I understand that one might want to measure those two examples with something other than fin kicks, so arm spans might be reasonable, but I've never seen a cannon so long that it would take even a single arm span.....and how does one estimate "2/3 of an arm span" with any accuracy? How does one do arm spans on either object without touching and possible disturbing it? Sorry, I think arm spans are from another era, like standing on the bottom. Since more and more folks are carrying a small reel/spool attached to their SMB/DSMB these days, they an easily stretch out a piece of line, put a small slip knot in it, and measure that with a tape after the dive.

 

[Aviyes]

Works great for a sandy or rocky bottom. Not so much for a reef system.

 

[Vitesse2l]

What is it that's being measured such that armspans are an acceptable unit? If the purpose of a dive is to 'do some measuring' then you'd probably be adequately equipped for whatever. If it's just to roughly size a discovery then near enough is good enough, surely. Like the old anchor I came across with a buddy. About 8' long shank, my height plus out stretched arm. There's no intention to count multiple arm spans is there? Maybe doing three or four might be ok. Like pp's have said, a knotted line is easily doable. Not having taken the course it's interesting to know the context of the original question.