Quiz - Skills & Environment - Determining Accurate Distances

[Pedro Burrito]

From the Skills & Environment Section of the PADI Dive Theory Exam:

When a calibrated measuring device is unavailable, measuring with ___ is best for determining accurate distances.

a. time measurement

b. arm spans

c. kick cycles

d. air consumption​

I will post a daily question from my exams to help newer divers and to encourage more experienced divers to interact gracefully and helpfully with the newer divers.

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Thank you for your patience while we try to give people something to discuss other than Covid-19 and/or Politics. I will post the answer covered by the spoiler tag later today.

 

[scubadada]

Personally, I have not found this method very useful, and have not used it since Navigation

 

[TMHeimer]

I learned that arm spans is the best while measuring during the Wreck course. My vote is in the minority again. I guess if you're going somewhere and not measuring a wreck or something for a short distance, I'd have voted kick cycles (you're not gunna arm span for 100'). That may turn out to be less accurate due to awkwardness doing it. Then again, with arm spans you're not affected much by current/surge as you'd be with kick cycles. Maybe a more specific wording of the question would be good.

 

[loosenit2]

seems like marking your reel/spool every 5-10 feet with a mark or knot would be whole lot more accurate than kick cycles.

 

[TMHeimer]

seems like marking your reel/spool every 5-10 feet with a mark or knot would be whole lot more accurate than kick cycles.

That would seem very accurate (unless maybe unwieldy for really long distances?).
Time and air consumption involves factors that could make these quite a bit less accurate than the other choices. So that leaves b & c as the most possible correct answers with the choices given.

 

[Storker]

b for short distances, c for longer distances. If c, then you need to have calibrated your kicks.

 

[NothingClever]

I think if you first have a reliable sense of estimating distance on land, you’re in good shape to estimate distance UW, regardless of natural or man-made terrain and degree of current.

I don’t think this is something you can train on once or twice and get it right on a periodic basis. It’s a skill that requires constant practice and must be trained using known distances.

For me it’s key that I first have a solid grasp of what 100m looks and feels like as a base unit of measurement. I already have a strong sense of distance on land (forest, desert, alpine and tropical rainforest) but if I did not, I would measure out 100m of cave cord, tie it off horizontally and swim it multiple times during a variety of conditions (no current-day, current-day, no current-night, current-night, etc).

For anything beyond about 200m that your brain and eyes can easily grasp UW, I think you’ll need to use aids to be accurate (marked intervals, maps, terrain association, etc).

 

[tursiops]

seems like marking your reel/spool every 5-10 feet with a mark or knot would be whole lot more accurate than kick cycles.

Would that not be a calibrated measuring device, and thereore not allowed by the question?

For anything beyond about 200m that your brain and eyes can easily grasp UW

Are you suggesting you can see 100m or 200m underwater?

 

[tursiops]

I use

Spoiler

kick cycles

a lot, with reasonable (10%?) accuracy. But if I change

Spoiler

fins or from flutter to frog kick

I need to re-calibrate. If there is a current, just measure going both directions and average the two results.

Also, you can difference the two results, divide by the difference in times it took to go each way and estimate the current. Example, outbound is (say) estimated as 60 ft, and it took 80 sec. Return is estimated as 40 ft and it took 50 s. The current speed is (60-40)/(80-50) = 2/3 ft/s, which is about 0.4 knots. (This is the projection of the current speed along the track you are swimming, not necessarily the full current speed.) Alternately, you can just hover, not kicking or sculling, for (say) 30s and see how far you drift from your starting point, and in what direction. You still need to be able to estimate how far it is back to your starting point! In the previous example, you'd drift 20 ft in 30s, if that was the total current.

 

[Pedro Burrito]

Spoiler: Answer

b. arm spans