Underwater Navigation

[Rick Inman]

It's an inexact science (e.g. "naviguessing") due to the variables involved ... current being a major factor because it provides a moving point of reference with respect to both your starting and ending points. In other words, you have to account for the effect that current will have on your body as you travel across the bottom.

I teach a method I refer to as "mental mapping" which, as pointed out by Charlie99, means that you build a mental picture in your head as you execute your dive. To do this you need the following pieces of information ...

- an imaginary "starting line" depicting headings for "out" and "in"
- an X on that line depicting your starting location (e.g. a buoy or anchor line)
- the depth you start your dive from

To navigate your way around, you draw "vectors" representing your heading relative to the starting line. I don't use kick cycles, as they're not a practical way to dive in the real world. I use my bottom timer to approximate the length of the vector ... and compass headings to represent the direction ... and build a "map" of the dive as I proceed from point to point. When it's time to return to the buoy line I know two things ... which side of the "starting line" I am on at the moment, and the depth I want to be at when I conclude the dive. And I also have a fair idea where I am at relative to that starting location. So depending on the terrain I can do one of two things to return to the line ...

- if there's a reasonably consistent slope, I'll simply swim to the starting depth and either turn left or right, depending on which side of my "starting line" I happen to be on.

- if there's an inconsistent or relatively flat bottom topography, I'll swim in a vector that best represents the return based on the mental "map" I have been building as I do my dive.

This sounds more complicated in writing than it really is in practice. I teach it in my AOW class by having my students follow a series of flags ... each marked with a heading to the next flag. The final flag simply says "Home", and at that point they need to make their way back to the buoy line. Most times they get it on the first try. The key is to build that "map" as you go and keep track of which side of your starting line you're on at any given time.

... Bob (Grateful Diver)

Great post as usual, Bob. This is the kind of stuff that reads more difficult than it is, and once you start doing it, you just kind of do it automatically without making a big deal about it. The human brain is an amazing computer, and with a little practice you can know in your head without looking at a gage, your average depth, current depth, current pressure, heading direction, time underwater and a bunch of other stuff, plus have a mental map of your location and know how to get back to your starting point.

 

[tfsails]

As both an instrument-rated pilot and a diver, I have found it MUCH easier to navigate an airplane at 200 kts when I can't even see my own wingtip than to navigate underwater when vis is less than optimal. Why?

The only answer I can come up with is that I tend to swim to the right (I'm left-handed) probably because my left leg's kick is stronger than my right's, and therefore I can (and do) miss my target easily, often, and to the right.

The best solution I can offer is set your heading on the compass on the surface, get lined up underwater, find a topographical feature you can see and swim to it. Once you get there, line the heading up again, find another feature, swim to it, and so on. This seems to work for me, and it takes any cross-current into account.

When I started diving, I was struck by how different underwater landmarks were from terrestrial landmarks. To me, using coral formations on a reef is like using trees in a forest. They all look the same to me. Seven years later I'm still a lousy u/w navigator.

 

[Sinbad the Diver]

Magnetic variation shouldnt be an issue over the tiny distances typically covered underwater on a dive.

The closer east\west to the US Great Lakes you are the greater the impact. If you are diving on the US coast of Maine it's almost 20 deg W. Even as far south as Florida it's 10 deg W. In either case it's more than likely the inacuracy of just trying to maintain a bearing while swimming will be a bigger factor, but if you start off 20 deg wrong and meander 20 deg the same direction, your pretty much lost

 

[Charlie99]

As Sinbad the Diver noted, deviation is important if you are taking the course off of a chart or GPS that has true bearings and then trying to swim those headings using a magnetic compass.

If you are using bearings that you measured with a compass, then everying is referenced to magnetic north, not true north, and the deviation doesn't have any effect.

I've never been in a situation where any bearing I've used underwater was a true bearing rather than magnetic, so deviation has no effect.

 

[String]

The closer east\west to the US Great Lakes you are the greater the impact. If you are diving on the US coast of Maine it's almost 20 deg W. Even as far south as Florida it's 10 deg W. In either case it's more than likely the inacuracy of just trying to maintain a bearing while swimming will be a bigger factor, but if you start off 20 deg wrong and meander 20 deg the same direction, your pretty much lost

Still not convinced. Underwater generally you're covering distances of no more than 100-200m maximum and most people cant hold a course to within 10 degrees. Then add current.
Compass navigation underwater in those conditions is at best an inaccurate science.

Generally with RIBs we dont expect anyone to be able to follow a course to anything under 15 degrees and thats on longish distance trips.


In either case from what i can gather from the original posts is the bearings are all taken off the same compass thereby neutralising the effects of variation as opposed to off charts and so on.

 

[String]

If you are using bearings that you measured with a compass, then everying is referenced to magnetic north, not true north, and the deviation doesn't have any effect.

Pedantic maybe but surely thats variation not deviation?

 

[Guba]

At this point, I'll attempt to interject humor...

Last weekend, my partners and I were putting out "targets" for an underwater treasure hunt. These consisted of a four inch piece of rebar, three feet of line, and a float which was to be suspended above the quarry's silt layer. We headed out with bags of targets to a known platform marked by a buoy only to find that we couldn't hit the platform for love nor money! We KNEW the bearing, since we had done the dive without the markers earlier in the day, but it took us two tries to get there. In addition, as we were moving to another site on a known bearing, our paths kept crossing or sometimes moving farther apart, though we all had the same bearing.
(Most of you are way ahead of me, but hear me out...) Finally, it dawned on me. We were carrying the bags of targets in our hands...where we also carried our compasses. Duh. In a moment of clarity, I took one of the targets out and held it close to my compass. Sure enough, not only were they ferrous--they were truly magentic. I think some of the pieces of rebar could have picked up a paper clip! We shifted the bags away from the compasses, re-oriented and had no more problems.
Next time, we use MY targets...they're weighted with lead.

 

[Sinbad the Diver]

Still not convinced. Underwater generally you're covering distances of no more than 100-200m maximum and most people cant hold a course to within 10 degrees. Then add current.
Compass navigation underwater in those conditions is at best an inaccurate science.

Generally with RIBs we dont expect anyone to be able to follow a course to anything under 15 degrees and thats on longish distance trips.


In either case from what i can gather from the original posts is the bearings are all taken off the same compass thereby neutralising the effects of variation as opposed to off charts and so on.

I don't disagree that it is a small part of it all. But, it does seem to make sense to at least be starting in the right direction so that you can remove the inacuracies that you can control, no matter how small. Though if you tend to swim to the right and you are going in a northerly direction the variation may work in your favor.

My original post was more to make the OP aware that there is something else out there to understand and consider and part of that was the fact that I recently took a SCUBA nav course and the subject was not mentioned at all. Having spent lots of time navigating on land in the SE US, I've seen plenty of people very lost because they forgot to factor that in. The best is when you set up a land nav test for a HQ company and have them skirt a nasty briar patch close enough that if they forget to factor declination in they will end up right smack in the middle of it and tear their pretty pressed BDUs into shreds...

 

[Charlie99]

Pedantic maybe but surely thats variation not deviation?

Let's see .......

Can Dead Men Vote Twice.
Compass Deviation Magnetic Variation True.

Yep. You're right.

 

[Sinbad the Diver]

Let's see .......

Can Dead Men Vote Twice.
Compass Deviation Magnetic Variation True.

Yep. You're right.

Based on your avitar you live in MA, you obviously understand that dead men can vote as often as they like...how else can you explain Ted Kennedy's continued presence in DC.