Question for expert navigators

[bilbobaggins]

Re: John Goode\'s best guess theory

[ QUOTE ]
'how we know the things we hold to be true are true?'

[/ QUOTE ]

Various tomes touch upon 'Navigational Errors', including the Admiralty Manual of Seamanship, the Manual of Air Navigation, Bowditch, and many others. What they are all about is helping the *professional* navigator explore the considerations s/he should weigh in the balance when making a judgment. Usually, the printed stuff is intended to act as aide memoire to the written notes and informed discussion that took place on an advanced professional training course. On such, it was always IMHO stressed by selected master navigator/instructors that the concepts were intended to inform reasoning and judgment, and could not be applied mechanically as 'a rule'.

The illustrations in AMS and MAN, etc., are there for informed and experienced guidance.

For example, in operational circumstances, a certain type of gyro-compass would 'wander' Left-Right by, say, up to 2º in an hour - or ~4nm in 120nm. That led to an 'across track' error vector of 2nm in 60nm traveled. Next, the different device that measured Along-Track distance run might be assumed accurate to 5% of distance run, or 3%, or better.

Consideration of those two components, carefully monitored - and a few other statistical concepts - determined whether the figure a pro navigator drew on his chart was a circle or an ellipse, and which way the ellipse' long axis was oriented.

Some navs had a plastic template with circles of various radii; other had various ellipses. The point was, the math involved in describing a precise ellipsoid was time-consuming and impracticable in the air or on the water, without stopping the ship! What was expected was a very well-informed best, and safe, estimate - and getting on with the job of getting the next precise fix.

There are many reasons to explain the gap between one's best stab at an EP and a good Single Position Line. Making best use - in good time - of that complimentary but usually frustrating info is what a pro navigator was employed for. Just now and then, a proper judgment saved a ship or an aircraft.

Electronic kit doesn't do that.

Peeps do.

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[Guest]

Boat move .... thanks

It is making me nervous at moment as time draws near !!

Weather's not too kind for it either .... still we shall succeed !!

 

[Cruiser2B]

Re: Position Probability Area!

What you describe from AMN1 is academic - it is the theory behind a navigator's assessment of the accuracy of any position - calculated, derived or assumed. AMN1 is after all an introduction to navigation. It does not answer Gunfleet's original question. For that you have to go to AMN vol 4, which is the guide to advanced navigation techniques and is not available to the public. Bowditch describes something similar, which also answers the original question. As to whether or not the techniques of applying accumulating errors to DRs or EPs is useful in this day and age to recreational yachtspersons, then I leave that to individual opinion.

 

[DeeGee]

Re: Position Probability Area!

[ QUOTE ]
What you describe from AMN1 is academic - it is the theory behind a navigator's assessment of the accuracy of any position - calculated, derived or assumed. AMN1 is after all an introduction to navigation. It does not answer Gunfleet's original question. For that you have to go to AMN vol 4, which is the guide to advanced navigation techniques and is not available to the public. Bowditch describes something similar, which also answers the original question. As to whether or not the techniques of applying accumulating errors to DRs or EPs is useful in this day and age to recreational yachtspersons, then I leave that to individual opinion.

[/ QUOTE ]Well, of course it is academic. The whole thing becomes academic once you try to get serious. But, telling me I have to go to a secret document... that takes the biscuit ! AFAIK, vol 4 is all about Naval vessel performance etc., and advanced navigation techniques (at the time of original publication) like GPS !! Do me a favour...

 

[andy_wilson]

It's not a multiplication, it's an addition.

Add together the radii of the circles of uncertainty to arrive at the maximum possible error. This is a situation that can only occur if the same error, in the same direction is compounded. the reality ought to be less as some errors may cancel out, but then you shouldn't count on it!

 

[Gunfleet]

<<Add together the radii of the circles of uncertainty >>
Thanks for the reply Andy but my question was where do these come from and how are they quantified. I admit the question was spurred by theoretical rather than practical interest but some answers, including yours, simply repeat the question with an added bit of navigation advice!

 

[Cruiser2B]

Academics

Vol 6 gives specific vessel performance. Personally I don't understand why vol 4 is not available to the public, but there you have it. Gunfleet did not ask what the accuracy of an individual DR is - he asked what compounded errors can be applied to successive DRs. The answer to that is the Pool of Errors, which is unfortunately only described in vol 4. This technique is not academic - submarines use it all the time, and the pub advises that it is suitable for surface ops when electronic (ie GPS) means are degraded and visual (ie astro) is limited. Useful to the coastal cruiser - not at all! But if your GPS craps out halfway across the S Pacific which is dotted with reefs, then the sensible navigator would certainly apply PoE to his astro-derived positions. I've already suggested Bowditch, which has a similar, though less precise, method to do this - and is available online to the public. Is that the favour you wanted?

 

[DeeGee]

Re: Academics

[ QUOTE ]
...Is that the favour you wanted?

[/ QUOTE ]Well, it will have to do, won't it? pssst, look behind you ! /forums/images/graemlins/shocked.gif

 

[bilbobaggins]

Re: Position Probability Area!

Very largely in agreement.

Now that we're in the 'shoal waters' of Probability of Position - which is really only of use to professionally-trained navigators who can spend time pondering such things - it may help to cloud the issue by recalling a relevant lesson, taught by a hugely-skilled and competent 'Spec N' Instructor, which has certainly saved my bacon on a couple of occasions.

After laboriously learning how to construct a 'fix' from a series of position lines ( 'two legs good; three legs better' ), and hoisting in a slow-growing confidence that our little aircraft was actually somewhere within the resultant large untidy triangle on our Lamberts' Conformal chart, we were then shown by our intrepid ShackleBomber Instructor how the likely errors in our position lines were distributed 'normally'.

And also how - indisputedly - our actual position COULD be outside that comforting triangle, and frequently was......

The maths graduates on the course were outraged. The rest of us were bemused. If we couldn't trust the fixing procedures being taught, how would we have any confidence in what we were doing, 1000 miles south-west of the nearest runway and tea-bar? It took a fair time before the significance began to dawn.... We were expected to 'evaluate' - all the time.

"Follow the procedures slavishly at first; with some acquired experience, follow them with reflection; later, follow them with a professionally-informed scepticism."

Our GPS fixes are also constructed from a series of position lines. The internal calculations done to produce a displayed position are also based on the same family of probabilities as the '2 Consol rdf lines and a sun sight' which an earlier generation of navs were pleased to have. The same caveats apply - things may not be as they seem, for reasons that are not always apparent.

The best approach is still a professional scepticism, comparing one system's output with another, and using any significant discrepancy as a 'warning bell' to initiate further examination and caution.

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[Cruiser2B]

Professional Scepticism

Sage advice!

 

[peterb]

Re: Position Probability Area!

At one time we had a Decca set which showed an estimate of "probable error". I well remember going through the Minkies with the set showing a probable error of 34 miles. In fact, visual navigation showed that it was well within a hundred yards.

Don't forget, the probability of being inside a 'cocked hat' triangle is only 25%, whatever the size of the triangle.

 

[bilbobaggins]

Re: Position Probability Area!

"Aye, yeh don't know yer born. Why, when ah was a lad....."

Recall being told by venerable nav tutor, just before first night astro exercise, that if my derived position lines could be plotted on the same chart as my route, I was maybe doing something right.

And another gentle put-down of a wet-behind-the-ears sprog air navigator, regarding techniques for the reduction of the DR Circle of Uncertainty..... "If you stand with your back to the wind, with low pressure on your left, you know you're in the Northern Hemisphere."

This from a deeply-experienced Shackleton navigator - and also an RAF Racing Skipper Examiner. He gave me my first copy of Reeds, which I read cover-to-cover ( Geeek! ). Must have listened harder than I knew, 'cos I survived several thousand hours of twin-jets and - over the years - thousands more hours on monomarans....

Ah, the ShackleBomber. The only aircraft in RAF service to have a calendar fitted instead of an airspeed indicator.... that would trundle off the runway at St Mawgan or Ballykelly, and descend to operational height.... the crew didn't file a flight plan like other aircraft to inform when it was coming back; they sent a postcard..... the long wire thingy hanging out the back is not an HF aerial - it's a mackerel spinner..... the first RAF aircraft that had two microwave ovens built in, and a fridge, as critical operational equipment....

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[andy_wilson]

That bit is easy to answer.

After every passage note the method you used to arrive at your EP, how far from THE EXACT previous position, how far adrift you are from your new EXACT position, and calculate a ratio of inexactness.

To do this you will need to navigate using the old fashoined way to compare with your hopefully accurate GPS fix.

Over time you will adjust your leeway allowance for your kind of boat at different points of sail and get this closer to the reality.

Then you will find just how far out your average 3 point fix (or whatever) is, and / or for DP, ultimately realise just how crap some helms are on different points of sail, for instance some luff up more than others in gusts, others get screwed down wind to much when running.

Far more important usually to know where you are NOT than to need to know exactly where you are.

Anyway, a cheap GPS will give you the answer to your question every time!

 

[duncan]

Re: Position Probability Area!

priceless contributions to this thread sir - thank you.

 

[peterb]

Re: Position Probability Area!

Ask the Day Skipper where he is. He gets out his 0.5mm clutch pencil, sharpens the lead, puts a miniscule cross on the chart and says "There!".

Ask the Coasstal Skipper where he is. He gets out a blunt 2(or higher)B pencil, puts a rough circle on the chart and says "About there!".

Ask the Yachtmaster where he is. He puts a horny thumb on the chart and says "Round about there!"

Ask the skipper of the Thames Barge where he is. If he's got a chart, then he puts the flat of his hand on it, and says "Round about there, somewhere!".

And which one would you most rely on?

It's not knowing where you are that matters, it's knowing where you're not.

 

[BrendanS]

Re: Position Probability Area!

If the dayskipper had GPS, and Thames Barge didn't, I'd go with Day Skipper.

 

[Guest]

you forgot .....

The Barge Skipper would first of all "sniff the air" ... then tell you where he is ... THEN hand on chart and say about there mate !!

 

[bilbobaggins]

Re: Position Probability Area!

[ QUOTE ]
Ask the Yachtmaster where he is. He puts a horny thumb on the chart and says "Round about there!"

[/ QUOTE ]

This superannuated sprog nav got into Racing 'cos he knew which side of a chart to use.... You know, the coloured bit! Trubble was, enuff time it was the wrong chart - but only I knew that!

And BTW, I carries a pencil too. A bluddy big thick carpenters pencil, for plotting sun sights. No need then to bother about '76% Bands of Error'.....



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[westernman]

It's not knowing where you are that matters, it's knowing where you're not.

It's not knowing where you are that matters, it's knowing in which direction to point the bow.

 

[zoidberg]

Sheesh! It's staggering to think I was contributing to this thread SEVENTEEN YEARS ago.

As for the carpenter's pencils mentioned in #58, I've improved my nav practices and now sharpen them at both ends - I keep one edge broad/flat and t'other sharp.
Someone will be along shortly to explain why....