Question about Navigation Course

Re: Very nice and all that ....

argh! The graph did not appear embedded but I am sure you can cut and paste the link into your web browser. Lakesailor, how do you get the image to appear automatically?

Thanks,
Cliff.
 
Re: Very nice and all that ....

here you go

48329822-M.jpg
 
Re: Very nice and all that ....

a lot of online image sites won't allow image linking to other sites because of the bandwidth. You have to put the image on to a site which allows linking
 
Re: Very nice and all that ....

[ QUOTE ]
a lot of online image sites won't allow image linking to other sites because of the bandwidth. You have to put the image on to a site which allows linking

[/ QUOTE ]

Brendan, thanks for that.

Cliff
 
Re: Very nice and all that ....

[ QUOTE ]


Larry, we're talking about different things. I am talking about a rate of change with respect to the ships heading, not how quickly this heading changes.

Cliff.

[/ QUOTE ]

...isn't that your definiiton of dD(C)/dC? or in english, the rate of change of D(C) with respect to C?

You are making this way too complex. Firstly, Deviation is measured at fixed points around the compass, and not as a continuous function. If you can derive a function for your deviation differentiation of that function will not lead to a simple equation as you suggest. Interpolating results between the measured points is very much an approximation that is "good enough" in navigational circs.
 
Re: Very nice and all that ....

Larry, I abandoned the whole calculus approach a good few messages ago, identifying it as a red herring to the discussion. I've gone now for a much more basic and simple graphical argument. I know you have issues with the graph approach but ...

Even if we take your approach, and we make discrete measurements of deviation for various headings (as is done in practice of course), we could end up with discrete sample points lying on my graph, and you still have more than one compass reading for the same magnetic reading. In fact, what if we made just three measurements - the ones I identified on the graph, we end up with three compass readings all corresponding to the same magnetic heading. There, that's no graph involved at all. So, I suppose that's the whole issue - is there a physical reason why this cannot happen? I'd be happy to understand why if so.

By the way as evidence that it could happen, I have seen similar looking graphs in aviation websites, but there it's far more complicated (because of pitch and yaw, etc).

Cliff.
 
Re: Very nice and all that ....

The graphical illustration is interesting, I would like to know if this could be engineered in reality. However, compass correcting is an art, and is time consuming. A properly set-up compass would never have a curve showing 0-15-5 degrees of error over only 25 degrees of swing. More normally, the result would be a curve close to a sine, with hopefully less than 10 degrees maximum error.
I'll dig out some text books and see what they say.
A yacht compass doesn't have the same capacity to have errors taken out that a big ships does, and is more likely to have affecting material placed close by from time to time, unlike a ships compass standing on the monkey island with large correction bars in place. A yacht compass will move with only a relatively small piece of material nearby.
Despite the amount of calculus and spherical trig we were taught at college, navigation remains an art not a science. While I find this discussion interesting, I think you need to try not to use your mathematicians head too much when learning that art!

Alistair
 
Re: Very nice and all that ....

Cliff, I don't follow your curves....what are the lines 'Deviation' and 'Magnetic'? You have two axes, 'Compass' and 'Magnetic', presumably you are trying to represent the actual magnetic heading for a given compass reading? In which case, why two curves? Surely you plot a point being x along one axis and y along the other, from observation (I have never heard of anyone trying to predict or fit a mathematical function to deviation).
 
Re: Very nice and all that ....

Lemain, just look at the green curve first. That's the magnetic vs compass curve. I then added the deviation vs compass curve in red / purple underneath. You don't need this but it shows you where the green curve comes from. Every point on the green curve is the sum of the compass coordinate (along the bottom axis) plus the corresponding deviation value. So for example, if you are at a compass heading of 010, deviation looks to be about 012 and therefore the magnetic reading corresponding to compass reading should be 010+012=022 as depicted by the green curve at compass reading 010.

That means the vertical axis is only properly labeled for the green curve. Sorry if that caused confusion.

Cliff.
 
hm possible error in axes notation

I think perhaps the vertical axis should be labelled "likelihood of nav instructor topping himself" and horizontal axis "number of irrelevant quasi-mathematical posts about nav which is v simple anyway and was explained 2 days ago on this thread", no?
 
Re: Very nice and all that ....

[ QUOTE ]
Every point on the green curve is the sum of the compass coordinate (along the bottom axis) plus the corresponding deviation value. So for example, if you are at a compass heading of 010, deviation looks to be about 012 and therefore the magnetic reading corresponding to compass reading should be 010+012=022 as depicted by the green curve at compass reading 010.

[/ QUOTE ]You have lost me. If the green curve is straightforward then it should be the Compass reading for given Magnetic headings. If it isn't, why isn't it?

If the green curve is the Magnetic vs Compass then how on earth does the slope ever go negative? What non-active device could result in a negative slope? Is this a real example or an invented one?
 
Re: hm possible error in axes notation

TCM, you seem to be getting very annoyed about this discussion. If you think its irrelevant, why do you keep coming back posting - just ignore it, as can anybody else who thinks so.

BTW, I take no offence from your remarks, in fact I got a giggle from them /forums/images/graemlins/smile.gif

I am not trying to be controversial or anything, I just think I noticed something that's interesting to me and decided to air it in case it is of interest to others and find out if there is anything to it or not.

Cliff.
 
Very good point. May I commend the truly idle navigator\'s approach

as practised by me:

1. Engage a professional compass adjuster. They usually have a fund of anecdote, so the time passes pleasantly as you motor about.

2. By the time he has finished, the steering compass should have no more than about 2 degrees of error on any heading, disrgarding heeling error, which we all do.

3. Do all your chartwork using the magnetic rose, and ignore the residual 2 degrees of deviation.

4. Er..very good point about "affecting material" placed nearby. Since most boats have the galley near the cockpit, the potential for someone to do something daft is ever present...and don't ask me how I know that!
 
Re: Very nice and all that ....

[ QUOTE ]
[ QUOTE ]
Every point on the green curve is the sum of the compass coordinate (along the bottom axis) plus the corresponding deviation value. So for example, if you are at a compass heading of 010, deviation looks to be about 012 and therefore the magnetic reading corresponding to compass reading should be 010+012=022 as depicted by the green curve at compass reading 010.

[/ QUOTE ]You have lost me. If the green curve is straightforward then it should be the Compass reading for given Magnetic headings. If it isn't, why isn't it?

If the green curve is the Magnetic vs Compass then how on earth does the slope ever go negative? What non-active device could result in a negative slope? Is this a real example or an invented one?

[/ QUOTE ]

The green curve is M vs C.

The purple curve is D vs C.

What I was describing in words was the fact that M = D + C

The curves are invented. And so my question boils down to whether this situation can happen in reality or not - a question physics.

As to how the slope can go negative physically, I think it could happen when one of the needles of the compass comes close to a strongly repulsive or attactive magentic material on the boat.

I am not saying this has to happen or even that it can happen physically. It'd be great if someone can make a convincing physical argument that says it can or it can't happen.

Cliff.
 
Re: Very nice and all that ....

[ QUOTE ]
The graphical illustration is interesting, I would like to know if this could be engineered in reality. However, compass correcting is an art, and is time consuming. A properly set-up compass would never have a curve showing 0-15-5 degrees of error over only 25 degrees of swing. More normally, the result would be a curve close to a sine, with hopefully less than 10 degrees maximum error.
I'll dig out some text books and see what they say.
A yacht compass doesn't have the same capacity to have errors taken out that a big ships does, and is more likely to have affecting material placed close by from time to time, unlike a ships compass standing on the monkey island with large correction bars in place. A yacht compass will move with only a relatively small piece of material nearby.
Despite the amount of calculus and spherical trig we were taught at college, navigation remains an art not a science. While I find this discussion interesting, I think you need to try not to use your mathematicians head too much when learning that art!

Alistair

[/ QUOTE ]


Alistair, you got my point exactly. I can scale the graphs down to smaller deviations but then they will have to deviate over an even smaller range of compass angles. Presumably you'd object to this even more strongly. So, the question remains as in my previous post - why can't this happen physically? If you have the physical insight to tell us why, please do.

Cliff.
 
Re: Very nice and all that ....

I think you are right; this could happen given a very weak magnet placed very close to the compass - e.g. closer than 10 times the size of the compass magnet.
 
Dont agree ...

Having done magnetism at College for MN tickets ... and honest it is so long ago - I don't want to try and remember !! But the 360 turn involves changing flux and varying pull on the needle ... so your scenario of the steady south pull etc. in fact doesn't happen ... therefore the needle doesn't care ... it will settle to similar to what it showed before the turn afterwards. Well something like that !!

And the trick of nav is to find the quickest easiest way to a solution that is at or near best possible ... Remember Navigation is the art opf knowing where YOU HAVE BEEN and where you are LIKELY to be later ...

Bringing too much science into it is asking for trouble !! IMHO
 
Deviation ....

Is a result of magnetic influences of all articles and structure combined to form your boat ... (that includes your tea mug and everything else ....) As the boat alters its heading it cuts the earths lines of flux at different angles creating a sitaution that the boats lines of magnetic flux are changing in their action and reaction to the earths. This gives rise to the different values of deviation measured at different headings.

It can be argued that deviation is a result of compass heading ... partly true - but actually due to the angle the boat cuts the angle of earths flux.
 
Re: hm possible error in axes notation

I'm well aware of the freedom to ignore your posts or otherwise. In the same way, of course - you have the freedom to ignore my posts too, or otherwise. I'm pleased - even flattered- if they gave you a giggle. I might draw a graph of the amount of giggling versus something else, but there again, i might not.

Also, altho not a big deal you don't *really* have the freedom to atempt to bully people off a thread, as you sort-of tried to do to me. It's great that you're very enthusiastic and so on - but it's not "your" thread, really. Anyone can bounce in.

I mean, the forum is not like, say, your navigation class might well be, for example, where perhaps everyone listens in awe to your suggestions about taking the second derivatives of a simple formula and dobn't dream of interuppting. Or when the whole class including the instructor stops what they're doing and hangs on your every word as you hold sway about another important new issue such as what happens when the deviation swings from plus 30degrees to minus 30 degress very quickly due to your imaginary magnetic sailing trousers. The forum isn't like that.

Anyway....one useful and VERY relevant issue arising out of this thread is surely - a recommendation for those seeking nav theory classes to consider sites such as www.cmonline.com - web based training and no blimmin classrooms at all. ahhhhhh.......
 
Am I glad I've been away for a few days, sailing!

Can we scotch the 'old wives' tale' that handbearing compasses are, in some mystical fashion, immune to deviating influences? They are every bit as vulnerable to deviation as any other magnetic-sensing compass, which relates to the nature and proximity of the deviating material.......

One acquaintance, who had a business making and distributing those anti-arthritic wrist bands, used to wear one of his more powerful products on each wrist. Of course, he knew that the active ingredient in each item was a strong magnet, but quite failed to make the connection with his poor performance in club racing - when he would charge off in any direction but the right one....

It is useful to consider the long list of everyday objects that *do* cause deviation in a yachts' compass - nickel-copper coins, personal stereos, some spectacle frames, torches and their batteries in a jacket pocket, some knives - and be vigilant against their effects when, now and then, really accurate steering is required.

And watch out for dangling your HBC on a cord around your neck, while helming in the fog. The magnetic field of one affects the other......
 
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