Help with navigation theory

[MikeFloutier]

Hi,

I'm just learning navigation theory and I'm grappling with Magnetic Variation.

I had thought I'd got it; ie add a west variation to a true course to get a magnetic course etc..

Now having read another resource, to bone up on position annotations, I came across this (see pic) which seems to say the reverse.

Could someone please put me right 

 

[VicS]

Hi,

I'm just learning navigation theory and I'm grappling with Magnetic Variation.

I had thought I'd got it; ie add a west variation to a true course to get a magnetic course etc..

Now having read another resource, to bone up on position annotations, I came across this (see pic) which seems to say the reverse.

Could someone please put me right 

You are right


Diagram confusing ... dont get it at all ... ignore

 

[MikeFloutier]

Many thanks Vic,

I thought that but like they say "always double check your position":encouragement:

Regards, Mike

 

[Phoenix of Hamble]

I've always used CADET And TVMDC

Compass Add East True and True Virgins Make Dull Company, oas a prompt for True Variation Magnetic Deviation Compass

From these you can work it out in your head

 

[johnalison]

The diagram is describing both deviation and variation, so it is important not to confuse them.

There are numerous ways of sorting variation out, such as "west is best etc" so I think it is up to each of us to find the way that suits us. Personally, I like to think of a diagram such as the one shown, but that is because I normally think visually, but other people will differ.

 

[PCUK]

This visual mnemonic has always worked for me - once you grasp it. Known as the CADET rule.
Converting from compass to true - add East. Converting from true to compass add west.
View attachment 47163

 

[Blue Drifter]

Like others I have always used True Virgins Make Dull Company, normally arranged vertically as below, where going down West is best and East is least (reversed coming up):

T - 290
V - 7W
M - 297
D - 3W
C - 300

Very simple to scribble down even when wet, cold and knackered.

 

[stephennoble]

When I learnt my instructor told us to think of our wet weather gear. Assuming the chart is down below and the compass in the cockpit. You put it on going up and take it off going down. So if you want to steer 000 on your chart add as you go up and steer that on the compass 003. Of course that is just the start of it and works in UK waters!

 

[roblpm]

Like others I have always used True Virgins Make Dull Company, normally arranged vertically as below, where going down West is best and East is least (reversed coming up):

T - 290
V - 7W
M - 297
D - 3W
C - 300

Very simple to scribble down even when wet, cold and knackered.

Not going to work for much longer!

http://britgeopeople.blogspot.co.uk/2014/01/somethings-happening-to-magnetic-north.html?m=1

 

[RichardS]

TeleVision Makes Dull Children AlWays

True Variation Magnetic Deviation Compass Add West

Richard

 

[knuterikt]

Hi,

I'm just learning navigation theory and I'm grappling with Magnetic Variation.

I had thought I'd got it; ie add a west variation to a true course to get a magnetic course etc..

Now having read another resource, to bone up on position annotations, I came across this (see pic) which seems to say the reverse.

Could someone please put me right 

A little detail, the example you posted is about going from compass to magnetic to true - exactly the opposite of what you are talking about (true -> magnetic -> compass)

 

[Falling Star]

Cadburys Dairy Milk Very Tasty

 

[laika]

I had thought I'd got it; ie add a west variation to a true course to get a magnetic course etc..

Now having read another resource, to bone up on position annotations, I came across this (see pic) which seems to say the reverse.

Is the confusion that they've used "+3" rather than 3E, so they're adding to magnetic to get true?

I'm sure mnemonics were hilarious for "chaps" in the 50s but I've never been able to remember them. I have to take the slightly longer route of visualising the "magnetic north" pointer on the compass rose. With west variation of 2 degrees, 0 magnetic is the same as 358 (or "-2") true, so I have to add 2 to true to get magnetic (and subtract from magnetic to get true)

and no matter how often you do it there's still the ...hang on...did I get that the right way round....?

 

[MikeFloutier]

My fault, I took the example diagram out of context. It was being used to illustrate plotting a position from bearings on fixed objects.

It didn't mention whether the Var was E or W, I just assumed it was West - doh!

Anyway thanks for all comments, it all helps me to think it through and settle it down in my mind.

 

[Mariner69]

I have always used:-

Error west compass best

This also works for time differences in astro.

 

[Piers]

I have always used:-

Error west compass best

This also works for time differences in astro.

Agreed. West is best, East is least.

 

[sailaboutvic]

So many good ways to remember , you could use a Breton plotter .

 

[[70521]]

Given that variation is so small at the moment, about 2 degrees west here I don't bother with it.

 

[knuterikt]

Given that variation is so small at the moment, about 2 degrees west here I don't bother with it.

The beauty of learning to do it the proper way is that it will work regardless of time and place.

 

[Doug_Stormforce]

The diagram you posted confuses a "course" and a "position line." These terms mean entirely different things but the author interchanges these terms freely.

As the bearing of the oil rig has probably been taken using a hand bearing compass there is no deviation correction to apply either, however the diagram has applied the same deviation to both the ships compass and the second compass, (unless he has used the ships compass to take a bearing at about 80 degrees to his heading).