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isogonic lines - semantics

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Paul A. Franz, P.E.

Joined: 02 Dec 2008
Posts: 280
Location: Bellevue WA

Posted: Tue May 19, 2009 3:37 pm Post subject: isogonic lines - semantics

On Tue, May 19, 2009 10:32 am, Michel Verheughe wrote:



 	  Quote: 	
		   So, heading and course are the same thing. The difference is that the Americans call

 it "heading" and the Brits call it " course" ... of course! (Yes, pun intended!)


	


Heading and course are not the same. Course and track are the same. Heading is what

you see on your DG or magnetic compass.



 	  Quote: 	
		   The actual path the aircraft is tracing in relation to the ground is called the track.


	


The 'course' and ground track are interchangeable and as far as I know that applies to

British English too.



 	  Quote: 	
		   Another difference between American and British English is that the Americans call the

 magnetic declination for magnetic variation. In fact, the magnetic variation is, as it

 name says, the yearly variation of the magnetic declination and the magnetic deviation

 is what deviates it from nearby magnetic structure.


	


Sectionals and WACs have isogonic lines which are lines of constant magnetic

variation. It would appear that magnetic variation and magnetic declination have the

same meaning.



<http://www.navfltsm.addr.com/basic-nav-plotcourse.htm>

<http://en.wikipedia.org/wiki/Contour_line>

<http://www.thefreedictionary.com/Isogonic+lines>



Sometimes it is called magnetic deviation too.



I would use the US Military definition as authoritative though

<https://ntc.cap.af.mil/ops/DOT/school/L23CockpitFam/magneticcompass.cfm>

and they called isogonic lines, lines of constant magnetic variation.



It would appear that variation, declination and deviation are all used for the same

meaning although the frequency of use when associated with the term isogonic favors

variation.

-- 

Paul A. Franz

Registration/Aircraft - N14UW/Merlin GT

Engine/Prop - Rotax 914/NSI CAP

Bellevue WA

425.241.1618 Cell



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_________________
Paul A. Franz, P.E.
Registration/Aircraft - N14UW/Merlin GT
Engine/Prop - Rotax 914/NSI CAP
Bellevue WA
425.241.1618 Cell
425.440.9505 Office

propellerdesign(at)tele2.
Guest

Posted: Tue May 19, 2009 9:55 pm Post subject: isogonic lines - semantics

To be American correct,



true course is what you get from map, compensate with variation and you get 

magnetic course, ad wind correction angle and you get magnetic heading.

ad magnetic deviation (magnetism in aircraft it self) and you have compass 

heading.

Heading is where the nose is pointing, not same as compass heading.



And why is there so little whiskey or whisky remaining in the compass?



tracking is the curved line the craft is making on its way from A to B and 

have most of the time nothing to do with the  true course, and with so 

little dampening fluid in the compass ...



Jan

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Michel

Joined: 10 Jan 2006
Posts: 966
Location: Norway

Posted: Wed May 20, 2009 1:53 am Post subject: isogonic lines - semantics

 	  Quote: 	
		   From: Paul Franz - Merlin GT [paul(at)eucleides.com]

 Heading and course are not the same. Course and track are the same.


	


Of course it is, my dear Paul. Of course, in your own language! My point was not to tell what is "correct" but to stress that language is something that evolves and has different meanings in different places. I hope you don't argue my quoting of the R.A.F, do you?



I have been involved with maritime education in English, for the past 26 years and if you wonder where I learnt English, being a native French speaker living in Norway, it is from my profession, where I am in charge to ensure the quality of our products, both graphically and verbally.



Which English to chose when you write for the international seafaring community? We chose British English. And, in Britain, course and heading are the same thing.



Regarding the magnetic declination vs. the magnetic variation, I know very well that, especially among American aviators, the variation is what is used. But then, I ask: How do you call what I call the magnetic variation, i.e. the yearly variation of the magnetic north?



The thing is: Seafarers are used to see that in the margin of their charts, as they may want to adjust it since their charts may be a few years old. Note that seafarers also use Mercator projections and not Lambert projections as airmen do.

I guess that the notion of a yearly variation has become obsolete among pilots since their charts must be up-to-date with the latest AIRAC that is published every 28 days. Flying with years old aviation maps is dangerous!



Cheers,

Michel Verheughe

Norway

Kitfox 3 - Jabiru 2200



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