# Magnetic Declination and handheld compasses



## GaiaOrion (Feb 21, 2009)

Hey guys,

I'm taking a Power and Sail Squadron course and we're getting into the nav/pilot part, using bearings, converting to and from true-compass. I remembered being in the field (ex-army and forestry grunt) with a handheld Silva compass that you would set the declination on and not worry about converting each bearing you took. 

Why is this not encorporated into sailboat binnacle compasses, or the handheld type? As I understand it , the binnacle type are adjustable for deviation, why not declination?
The only problem I see could be in the fact that a sailboat deck is rarely "parallel to the ground/sea, so could there be a physical construction problem?>
Apologies if there is an obvious answer I have missed here.

M.


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## sailingdog (Mar 19, 2006)

One reason that comes to mind off the top of my head is that hand-bearing and binoculars with integral compasses can't be offset that way and are often used for giving bearings for LOPs. If the boat's compass was adjusted so that it read true, rather than magnetic, the chances of making a mistake are far higher.


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## GaiaOrion (Feb 21, 2009)

*LOP need to be corrected ...*

Thanks ....but ,

in reading my course manual, I see that using a handheld, or I suspect bino's for an LOP requires correction.....so I guess I don't quite understand your logic.

M.


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## celenoglu (Dec 13, 2008)

The difference of true north and magnetic north changes depending on where you are on earth. If you are planning to use your boat in a very specific and small area the compass can be adjusted but if you move to another point on earth you will not be sure of true or magnetic north.


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## svHyLyte (Nov 13, 2008)

Declination is the horizontal angle between true north and magnetic north at a given location. Whereas declination changes as one's location changes, and at a given location changes over time ("variation"), how could one resonably "set" it for a voyaging yacht? (Although one might for a yacht confined to a given area). Some of the old compasses, particularly those of the Grid type, did have an azimuth ring or bezel that one could rotate but that's rather a lot of mechanization for a rather simple matter, no? I recall that the rotating bezel on the standard US Army field (lensatic?) compass merely rotated a reference line in the covering lens but not the underlying arc.

For our part I find having to shift from True to Magnetic and back rather tedious and so use a plotting board that allows one to adjust the north referance lines so that one is always reading magnetic rather than true bearings off the chart. This does not, however, take into account deviation which is the local error between magnetic bearings and indicated magnetic bearings for a given yacht for which one must make up a deviation card to have handy at the helm.

FWIW...


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## PalmettoSailor (Mar 7, 2006)

I'm ex-Infantry also and I don't remember that. I remember "GAMES". ie Grid(map)azimuth to magnetic, easterly, subtract. So if the declination is westerly you add an the opposite to go from magnetic to grid.

I don't remember the issue lensatic compasses having any method to set declination and I also don't remember it on the Silva Pathfinders that were even more common than the Army issue model.


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## gershel (Feb 4, 2001)

I think the correct term is "DEVIATION". Declination is used to determine locations of stars & planets.
Marc


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## Omatako (Sep 14, 2003)

*Are we sure this is "Declination"?*



gershel said:


> I think the correct term is "DEVIATION". Declination is used to determine locations of stars & planets.
> Marc


Yes, I think the OP is actually talking about deviation. If not, sorry to add to the confusion.

If it is deviation we're talking about then the most logical reason I can see for deviation not being "programmed" into a steering compass is because the deviation is different for different points in the compass. My steering compass has zero deviation at N and 5 degrees of deviation at 135 degrees and so on.

Also deviation can change with changes in the boat. Any magnetic source that changes (like your engine) will cause the compass to need being re-swung (deviation corrected).

AFAIK hand bearing compasses generally don't have any deviation calcs required and are usually interpreted as is.

Variation refers to the changes in the earth's magnetic field from place to place and has no relationship to deviation.

One of the old tricks that I learned years ago to help me remember is the mnemonic *T*imid *V*irgins *M*ake *D*ull *C*ompany where:

*T* (true) + or - *V* (variation) = *M* (magnetic) + or - *D* (deviation) = *C *(compass)


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## celenoglu (Dec 13, 2008)

Variation is the difference of the magnetic north to true north. This is some degrees to the east or west depending on your location on the earth.

Deviation is the difference of your specific compass on your boat due to some magnetic objects on the boat such as your engine etc. 

Deviation differs in angle in different directions of your boat because the metal affecting your compass, changes its position compared to magnetic north. This can be corrected by small magnets on the case of compass. If you cannot correct it you keep a able of changes for different angles of the compass to correct it.

Variation on the other hand is due to the position of the magnetic north. This can be also corrected to show the true north but only if you are sailing in the same small area of the ocean in a year. Variation changes to some extent by time also. There is a yearly increase or decrease amount shown on the mpas of the area. For example the variation is approximately 4 degrees east in Aegean sea.


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## jackdale (Dec 1, 2008)

Declination and variation are sometimes used interchangeably. Declination is used by some geologists that I teach in my nav classes.

Jack


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## svHyLyte (Nov 13, 2008)

A good discussion on the subject is available at Magnetic declination - Wikipedia, the free encyclopedia . This includes reference to an adjustable compass, below. (Note reference, in red, to E or W Decl(ination).


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## nolatom (Jun 29, 2005)

Omatako said:


> Yes, I think the OP is actually talking about deviation. If not, sorry to add to the confusion.
> 
> If it is deviation we're talking about then the most logical reason I can see for deviation not being "programmed" into a steering compass is because the deviation is different for different points in the compass. My steering compass has zero deviation at N and 5 degrees of deviation at 135 degrees and so on.
> 
> ...


I think a lot of us learned this in the old non-politically-correct days (what's the PC version, if there is one?). The version I learned in Power Squadrons many decades ago was:

"True Virgins Make Dull Companions, Add Whiskey" (meaning add Westerly error)

When going from magnetic compass to true, it was:

"Can Dead Men Vote Twice, At Elections" (add Easterly error)

PS: The answer to the latter question, in Louisiana, has always been, "Hell yes"...


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## Barquito (Dec 5, 2007)

So, in summary: 1) We usually work in magnetic (except tide tables), therefore, don't have to convert to true much, 2) each boat has it's own deviation card depending on where they have their giant subwoofers mounted in relation to the ships compass. (although I understand most don't bother correcting for deviation b/c it is within the error of steering anyway).


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## CaptKermie (Nov 24, 2006)

Both declination and variation refer to the difference between magnetic north and true north, the only distinction being that seafarers use the term "variation" while landlubbers refer to the term "declination".
I have one of those little Silva Ranger land compasses and it is still set at19.5degrees declination from 30 years ago when I first set it. The setting does not change how the needle moves but instead changes one of the circular references, namely the arrow and how it lines up from the needle. The needle still points to magnetic north but when it is lined up with the preset arrow you can easily see where true north is.


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## JiffyLube (Jan 25, 2008)

GaiaOrion said:


> Hey guys,
> 
> I'm taking a Power and Sail Squadron course and we're getting into the nav/pilot part, using bearings, converting to and from true-compass. I remembered being in the field (ex-army and forestry grunt) with a handheld Silva compass that you would set the declination on and not worry about converting each bearing you took.
> 
> ...


Power Squadron classes teach you to convert to and from true-compass for practice, so that by the end of the class it will be drilled into you head on how to do it, and the importance of doing it. P.S. classes are designed for beginners that want to know how to chart, and they assume that you don't know anything about charting. For people that understand charting, converting the way the P.S. teaches may seems overlay complicated, but it sets a good foundation for the beginner. After a person gets more comfortable with charting, some of the things learned can be done in the head thus simplifying the whole process.


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## tempest (Feb 12, 2007)

GaiaOrion said:


> Hey guys,
> 
> The only problem I see could be in the fact that a sailboat deck is rarely "parallel to the ground/sea, so could there be a physical construction problem?>
> Apologies if there is an obvious answer I have missed here.
> ...


Irregardless of the angle of heel of the vessel, the compass card floats and remains level.

As others have stated, Variation or Declination varies depending upon where you are on the planet. Deviation varies depending on the vessel. ( local (your boat.).magnetic disturbances)

All vessels should make a deviation card.

I work in magnetic..it's important to remember that all charted ..current directions are given in " true degrees"


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## XTR (Feb 28, 2007)

Someone else pointed out the important point here.

Boats move!

The declination or variation is difference in magnetic north and true north changes as you move on the globe. 

The variation in Virgin Islands is about 13 degrees, around the Chesapeake it's in the 7 to 10 range, and it would If you are sailing from Red Hook to Annapolis and you were going to set your compass for variation so that it read true you'd probably need to do it at least every 2 days to be w/i a degree or so and all that tweaking begs to have two people adjust it on the same day and have you end up on Diamond Shoals instead of Norfolk. 

Unlike map reading when hiking you typically don't have the compass sitting on the chart with the lines aligned with the grid lines like you do on maps.

Every chart shows it's variation on the compass rose, you just get accustomed to correcting, and for vessels it's a better way.


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## sck5 (Aug 20, 2007)

""Can Dead Men Vote Twice, At Elections" 

this reminds me of my favorite political quote from Morris Udall (i might not have it exactly right)

"When I die I want to be buried in Chicago so I can stay involved in politics"

another one 

"I have learned the difference between a cactus and a caucus. On a cactus, the pricks are on the outside. "


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## Boasun (Feb 10, 2007)

The politically correct for Magnetic Corrections is:
*T V* *M*akes *D*ull *C*hildren at best (add west).
*T V M D C* -->+W -E
But then I am not politically correct in any shape or form.


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## GaiaOrion (Feb 21, 2009)

*Thanks and irregardless?!%$#@!??!!*

Hey guys, thanks for all the advice and True Virgins do Make Dull Company....
and I'll be sailing in relatively the same area for a while, so changing the variation won't be necessary until my basic sailing skills are improved enough to take me where another rose prevails.

Anyhoo, please, guys, irregardless ????? check out the dictionary for lubbers and salts alike !

Mike.


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## tempest (Feb 12, 2007)

Hey Mike... you say potato, I say potato....irregardess..correct or not....has been around for a long time.... Irregardless - Wikipedia, the free encyclopedia

You asked a question, I tried to help....and your response...is to make fun of my grammar and spelling rather than work on your navigation skills...?

I tried to answer your sailing question....and overlooked your use of...
encorporated...instead of incorporated.....because this is a sailing forum not an english class....I won't make that mistake again.


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## GaiaOrion (Feb 21, 2009)

*Tempest,*

Tempest, no misnomer.

thanks for the advice, on the nav issue.

Mike.


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## hellosailor (Apr 11, 2006)

KISS.
In other words, use magnetic for everything and make your final corrections once and only once in the process. If you could "adjust" every compass or other device on the boat, sooner or later they'd wind up being adjusted differently from each other, or misadjusted. And each one would cost a tad more because of the extra parts.
Simpler to keep everything magnetic and work in magnetic, and if you're in mixed company state "Magnetic" or "True" whenever you pass along a bearing.


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## thekeip (Aug 8, 2007)

No one answered the question about declination, which is a phenonomon which affects card type magnetic compasses in the vacinity of the earth's poles. Has nothing to do with variation or deviation...has everything to do with tilt. compasses commonly used in high latitudes are the gyro variety and are always True


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## Boasun (Feb 10, 2007)

thekeip said:


> No one answered the question about declination, which is a phenonomon which affects card type magnetic compasses in the vacinity of the earth's poles. Has nothing to do with variation or deviation...has everything to do with tilt. compasses commonly used in high latitudes are the gyro variety and are always True


This is also known as a dipping. As you get closer to the magnetic poles the compass card. The card tendency to point to the poles will cause that dip. 
The poles are deep under the mantle of the earth and not on the surface, thus the card's magnetics will point down toward the poles and the drag caused by the dipping will decrease the responsiveness of that card.
When you are on the Magnetic equator, the compass card is level with the surface of the earth. But as you near the poles the card will dip toward the poles.


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## BeneteauMark (Nov 7, 2002)

I learned all the C-D-M-V-T-A-E and T-V-M-D-C-A-W stuff in the Navy, and it was useful when checking the ship's gyro compass. However, I've never seen the advantage to navigating in true on a recreational vessel on anything short of an ocean crossing. The way I see it, every conversion between magnetic and true is an opportunity to make a mistake. For me, working in true is all downside and no upside.

Everything on my boat reads in magnetic (steering compass, hand bearing compass, bino compass, etc.) I can configure my nav software and GPS to work in magnetic as well. After my first season of doing all that math as I navigated, I learned to stop worrying about true north and love magnetic. (apologies to Stanley Kubrick)

Mark


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## tempest (Feb 12, 2007)

Bosun, 

Would the dip...then be more of a deviation problem, and correctible for that particular lattitude by compensating magnets? Or is that too complex a solution?


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## jackdale (Dec 1, 2008)

The Portland Plotter makes the conversion between true and magnetic a breeze.










The "0" postion on the plotter is in true. Just read the degree under the appropriate variation. Voila.

I have used parallel rules, Douglas protractors, triangles, course arm plotters, etc.. The Portland plotter is my tool of choice.


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## btrayfors (Aug 25, 2006)

Hard to believe....

42 posts full of "answers" containing correct, partially correct, wrong, and misleading responses to the OP's question.

The term "declination" can, indeed, be exactly the same as "magnetic variation". If you're going to use it, though, in the nautical sense it's best to use the term, "magnetic declination".

Why?

Because in the nautical world the term, "declination" usually refers to the VERTICAL measurement, in degrees, of a celestial body north or south of the celestial equator. It's used in each and every celestial sight reduction problem, because it's one of the two coordinates needed to fix that body's location (the other being GHA or, for stars, SHA). It is abbreviated as: dec. Thus the position of the sun might be: GHA 123 degrees and its declination - 12.5 degrees.

I'd also be careful about the use of the term "dip", since that usually refers to a sextant correction...also needed for each sight.

*Magnetic variation* is a given...something you can't do anything about and which varies according to your position on the earth's surface. It also varies somewhat from year-to-year for the same geographic location.

*Compass deviation* refers to errors in your particular compass, due usually to nearby magnetic fields. Every mariner should "swing the compass" and construct a "compass deviation card" so you know that, e.g., when your compass reads a heading of 010 degrees there's actually a -3 degree error and the heading is actually 013 degrees.

Bill


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## WSailing (Jun 15, 2009)

*The Magnetic North Poll Moves!*

When I learned about the angle between the Magnetic North and True North as a young teen it was 22.5 degrees here in the Pacific Northwest. When I reviewed the topic now that I have aged it is 17.5 degrees.

In the Pacific Northwest the Magnetic North Pole is moving about .4 degrees a year.

Walt


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## tempest (Feb 12, 2007)

now now Bill,

Since the op referenced use of a compass instead of Sextant..I think we all ruled out..The verticle declination of celestial bodies.. 

But yes..we should should precise terminology..if not correct spelling ;-)


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## sailingdog (Mar 19, 2006)

btrayfors said:


> Hard to believe....
> 
> 42 posts full of "answers" containing correct, partially correct, wrong, and misleading responses to the OP's question.
> 
> The term "declination" can, indeed, be exactly the same as "magnetic variation". If you're going to use it, though, in the nautical sense it's best to use the term, "magnetic declination".


I'd point out that the OP did use Magnetic Declination in the OP.  And that any confusion on the part of replying posters is purely their own fault, since he was clearly discussing compasses here, not sextants.



> Why?
> 
> Because in the nautical world the term, "declination" usually refers to the VERTICAL measurement, in degrees, of a celestial body north or south of the celestial equator. It's used in each and every celestial sight reduction problem, because it's one of the two coordinates needed to fix that body's location (the other being GHA or, for stars, SHA). It is abbreviated as: dec. Thus the position of the sun might be: GHA 123 degrees and its declination - 12.5 degrees.
> 
> ...


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