Magnetic Declination and handheld compasses - SailNet Community
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post #1 of 32 Old 02-13-2010 Thread Starter
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Magnetic Declination and handheld compasses

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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post #2 of 32 Old 02-13-2010
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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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post #3 of 32 Old 02-14-2010 Thread Starter
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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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post #4 of 32 Old 02-14-2010
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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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post #5 of 32 Old 02-14-2010
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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.

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post #6 of 32 Old 02-14-2010
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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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post #7 of 32 Old 02-14-2010
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I think the correct term is "DEVIATION". Declination is used to determine locations of stars & planets.
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post #8 of 32 Old 02-14-2010
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Are we sure this is "Declination"?

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Originally Posted by gershel View Post
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 Timid Virgins Make Dull Company where:

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


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post #9 of 32 Old 02-15-2010
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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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post #10 of 32 Old 02-15-2010
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Declination and variation are sometimes used interchangeably. Declination is used by some geologists that I teach in my nav classes.

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