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Old 03-06-2018, 11:29   #31
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Re: true vs magnetic bearing

Use it all the time, helmsman is steering to the compass so makes it easier for the on-watch crew to maintain course.
Remember this (It's even politically correct) True Virtue Makes Dull Company, Add Whiskey. The Variation is printed on the chart rose, the Deviation you can figure out by steering a series of courses, in flat water, no current, using a GPS, and make a Deviation card for you particular boat and compass. ( Even listing the deviation for the cardinal points will be close enough for recreational boat use).
If you still need help, let me know.
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Old 03-06-2018, 11:30   #32
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Re: true vs magnetic bearing

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Originally Posted by Cadence View Post
I'd guess at most lat. it doesn't make much difference. Maybe if you are sailing to a pole.
Check the chart at post #20.

I think "declination" is specifically the angular difference measured at any point between the true north pole and the magnetic north pole. "Variation" adds local magnetic disturbances on top of that, to give the angle between the true north pole and the apparent magnetic north pole. Both terms seem to be used interchangeably now.

BTW, the MNP isn't in Canada anymore.
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Old 03-06-2018, 11:42   #33
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Re: true vs magnetic bearing

"East is least and west is best" when correcting a compass, or did I get that back word? After over fifty years in aviation and nearly as many running boats you'd think I'd remember.

When you learn to fly you have to convert True to Magnetic. Every runway and every airway is labelled in Magnetic and every airplane I flew up to, and including the DC-10, had a whisky compass.

When you sit for a Coast Guard license exam you have to take Magnetic bearings and convert them to True for coast pilotage.

Why, with today's electronics, do we still have to do this? Just make the computers, both in airplanes and boats, do everything in True and be done with it. But like celestial nav, everyone should have a little Magnetic nav skills just in case
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Old 03-06-2018, 11:46   #34
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Re: true vs magnetic bearing

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Originally Posted by xxhat View Post
I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?


Thanks,


jim
Individual practices vary a lot among recreational boaters, but per classic navigation practices:

Bearings are plotted in magnetic, because they are taken with a magnetic compass. For handbearing compasses, "deviation" (not "variation") is assumed to be zero. This is based on the assumption that user is smart enough to move away from significant sources of deviation aboard the boat when taking the bearings.

Course lines are plotted and labeled in degrees true. This makes it easier to plot and less error prone (due to eliminating conversions to from magnetic & compass).

You can take bearings with the ship's compass (PSC...Per Steering Compass), but its awkward and error prone on most vessels. The ship's compass is intended for steering a heading and is best suited to that use.

Currret set is given in true.
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Old 03-06-2018, 11:51   #35
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Re: true vs magnetic bearing

With gps, I just set up to follow the gps correct course, and note what the magnetic compass indicates. Now I have a number to shoot for when steering by compass.
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Old 03-06-2018, 12:06   #36
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Re: true vs magnetic bearing

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Originally Posted by Cadence View Post
I'd guess at most lat. it doesn't make much difference. Maybe if you are sailing to a pole.
Not so. Look at an isogonic chart. It shows variation over the surface of the earth. There are areas of significant variation which are not near poles.
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Old 03-06-2018, 13:29   #37
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Re: true vs magnetic bearing

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Originally Posted by belizesailor View Post
Not so. Look at an isogonic chart. It shows variation over the surface of the earth. There are areas of significant variation which are not near poles.
How do you correct for it on a regular bases. Most people probably sail in a particular area so deviation is probably insignificant. I guess if your in the middle of a significant deviation, at sea, it's drag out the sexton. Or is it relying on GPS? I doubt to many even use a magnetic compass today or if they have one there is no telling what Ferris metal is close by and unaccounted for. JMHO
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Old 03-06-2018, 13:43   #38
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Re: true vs magnetic bearing

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Originally Posted by Cadence View Post
How do you correct for it on a regular bases. Most people probably sail in a particular area so deviation is probably insignificant. I guess if your in the middle of a significant deviation, at sea, it's drag out the sexton. Or is it relying on GPS? I doubt to many even use a magnetic compass today or if they have one there is no telling what Ferris metal is close by and unaccounted for. JMHO
Looks like you are mixing up terminology. "Variation" is the difference in geographic N versus magnetic N. "Deviation" is due to local influences aboard the boat.

Like you say, most are sailing in the same area most of the time so need to constantly correct for variation. Most GPS' figure it out automagically and some will display the info.

Fluxgate compasses, as commonly used in auto pilots, can be swung mostly automatically to adjust for deviation and figure it out or can be set manually if not.

No need to drag out the sextant, just look up variation on the chart....and adjust for annual change which is also on the chart.

Also see process for swinging a compass (adjusting a compass for deviation, not variation, and making a "deviation card").

Deviation of ships compass is ideally already known and documented on the deviation card. If not, its easy to approximate.

Variation is typically compensated for in your navigation calculations.

Good stuff to know when the electronics fail...especially well offshore.
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Old 03-06-2018, 14:18   #39
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Re: true vs magnetic bearing

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Originally Posted by belizesailor View Post
Looks like you are mixing up terminology. "Variation" is the difference in geographic N versus magnetic N. "Deviation" is due to local influences aboard the boat.

Like you say, most are sailing in the same area most of the time so need to constantly correct for variation. Most GPS' figure it out automagically and some will display the info.

Fluxgate compasses, as commonly used in auto pilots, can be swung mostly automatically to adjust for deviation and figure it out or can be set manually if not.

No need to drag out the sextant, just look up variation on the chart....and adjust for annual change which is also on the chart.

Also see process for swinging a compass (adjusting a compass for deviation, not variation, and making a "deviation card").

Deviation of ships compass is ideally already known and documented on the deviation card. If not, its easy to approximate.

Variation is typically compensated for in your navigation calculations.

Good stuff to know when the electronics fail...especially well offshore.
You nailed that. People want everything on one screen. When the screen takes a dump they have nothing.
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Old 03-06-2018, 14:22   #40
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Re: true vs magnetic bearing

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Originally Posted by xxhat View Post
I would appreciate it if someone explains how they use true or magnetic bearing for navigation.


I believe to use magnetic bearing I need to find the deviation of my compass ?


Thanks,


jim
This is going to hurt because it's nearly 40 years since I did my coastal navigation course.
The reason we have magnetic variations is because the earths magnetic field affects compasses. This variation is different all over the world depending on where you are and over time the variation in a particular place also changes.
So long as you have a relatively new paper chart (up to date variation reading) you will find the magnetic variation for the chart on the compass rose displayed as ie 2 or whatever number degrees w or E. Reading through these posts you will now be familiar with the saying West is best and East is least. This is just an easy way of remembering that westerly variations are added to a compass reading and Easterly variations are subtracted from a compass reading with both calculations giving you your true bearing.
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Old 03-06-2018, 14:38   #41
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Re: true vs magnetic bearing

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Originally Posted by Cadence View Post
You nailed that. People want everything on one screen. When the screen takes a dump they have nothing.
Modern electronics are pretty reliable, but of course failures happen...sure would suck to be way offshore without a clue.

Back in the days of Selective Availability, we lost GPS signal about half way between Mass. and Bermuda...nothing, zip...not on the ships GPS or hand held units carried as backup. As luck would have it, several of us were practicing our celestial nav. We had plotted our celestial fixes along side GPS fixes on a chart and kept a paper nav log. So, we were in good shape and found our way to Bermuda using celestial fixes and DR...very satisfying. Ashore we learned about Operation Desert Storm!

A lightening strike could also create a similar situation. Years ago a boat relying on electronics got tapped on the way to the Bay Islands, only his SSB still sorta worked...enough to get talked in close enough for him to be sighted and guided in.

I plan to crew for a friend on a Pacific crossing next year, Im packing my sextant! ...and my smart phone, and my tablet, and my InReach (all w built in GPS)! Maybe I'll build a little Faraday Cage for them all that fits in my bag! 😆
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Old 03-06-2018, 14:48   #42
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Re: true vs magnetic bearing

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Originally Posted by Paul L View Post
In the days of using paper charts, using childbearing compass for fixes and steering by the magnetic compass I used magnetic headings for most everything. Now with a chartploter giving me a continuous super accurate fix, I work in true readings.

If I wanted the approx bearing of say a flair I saw, then I'd look over the compass and get a magnetic bearing. I would convert to True if I needed to plot it.
They say 'what's in a word' but as you are responding to a neophyte who is genuinely trying to educate himself, I think it behooves all responders to at least proof read their response to be sure 'autocorrect' has not created more doubt as to the writers response. ie: childbearing compass? & flair? Come-on guys, it may sound picky to some of you but surely you can do better.
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Old 03-06-2018, 14:49   #43
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Re: true vs magnetic bearing

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Originally Posted by Tbonem17 View Post
This is going to hurt because it's nearly 40 years since I did my coastal navigation course.
The reason we have magnetic variations is because the earths magnetic field affects compasses. This variation is different all over the world depending on where you are and over time the variation in a particular place also changes.
So long as you have a relatively new paper chart (up to date variation reading) you will find the magnetic variation for the chart on the compass rose displayed as ie 2 or whatever number degrees w or E. Reading through these posts you will now be familiar with the saying West is best and East is least. This is just an easy way of remembering that westerly variations are added to a compass reading and Easterly variations are subtracted from a compass reading with both calculations giving you your true bearing.
Whoopsie..!!

Far better to say 'error east - compass least ... error west - compass best'

Thus if the variation is east.... you subtract the variation from the true course to find the magnetic course.

And

If the variation is west ... you add the variation to the true course to find the magnetic course.

The opposite applies if you are converting a magnetic bearing to a true bearing...
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Old 03-06-2018, 14:50   #44
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Re: true vs magnetic bearing

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Originally Posted by Kelkara View Post
Also worth noting that on land "variation" is called "declination" ... same thing just a different name.

Sorry to correct you
Variation is the angle between Magnetic North and True North and can be either East or North.
It is shown in the Magnetic Rose on the chart, (it does change over time, ie is not constant) or as isogonals on small scale charts.


Declination, is the angle of Dip of the Earths Magnetic Field, this magnetic field is only parallel to the Earth's surface at the Magnetic equator, and is vertical to the Earth's surface at the Magnetic Poles, which explains why a Magnetic Compass cannot be used at high latitudes.


The other factor affecting a magnetic Compass is Deviation.
Deviation is the angular between Compass North and Magnetic North and is caused by the effect of Soft and Hard Iron Magnetism in the vessel, and this varies with the vessels heading due to soft Iron Magnetism (Magnetism induced by the Earth's Magnetic Field in magnetic materials on the vessel ) Deviation can cause the Compass Heading to be Either East or West of the Magnetic Heading.

A compass Swing is done to map the Magnetic Deviations on the vessel and prepare a Deviation Card



So we steer Compass heading which we must correct to a Magnetic Heading by applying the Deviation Correction.
A handy mnenonic to remember this is:
Deviation East Magnetic Least (Compass North is East of Magnetic North)

Deviation West Magnetic Best


If we wish to plot the course on a Chart we must convert the Magnetic Heading to a True Heading

A handy mnenonic to remember this is:
Variation East Magnetic Least ( Magnetic North is East of true North)

Variation West Magnetic Best
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Old 03-06-2018, 14:58   #45
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Re: true vs magnetic bearing

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Originally Posted by hamburking View Post
Yes, please remember that Canada owns the north pole. So be nice to Canadians...we let you use our pole for free...for now.
OK Canucks, that being so, perhaps we shud warn Mr.Trump if he continues to screw with NAFTA we will retaliate by moving the pole! That shud get his attention.
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