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Old 16-07-2011, 13:35   #16
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Re: True North or Magnetic North ?

Not much of this has to do with your problem, but...

For most people with small boats sailing for moderate distances, the annual change isn't a big hairy deal and is pretty darn small compared to people's ability to hold a compass course and compensate for tides, currents, and leeway. Also, one caution might be that sometimes the GPS is far more accurate than some charts, especially for more remote areas. And, at very fine resolution, one of the challenges for cartographers has been that mapping techniques have run into the fact that there Earth is not a uniform sphere or even a uniform "squashed sphere".
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Old 16-07-2011, 13:54   #17
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Re: True North or Magnetic North ?

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Originally Posted by allanpeda View Post
Can GPS units provide the present variation
Yes they can, and most do.

I always work in True. All instruments are set to display in true. The only instrument that doesn't display in true is the steering compass. Everybody has a preference but you must know exactly which is which, depending on where you are the difference between magnetic and true can be huge. I often sail across an area where variation goes from west at one end of the voyage to east at the other end (only a few hundred miles) so getting your sums correct is very important! YMMV
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Old 16-07-2011, 15:24   #18
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Re: True North or Magnetic North ?

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Originally Posted by storyinframes View Post
Its best to standardise all bearing and course calculations to True North.
A simple formula to convert all your magnetic/compass reading to true north is

C d M v T [ compass to magnetic, use deviation card; magnetic to true, use variation]

C compass
M magnetic
T true

d deviation
v variation

Every boat has its unique deviation chart depending upon the number of metallic additions and alterations.
Variation is on compass Rose. See the deg/min changing every year and since which year. Calculate, and you can add/subtract accordingly to get the correct true reading.
My RYA instructor, who incidentally was a church-going lad, drilled into us
True Virgins Make Dull Company.
T V M D C
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Old 16-07-2011, 15:30   #19
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Re: True North or Magnetic North ?

Mary-
"I don't have any setting on the GPS"
It would be a most unusual (let's call it, primitive or really dirt cheap) GPS that had no menu option to display magnetic versus true, or no accomodation for any aspect of that. Certainly not one intended for navigation in any way.
I'd suggest checking again or contacting the maker, they may have done a good job hiding the menu option you need.

Since a GPS knows where it is, or at least where it probably is, it can automagically deal with deviation and variation if the software has been written to do so. At least, it can come pretty close for a number of years.
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Old 31-10-2011, 00:07   #20
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Re: True North or Magnetic North ?

Come on guys, OpenCPN would be MUCH better if you could select either "Magnetic" or "True". This is what ALL the pro charting software uses, IE, Maxsea.
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Old 31-10-2011, 00:14   #21
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Re: True North or Magnetic North ?

Quote:
Originally Posted by Bash View Post
I vote we switch to "magulation." Much less confusing.
I've programmed everything in my nav system to magulate on magnetic. We tend to think of true north as a myth invented by druids.
+1

We also leave true north to the Druids. A useless abstraction.
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Old 31-10-2011, 07:39   #22
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Re: True North or Magnetic North ?

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Originally Posted by Quincofish View Post
Come on guys, OpenCPN would be MUCH better if you could select either "Magnetic" or "True". This is what ALL the pro charting software uses, IE, Maxsea.
G'Day Dave,

Have to agree with you. I added this feature to the "wish list" some while ago, and hope that it rises to the top soon. To me this is a more significant addition than some of the bells and whistles that are being investigated. But, with open source software, the guys doing the work get to set the priorities, and I can't argue with that!

Cheers,

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Old 31-10-2011, 07:50   #23
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Re: True North or Magnetic North ?

If you want to be polically correct, you could use:
T
V
Makes
Dull
Children
at best (Add West)

I always plot in True bearings on paper charts.
And that is the way I teach my students who want their USCG Licenses.

Plus I learned and prefer: True Virgins Makes Dull Companions at Best.
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Old 31-10-2011, 10:15   #24
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Re: True North or Magnetic North ?

"We also leave true north to the Druids. A useless abstraction. "
Not so useless: True North is unvarying, while magnetic north tends to wander, and over time will actually reverse to South.
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Old 31-10-2011, 13:39   #25
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Re: True North or Magnetic North ?

Quote:
Originally Posted by hellosailor View Post
... ... ... and over time will actually reverse to South.
True enough - but not within the timescale of us sailing from A to B
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