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Old 14-07-2011, 17:05   #1
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True North or Magnetic North ?

I just started using OPEN CPN.
All my course and bearings are off by 10 degrees. I'm assuming it only calculates true north? How can I fix that? Is it the GPS reciever or OPEN CPN?

I'm using a skytraq GM65 reciever.

Add 10 degrees and it agrees with my Garmin handheld?
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Old 14-07-2011, 17:36   #2
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Re: True north or Magnetic north?

If, as you presume, the Open CPN is utilizing true North and GPS is set to magnetic, you could always reset your GPS to True. However, check and see if ten degrees is the proper declination in your area.
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Old 14-07-2011, 18:23   #3
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Re: True north or Magnetic north?

Variation.

Agreed with Astrid, if your steering compass is magnetic then it makes it easier to do everything (chart plots, GPS COG units etc) with respect to magnetic north.

Another variable is that you may have some compass deviation that you are not aware of. It's an easy Google to look up how to swing a boat to make a deviation table for your steering compass.
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Old 14-07-2011, 18:32   #4
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Quote:
Originally Posted by David M
Variation.
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.
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Old 14-07-2011, 18:43   #5
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Re: True North or Magnetic North ?

Quote:
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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.
Funny!
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Old 15-07-2011, 08:22   #6
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Re: True North or Magnetic North ?

Whew! For a minute, I thought you said "mangulation", which is what can happen when I forget to wear pants on deck when it's time to tack.

Ow.
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Old 15-07-2011, 11:59   #7
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Re: True North or Magnetic North ?

Welcome Aboard Cruisers Forum
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Old 15-07-2011, 17:52   #8
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Re: True North or Magnetic North ?

Why do the bearings on OPEN CPN differ by ten degrees from my cruising guide? All OPEN CPN figures differ by 10 degrees from posted route.

My handheld Garmin agrees with the posted routes, but the CPN is off by 10 Degrees.

The cpn maps seems to be using true north orientation.
Where do I find maps in magnetic north. Can you reorient the maps?

I don't have any setting on the GPS and I don't see any in OPEN CPN.

Any ideas?
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Old 15-07-2011, 17:54   #9
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Re: True North or Magnetic North ?

I should have used the term course instead of Bearing.
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Old 16-07-2011, 02:29   #10
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Re: True North or Magnetic North ?

Mary,
I'm afraid you will not find such charts. At least I have until now never seen a chart like that, paper or electronic.
Cartographers tend, for obvious reason, to like the geographic north pole, not the magnetic one that moves around constantly and so would prevent the creation of any chart.

Pavel
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Old 16-07-2011, 06:27   #11
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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.
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Old 16-07-2011, 06:52   #12
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Re: True North or Magnetic North ?

Since a GPS knows both the date/time, and where you are, and since the variation is a (time variant) function of location, can GPS units provide the present variation?

Or do they already? Me, I never leave site of land, but I am curious.
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Old 16-07-2011, 07:03   #13
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Quote:
Originally Posted by allanpeda
Since a GPS knows both the date/time, and where you are, and since the variation is a (time variant) function of location, can GPS units provide the present variation?

Or do they already? Me, I never leave site of land, but I am curious.
Deviation and variation have more to do with Course and Bearing than a 2D trisected location like GPS. I guess so. My 2 paise.
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Old 16-07-2011, 07:13   #14
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Re: True North or Magnetic North ?

No you can not reorient Charts to Magnetic North. BUT! By checking the compass rose on the charts you will find that the inner ring is for magnetic north. Unfortunately due to the annual creeping eastward of the magnetic north pole that inner ring could be off.
Best way, like I teach my students; convert from your magnetic compass to True
Remember that, Course is the direction you are going and Bearing is the direction you are looking at something... And to convert you have to use the deviation from your course for converting magnetic bearings to True.

Ship's course has deviation
Bearings do not have deviation.

Thus remembering the above will help you from making mistakes during conversions. Because it is very important to use the ship's course deviation to convert your bearings.
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Old 16-07-2011, 12:58   #15
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Re: True North or Magnetic North ?

Mary, what kind of GPS do you have.
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