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Old 12-10-2009, 05:53   #31
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Originally Posted by BillyDoc View Post
Is there any way to determine magnetic deviation at a location ...
World Magnetic Model 2005 Calculator (WMM-2005)

World Magnetic Model Calculator

NGA: (U) DoD World Magnetic Model 2005 (UNCLASSIFIED)

WMM-2010 will be available in December, 2009.

Many people incorrectly use deviation when they mean declination or variation.
The angle between magnetic north and true north is called magnetic declination. Because the word "declination" also has an astronomical usage - the angle of a star or planet above the celestial equator, mariners prefer the term “variation”.
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Old 12-10-2009, 06:29   #32
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Quote:
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...

Many people incorrectly use deviation when they mean declination or variation.
The angle between magnetic north and true north is called magnetic declination. Because the word "declination" also has an astronomical usage - the angle of a star or planet above the celestial equator, mariners prefer the term “variation”.

...
As far as I know:

Magnetic Declination: angle between true north and magnetic north (this angle is due to the non coincidence of magnetic poles w.r.t. geographic poles). This angle depends on the position on earth and on time (it slowly varies during time). Can be computed from data (local declination & annual variation) present on the maps.

Magnetic Deviation: angle between the magnetic north and the compass north (this angle is due to the earth magnetic field distortion produced by magnetic materials present around the compass). This angle can be partially compensated (compass calibration) and depends on the boat heading (since changing the heading changes the relative position of the disturbing magnetic materials w.r.t. the earth magnetic field around the boat). Can be computed thanks to a table present (sometimes...) on the boat that gives deviation w.r.t. heading.

Magnetic Variation: simply the sum of the two above angles. It is the total angle between the true north and the compass north. Of scarce usefulness since Variation depends on too many factors and shall be always calculated summing the two above values. Anyway it has a theoretical meaning...

Ciao, Marco.
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Old 12-10-2009, 08:31   #33
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Gord, Marco, you are both right, I misspoke. Variation it is! For the total "discrepancy" that is.

Gord, your links are most useful! Thanks. And I note that the source code for the DoD magnetic model is in the public domain and in "C" so it would be easy to add to openCPN as a feature. Just click on a key and the subroutine would note the present "myship" location and return the deviation in a separate box (or whatever). From that the variation can be determined locally.

I'll make a post in the feature request thread for this now.

Thanks again!

BillyDoc
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Old 12-10-2009, 10:08   #34
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I find that a lot, if not most of the scanned raster charts has the traditional information about variation.
I suppose you are referring to the different vector charts.

Thomas
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Old 12-10-2009, 10:36   #35
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UPDATE: Compatible GPS for laptop

I wanted to update what I bought for my laptop as far as a GPS and how it worked.

I bought a GlobalSat BU-353 GPS mouse from amazon ($40) and installed the drivers from their online site.

After plugging it in it quickly came up on OpenCPN!

Last weekend was the real test. We sailed from San Francisco Bay to Half Moon Bay with an overcast sky and I used the GPS/laptop in the cabin at the nav station! Next day we sailed from Half Moon Bay to Moss Landing and it worked perfectly.

This GPS mouse worked great.
Thank you to the developers of this software.
Steve

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How do I choose a compatible GPS and an AIS to work with OpenCPN?

I know they must be usb connectible. I do not own anything right now.
(Other than a laptop)
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Old 12-10-2009, 11:28   #36
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I find that a lot, if not most of the scanned raster charts has the traditional information about variation.
I suppose you are referring to the different vector charts.

Thomas
Well, I googled for a while about Magnetic Variation and I discovered that in US and UK (I guess in all english speaking countries) the Magnetic Variation is used in nautical and aeronautical navigation as a synonymous of Magnetic Declination. I must say that in Italy, France and Germany this is not absolutely true and Declination and Variation are NOT synonymous.

In International charts, the different wording around the world for the magnetic declination has been solved by not writing any word! In those charts they just put the 360 degree true circle rose with a concentric declined magnetic rose with a text like:

1°05'E 2000 (8'E)

meaning 1°05' Declination East at January 2000, the declination goes East 8' each year (or if you prefer: 1°05' Variation East at .....)

Anyway in all countries, outside navigation world, the word to mean the angle between true north and magnetic north is "magnetic declination". This convention is true also for WMM.

Ciao, Marco.
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Old 13-10-2009, 04:00   #37
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Deviation is the error induced in a compass by local magnetic fields (such as a steel boat or electrical conduit) [...] The term magnetic variation is equivalent, and is more often used in aeronautical and other forms of navigation.

Declination at any point on the Earth is the angle between the local (global!) magnetic field—the direction the north end of a compass points—and true north.

(The German terms are Ablenkung and Missweisung)
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Old 13-10-2009, 06:02   #38
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Deviation is the error induced in a compass by local magnetic fields (such as a steel boat or electrical conduit) [...] The term magnetic variation is equivalent, and is more often used in aeronautical and other forms of navigation.

Declination at any point on the Earth is the angle between the local (global!) magnetic field—the direction the north end of a compass points—and true north.

(The German terms are Ablenkung and Missweisung)
Hi ipnd, actually the link you suggest for deviation and declination says something slightly different. The first link (deviation) says:

"In nautical terminology deviation is the error caused by the metal parts of the ship, an error dependent on the magnetic heading (the angle the ship makes to magnetic north) whereas declination, also called variation, is the difference between true north and direction of the earth's magnetic field at that point without the ship. Of course variation in this sense is due to the location of the magnetic north pole as well as local geological conditions."

1) variation is a synonymous of declination (not deviation). This is not true for Italy, France and Germany (at least) where declination is never called variation.

2) declination is the difference between true and magnetic north "without the ship", i.e. it is not the total magnetic distorsion angle. Since it is independednt by the ship (and bu local distorsions), declination can be modelled at earth level (e.g. WMM).

Ciao, Marco.
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Old 13-10-2009, 06:17   #39
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Oooops, you're right.. Variation is synonymous with declination! Got a bit overexcited on the cutting&pasting apparently
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Old 13-10-2009, 12:06   #40
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I took it upon myself to advertise the new opencpn release at the SSCA forum - and promptly got two Mac users asking for the current DMG. This has not been compiled for a few versions.

Are there any Mac users around prepared to contribute one?

Thanks for your attention.
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Old 15-10-2009, 14:11   #41
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Well, some S57:s do contain data about variation. Check the dump below of a South China Sea Chart from SCSHC. Look just to the left of the upper left corner of the pop-up box.

Thomas

PS
Entertaining discussion about "declination" above which for me, with extensive celestial navigation experience, ... only can mean one thing.
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Old 15-10-2009, 14:35   #42
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Well, some S57:s do contain data about variation. Check the dump below of a South China Sea Chart from SCSHC. Look just to the left of the upper left corner of the pop-up box.

Thomas

PS
Entertaining discussion about "declination" above which for me, with extensive celestial navigation experience, ... only can mean one thing.
2 things.

1) for Dave: why ENC has the declination (variation) symbol while the CM93 doesn't? Is it a OCPN problem in handling CM93 or the CM93 does not contain that object?

2) For Thomas, it's a pitty that for you a word just mean one thing

"declination" has at least 7 meanings (and one is "variation")

Declination Definition | Definition of Declination at Dictionary.com

then the meanings of "variation" (one is "magnetic declination")

Variation Definition | Definition of Variation at Dictionary.com

Just for fun
Ciao, Marco.
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Old 16-10-2009, 14:50   #43
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I've been told repeatedly that there is a relatively current version of OpenCPN intended to be run on Macs. From seeing the post several above this one, should I assume that is false? And if there is not a Mac version available now, are there specific plans for introducing this version...or is it just on a wish list for the future?

Many thanks...

Jack
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Old 16-10-2009, 15:53   #44
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See post #2 in this thread ... here, for more information.
The latest Mac version is 1.3.0 released in early May 2009.

I think that the real problem is lack of experienced Mac users that can help out with testing and Mac-specific issues. Maybe you can help out, or know of some Mac-people that would be interested in contributing ??

Thomas
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Old 16-10-2009, 19:59   #45
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Thanks, Thomas. I did indeed miss that reference to the Mac and that's a new URL for me.

Jack
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