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» Online Classroom   »   » Public Discussion of Cel Nav   » Magnetic Declination

Author Topic: Magnetic Declination
Michael C

 - posted August 30, 2007 03:05 PM      Profile for Michael C           Edit/Delete Post 
How do you determine Magnetic declination?
The magnetic North and South poles are always moving.

Thank you


 - posted August 31, 2007 03:31 PM      Profile for HHEW           Edit/Delete Post 
As far as I know, magnetic declination is an old term for what is now generally called magnetic variation - the difference in degrees between true North and the magnetic lines of force running between the magnetic poles of the earth. Yes, north and south magnetic poles wander, but very slowly. Here's the URL to one internet source of info: http://geology.isu.edu/geostac/Field_Exercise/topomaps/mag_dec.htm. If you Google "magnetic declination" you'll find quite a few others.

If you're asking how you determine magnetic variation as a matter of practical navigation, you look at your chart. They all have two printed concentric compass "roses" on them - the outer one is the true compass, the inner the magnetic.

Printed near the center of the inner rose will be the amount of the variation as of a given date AND the amount of the annual change in the variation - usually a few minutes of arc.

Variation and its change are expressed as a number of degrees and minutes East or West. East variation means the magnetic rose is twisted clockwise vis-a-vis the true rose, West variation means it is twisted counter-clockwise. If you look at the roses on a chart it's very easy to see that with W variation the magnetic compass will read higher than the true compass; with E it will read lower. Hence the mnemonic "Variation West, compass reads best, variation East, compass reads least."

Depending on the scale of the chart, there will be from one to half a dozen roses on it. If you're on a small-scale passage chart covering half an ocean, you use the rose on the chart that is closest to your DR or your most recent fix to tell the helmsman what to steer.

David Burch

 - posted August 31, 2007 05:23 PM      Profile for David Burch           Edit/Delete Post 
i agree to not knowing if you mean how do we as a mariner underway know what it is, or how does the scientist measure it so he can tell us.

As noted there are several ways to get the former, including the wonderful freeware program we have in the shareware section of the library. That program is the same one that all the GPS companies use when they tell you the variation. It is a simple program you can load onto your PC. Many echart programs have it built in

we somewhere in the library as well have a hi-res pdf of the world showing the variation as well, but it is as pointed out on every nautical chart.

From: Starpath, Seattle, WA
Michael C

 - posted September 01, 2007 06:50 AM      Profile for Michael C           Edit/Delete Post 
Thank you. I appreicate the help.


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