# What is the differance: True North and Magnetic North



## navycop (Oct 21, 2012)

I am looking to use my iphone for geocaching (finding containers hidden in the woods). I can set the compass feature for either true or magnetic north. I heard somewhere that one of them needs to have declination added or subtracted.


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## terryf (Oct 21, 2012)

True north is exactly what it means, the direction to the north pole.

Magnetic north is what your handheld compass shows you and is a result of the magnetic field inside the earhths core. You must determine the declination from time to time as it changes for each compass at each location.

You can go here to get a rough idea.
NOAA's Geophysical Data Center - Geomagnetic Data

As far as geocaching goes it shouldn't matter as you are not using you compass but rather a latitude and longitude. If your GPS can do both its generally accepted to use true north.


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## philipff (Oct 21, 2012)

The difference is called Variation and is different in each part of the county and world.  Magnetic settings are a whole lot easier to follow.  Good luck searching!  Philip


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## Smitty37 (Oct 21, 2012)

*Check the map*

If you are given compass directions and distances to follow you want to set it so you are using the same type readings as directions are given in (most commonly that would be magnetic, but if they used GPS it could be either.


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## terryf (Oct 21, 2012)

Smitty37 said:


> If you are given compass directions and distances to follow you want to set it so you are using the same type readings as directions are given in (most commonly that would be magnetic, but if they used GPS it could be either.



No compass directions are given in geocaching unless its a multi stage cache and even then its very unlikely.

They use lat longs almost exclusively.


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## Rick P (Oct 21, 2012)

"Navycop" I had assumed you were or are in the service.........you don't know how to read a compass!?!?!

The decliniation from ture north can be very significant! Here it's 18 degrees 3 minutes East, that will definately effect where you end up in as little as a couple of miles 1 degree is one inch at 100 yards!!!!! I follow magnetic north that puts me off 18 inches for every 100 yards traveled!


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## navycop (Oct 21, 2012)

Rick P said:


> "Navycop" I had assumed you were or are in the service.........you don't know how to read a compass!?!?!!



Well there really is no need to read a compass on a ship, unless you are on the bridge or in SARE...


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## Jim Burr (Oct 21, 2012)

navycop said:


> Rick P said:
> 
> 
> > "Navycop" I had assumed you were or are in the service.........you don't know how to read a compass!?!?!!
> ...


 
16 years in CIC...I know exactly what you mean!! Leave the heading to the guys driving the ship!!


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## jcm71 (Oct 22, 2012)

Anyone remember CDMVT and TVMDC  or Can Dead Men Vote Twice and True Virgins Make Dull Company.


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## Sylvanite (Oct 22, 2012)

Rick P said:


> ... 1 degree is one inch at 100 yards...!


Actually, one minute of angle (1/60 of a degree) is approximately one inch at 100 yards.  A full degree would be around 60 inches, or 5 feet at 100 yards.

Regards,
Eric


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## Finatic (Oct 23, 2012)

The difference between True North and Magnetic North is "Variation". In New England it is about 15 degrees and will incresas by x minutes per year.
Check a nautical chart in your area as it will be either larger or smaller and can be east or west of true north.


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## cwolfs69 (Oct 23, 2012)

I always dreaded CBDR with a large blip.
(constant Bearing Decreasing Range)


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## joefyffe (Oct 23, 2012)

I am certainly glad I'm not traveling with you guys, I might end up in San Francisco, Michigan!  :biggrin::biggrin::biggrin:


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## Smitty37 (Oct 23, 2012)

terryf said:


> Smitty37 said:
> 
> 
> > If you are given compass directions and distances to follow you want to set it so you are using the same type readings as directions are given in (most commonly that would be magnetic, but if they used GPS it could be either.
> ...


If your I phone can give you either magnetic or true - my guess is that you won't need to worry about variation.  The gps signals are based on true north and the iphone can computer the variation based on the phones actual location.  Do you need to use a hand held compass to determine the direction to walk?  If you do get magnetic readings from your iphone and don't sweat variation.


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