Cache Location and Magnetic North

Onion

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	Cache Location and Magnetic North « on: June 18, 2012, 05:24:01 pm »

Greetings,

Hope I get this in the right spot as I couldn’t find a post to answer my question. Been mulling over a Topo map and playing with Google earth trying to get a specific area located. I came across some information about magnetic north changing over the years as the magnetic field changes. I also read that Google earth uses true north and not magnetic north which I have to account for as I am confident this spot was chosen using compass headings relating to magnetic north, which is what got me thinking.

So let’s say a 100 years ago I buried a cache and used a compass pointing to magnetic north to set my marker SW that pointed me in the direction to go. Then later I could use the marker to point me in direction using line of site in case I had left my compass at home. Now it is a hundred years later and I return to claim my cache. When I get out of the truck I realize as I have before, I left my compass at home. No worries as I had left a marker pointing me in the SW direction. So when I get to my marker it is a pile of rubble. So I return home for my compass and head back so I can line myself up with magnetic north to get my SW line again. This is where my question comes in.

Am I still pointing in the right direction?

I read that magnetic north changes slightly each year and varies by how much in relation to where you are between the poles. In this location I found between 1963 and 1998 magnetic north decreased by 2.5 degrees. So a 100 years later and using magnetic north to find my SW line again and going my specific distance I could be significantly off if I dug in that specific spot correct? Or do magnetic north and the direction to travel stay the same due to the rotation of the earth even though a 100 years have passed? Maybe I am over thinking the process and there is no change or incorrectly digested the information I have read. I am just trying to account for any variables that may affect my calculations in finding the proper direction and spot as I have limited time and when I get there I want to make the most of it. Anyone have a thought on this?

Thanks!

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GoldDigger1950

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	Re: Cache Location and Magnetic North « Reply #1 on: June 18, 2012, 05:47:02 pm »

The phenomenon you are asking about is called magnetic declination and it can be found anywhere that has information regarding fliying airplanes since they never, ever used magnetic north. On all geodesic survey maps you will see an angle next to the arrow for magnetic north. That angle varies around the world based on your longitudinal positioning. At two places on earth, the magnetic and true north poles are zero degrees apart. In 2011, the poles were approximately 560 miles apart.

NASA and the NOAA keep track of the declination as well.

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« Last Edit: June 18, 2012, 05:50:50 pm by GoldDigger1950 »	Logged

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	Re: Cache Location and Magnetic North « Reply #2 on: June 18, 2012, 05:55:58 pm »

Ok...now we're getting complicated. Simply put the magnetic pole itself wanders over time.

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http://www.nasa.gov/vision/earth/lookingatearth/29dec_magneticfield.html

Now onto your specific question, within a limited geographical area, as long as you correct for the change (easy to calculate if you have two known points) and known original bearing, you should be ok.

BA

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GoldDigger1950

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	Re: Cache Location and Magnetic North « Reply #3 on: June 18, 2012, 05:58:37 pm »

	Quote:	Posted by BitburgAggie_7377
Ok...now we're getting complicated.

Come on, mate. It's just an angle on the map. You can handle it.

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	Re: Cache Location and Magnetic North « Reply #4 on: June 18, 2012, 06:00:20 pm »

GD, my comment wasn't aimed at your response, but at Onion's original question. You and I were working on our responses at the same time (and I really didn't feel like re-writing mine just because you hit post first

)

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	Re: Cache Location and Magnetic North « Reply #5 on: June 18, 2012, 06:02:15 pm »

Your using a map, you will need to add or subtract the difference for true north. Most compasses come with instructions and can explain it better. The bezel dial also moves to help set your heading and stay on it. If you type in magnetic north on a google or wiki search, you should get a declination chart that is easy to read. Just 2' degree at a 1000 yards will have you way off target. When you find your azmuth, find a object off in the distance in your line of sight, and use that target to walk to counting your steps(pace count).

If your gonna bury something, use two points of reference, something that doesn't move or die. You can then use your position on a map to figure where you are and where you buried it.

Opps we all posted at the same time.

Just give it to me and Ill hold it. I got 100 years right?

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« Last Edit: June 18, 2012, 06:04:26 pm by Occassionally »	Logged

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	Re: Cache Location and Magnetic North « Reply #6 on: June 18, 2012, 06:05:11 pm »

I kind of figured you were talkiing about his post but I couldn't resist the dig. It's that inner "high school brat" in me that peeked out.

Sorry, my friend. Forgive me.

Posted on: June 19, 2012, 02:03:39 AM

	Quote:	Posted by Occassionally
If your gonna bury something, use two points of reference, something that doesn't move or die. You can then use your position on a map to figure where you are and where you buried it.

I found an old diary once that advised the reader to turn left at the yellow house. Written in 1866, one can only imagine how many different colors of house there have been since then.

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	Re: Cache Location and Magnetic North « Reply #7 on: June 18, 2012, 06:16:43 pm »

And in terms of your original question....where you are deciphering locations based on compass bearings made 100 years ago------If your waybill includes the bearings of two prominient landmarks with in sight of the target location, the relative angle between the target and each of two landmarks will remain the same. So if you draw a straight line from you to the first landmark, and then proceed along that straightline of travel until the angle from you to the second landmark is the same as it is in the original mapping, you'll be at your spot. The only thing that would change that significantly would be a major earthquake or similar landscape altering event (like humans taking out a mountain in the search for precious metals or building material)

BA

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	Re: Cache Location and Magnetic North « Reply #8 on: June 18, 2012, 06:24:38 pm »

Glad I didnt mention "back azmuth" lol. Poor guy is probably confused now.

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	Re: Cache Location and Magnetic North « Reply #9 on: June 18, 2012, 06:26:48 pm »

	Quote:	Posted by Occassionally
Glad I didnt mention "back azmuth" lol. Poor guy is probably confused now.

It's definitely a subject that lends itself better to pictures than to word pictures.

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