Magnetic Declination....

[Jamisjockey]

Tomorrow I'm going hiking and fishing.  The trail is well marked, but the mountain range is known for swallowing people.  So I bought a topo for the quad I'll be in, and a shiny new compass.  I also know that the major road there cuts SW/NE, so If I put the compass on E it'll get me there, eventually.
But it's been years since I took land nav.  I remember the basics of triangulation, etc, but I can't remember how to figure mag. dec.  The map shows MN 13* right of TN.  Do I add, or subtract that from my bearings when figuring a route?

[K Frame]

Here's a decent page on setting declination.

http://geology.isu.edu/geostac/Field_Exercise/topomaps/decl_setting.htm

If you're in Utah, you need to set your delination approximately 13 deg. East. Set it 13 deg. West and you'll end up in Oregon.

[280plus]

Doesn't your map show both TN AND MN? Just use MN and forget about all the figurin'?

[Jamisjockey]

Doesn't your map show both TN AND MN? Just use MN and forget about all the figurin'?

It sure does, I think that seems to be the simplest plan.  I won't be getting far off the beaten path, but just in case....

[280plus]

Yup, before you start out at your starting point lay your map out, orient MN with the needle on your compass and get a good fee for what direction is back out. Say to a road or a brook to a road. That way you know that no matter what else happens all you have to do is head "east" for example and you will be back in civilization.

Oh wait, duh, I see you already know that. I'll shut up now!

How's that go? "I was never lost in the woods but I sure was mighty bewildered a few times."

Good luck! Have fun and above all be safe!

[Tallpine]

IIRC, at (the meridian that goes through) Chicago, MN == TN

Everything in the US west of Chicago has east declination (MN is to the east or right of TN) and east of Chicago declination is west or left of TN.

Your compass should have a moveable center with an outline of the arrow that you can adjust relative to the outer compass ring.  Declination is about 16 deg east here in central MT, so 13 E sounds about right for Utah (that's where you are, right?).

So you should adjust that center section (hold it above and below with thumb and finger while turning the outer ring) so that the outline of the arrow points 13 deg to the east of North on the outer ring.  Then when you use the compass, turn it until the arrow is exactly within the outline, and North on the outer ring will be true north.

Of course, you can turn the outer ring (and the inner part with the declination adjustment will move with it) relative to the square(ish) compass base and use it to align with a map.

If none of the above makes any sense, just hike UPSTREAM when you go fishing and then DOWNSTREAM back to your vehicle.

(be sure to carry a deck of cards, too )

[cosine]

(be sure to carry a deck of cards, too )

But what if he doesn't know how to play Solitare?

[One of Many]

Magnetic Declination (MD) is not constant.  There should be some indication of the rate of change of MD on the map.  Check the date of issue of the map (when the MD was correct) and determine the number of years of correction needed, multiplied by the yearly rate of change of MD.  Once you have a current MD, you can compensate for magnetic north to true north.

[Marnoot]

Here's a site to compute magnetic declination based on Lat/Long coordinates and date:

http://www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp

Grab the coordinates off of Google Earth if you don't have them. Enter as 'H M D' or 'HH.HH'

[mfree]

Don't forget the fiber optic line.

Surely by now you know the joke

[Tallpine]

"Magnetic Declination (MD) is not constant"

yeah, but I didn't want to complicate matters anymore than I already had ...

so now you're lost and need your compass to find your way out of the woods, but first you need to know where you are in order to compute the declination


dang Canadians can't even manage the magnetic north pole properly and keep it from moving around

[280plus]

Yea, and they get their boots all mixed up with their boats...

[280plus]

BTW if you REALLY want to learn how to use a compass get a copy of "The Scout" by Ion Idriess. He goes so far as to tell you how to figure out what your declination is using the sun your compass and nothing else. It's actually quite easy once you get the hang of it. I'd have to refresh my memory though. If anybody's interested I'll post the procedure here when I get a chance over the next couple days

[K Frame]

"Don't forget the fiber optic line.

Surely by now you know the joke"

Uhm...

The punchline involves Chuck Norris, right?

[280plus]

It is a known fact that I know very few old jokes that everybody else knows. So if you please...

[mfree]

Oh, well... as it goes, if you're lost in the woods just bury a few feet of fiber optic line. Within the day a crew with a backhoe will be along to dig a trench over it.

[cosine]

It is a known fact that I know very few old jokes that everybody else knows. So if you please...

Don't feel to bad, I never heard of the one about the fiber optic line either until mfree posted it.

[Leatherneck]

Can Dead Men Vote At Elections?

Compass=>Deviation=>Magnetic=>Variation=>Add East.

TC

[Art Eatman]

Tallpine, the declination in Georgia is zero.  SFAIK, the zero line would go on up through the general D.C. area...

Easier for me, out here in ten-degree country of West Texas, is to remember that true north is LEFT of magnetic north.  So, in Utaw, magnetic north is 13 degrees to the right of actual.

, Art