View Full Version : declination deviation
johnny quest
12-07-2007, 09:41
does anyone already have the declination of different spots on the trail? i can compute it but if someone had already done the heavy lifting....
Magnetic deviation should be on your maps; if not, they're not worth anything except for driving.
http://www.ngdc.noaa.gov/seg/geomag/icons/us_d_contour.jpg
http://www.ngdc.noaa.gov/seg/geomag/icons/us_d_contour.jpg
Need ° corrections to bring it up to date too. I don't have an almanac, but an up to date one should have them listed.
johnny quest
12-07-2007, 10:15
yeah, they are on my maps, but a quick check of each ending map showed maine to be spot on while georgia was significantly off. either my figgering was wrong or theirs was, or jules verne played a fast one with the earth's hollow core.
i dont know about driving, but its worth something to me cause i want them. just thought someone maybe kept track of them.
Each line is one degree (pos blue/neg red) of declination...
johnny quest
12-07-2007, 10:25
right. ive figured it all by now. it didnt take that long after all. by the way, i double checked. the southern end of the atc maps seemto be off by 3 or so degrees.
What a date were the AT maps made?
If you want the most up to date stuff... http://geomag.usgs.gov/observatories/data/realtime/
but I think for general purpose, the first link will suffice..
johnny quest
12-07-2007, 10:40
what i did is 12 pick spots along the trail, south to north, select a month and day in 2008, and used the engine here. i think that will do fine. thanks
take-a-knee
12-07-2007, 13:57
What a date were the AT maps made?
That does matter 'cause the magnetic poles move around from time to time.
johnny quest
12-07-2007, 14:01
the maps are dated 2004 but i wonder if the dec info was that recent. seems odd that some would be on and some werent. oh well, i will use the numbers i came up with for 2008
Uncle Silly
12-07-2007, 14:11
Declinations for various spots along the trail were given in one of the guide books (Wingfoot's, I think, tho it could've been the Companion). I copied them down in my trail journal but don't have them handy. Maybe someone else has one handy and can copy the list here -- I'd be interested in seeing them compared to JQ's calculations.
johnny quest
12-07-2007, 14:31
keep in mind that the jules verne device inside the earths core moves around. the noaa website i used allows you to pick not only a location but a date. i charted decs up the trail from march to october of 2008. as an added bennie, they wil tell you how much the dec changes a month per location. so the readers digest version of that is that if you want minutely correct dec info you need to get it for the time and place your interested in. i will be happy to share what i came up with however for someone to check.
NICKTHEGREEK
12-07-2007, 14:37
Magnetic deviation should be on your maps; if not, they're not worth anything except for driving.
The rates (mils/yr since map date ) may be way off if the maps are old, the mag North pole is virtually racing toward Siberia. That said it pretty much doesn't mean shinola for most short distance nav.
The Old Fhart
12-07-2007, 14:55
A map of North America showing isogonic lines (lines of constant declination) will give you a good overview of what happens along the A.T.. As to why this could be important, especially in the NE where the declination is around 20° west of TN (true north, not Tenn.), you can figure it out mathematically that if you went one mile and were off 5.5°, you would be 500 feet off, or about 0.1 mile.
2778(click on image for full size view)
If you're using a map and compass for navigation to plot or find your location, this correction could be critical. Even though with a GPS, the electronics do all the calculations and will pinpoint where you are, you should have, and know how to use, map and compass.
CoyoteWhips
12-07-2007, 15:26
Is it possible to check your compass against the night sky once a week or so?
...or, if you happen to sitting in the same place for a couple of hours on a nice day, the movement of the sun?
take-a-knee
12-07-2007, 15:42
Is it possible to check your compass against the night sky once a week or so?
...or, if you happen to sitting in the same place for a couple of hours on a nice day, the movement of the sun?
The best way to check your compass is to hold it level against a wooden fence post and note the reading, needless to say, this needs to be done before you dash it against the ground. You can do the same thing with several compasses and compare results to see if one or more is off. Also be sure that, if your compass has a means to adjust for declination, like a Silva Ranger, that this is returned to zero.
johnny quest
12-12-2007, 13:07
this is what i came up with for 2008 trail declinations. i invite anyone ...as rediculous as myself who wants to... to check my work.
BLAIRESVILLE GA 2/28/08 4° 46’ W
HOT SPRINGS NC 3/28/08 5° 50’ W
DAMASCUS VA 4/15/08 6° 47’ W
DALEVILLE VA 4/29/08 8° 20’ W
BERRYVILLE VA 5/15/08 10° 9’ W
HARRISBURG PA 6/10/08 11° 12’ W
NEWTON NJ 7/15/08 12° 46’ W
PITTSFIELD MA 8/10/08 14° 6’ W
WOODSTOCK VT 8/29/08 14° 53’ W
LANCASTER NH 9/10/08 15°41’ W
FARMINGTON ME 9/29/08 16° 25’ W
BANGOR ME 10/10/08 17° 1’ W
HOULTON ME 10/15/08 17° 52’ W
CoyoteWhips
12-12-2007, 13:20
That is an impressive effort to me, I typically use my compass to be sure I'm not walking in the wrong direction. An error of ±45° will normally work for me.
johnny quest
12-12-2007, 13:27
nah...the website calculator did all the heavy lifting. im just the geek that went thru it.
i love land nav. that is why i want to know the correct dec. but i think if you were in maine somewhere and had to get off trail for an emergency...the proper dec would come in handy. its pretty extreme up there. and ive noticed some of the decs on the maps arent up to date.