View Full Version : Compass Declination for GA
This is probably a little much but......
I am bringing a map and compass with me when my fiance and I do the GA section of the AT in about a week and a half.
I am definately a beginner with using a compass. I am familiar with the basics for using a compass:
Align the edge of the compass with the starting and finishing point.
Rotate the compass housing until the orienting arrow and lines point N on the map.
Rotate the map and compass together until the red end of the compass needle points north.
Follow the direction of travel arrow on the compass, keeping the needle aligned with the orienting arrow on the housing.
I have the ATC map. It has an arrow pointing to MN on the map and then one that is angled just a tad to the right with GN, but it does not mention how many degrees, or a declination setting.
So, what is the declination for GA?
Im sure that it wont really matter for the AT but Im interested. And from what Ive studied I shouldnt even really need a map and compass, but hey, if I dont have it I would for some reason need it!
Anyway, thanks in advance!
The Weasel
08-25-2009, 19:46
If you really want to know (actually, I don't think you do, and when you go to this site, you'll see why)......
http://www.ngdc.noaa.gov/geomagmodels/IGRFGrid.jsp
TW
So this is the results from Amicalola:
Results for declination on date: 2009.6492919921875-84.10434.394-4.722
Uuuummmmmm..........
So this is the results from Amicalola:
Results for declination on date: 2009.6492919921875-84.10434.394-4.722
Uuuummmmmm..........
So is the correction made 4.7 deg CW (right) or CCW (left)?
Not right or left, east and west.
"East is least, west is best".
Dances with Mice
08-25-2009, 20:19
Stick: The correction for GA from mag north to true north is practically negligible. I appreciate you bringing a map & compass but be aware that all you need to know is which way is "north-ish". That way in the morning when you hike back to the trail from a shelter you'll know which way to continue hiking.
Alrighty then.....I think Ill stick with the basics and be happy!!
Thanks Guys!!
Hikerhead
08-25-2009, 21:33
Just follow the north star in the big dipper. It's always there, I've never been lost. :)
This might help:
http://www.thecompassstore.com/decvar.html
sliderule
08-25-2009, 22:00
I have the ATC map. It has an arrow pointing to MN on the map and then one that is angled just a tad to the right with GN, but it does not mention how many degrees, or a declination setting.
So, what is the declination for GA?
If the map is oriented to magnetic north, declination is automatically accounted for when measuring bearings, etc., with the compass. If declination is not listed, measuring the angle formed by the MN and GN pointers will provide that information. Declination is "West" if MN points to the west of GN. And vice versa with regard to East.
Here in the Atlanta area, we have always taught our Boy Scouts that the angle of declination is about -4.5%. Thanks for the map, Summit - it was good to see validation.....
Most maps have a line pointing to magnetic north.
Orient to that.
flemdawg1
08-26-2009, 12:48
Unless you have a really old map, or a crappy map, it should have the declination on it.
JaxHiker
08-26-2009, 14:59
My USGS map for Cowrock says 3 degrees I believe.
Pedaling Fool
08-26-2009, 15:30
My USGS map for Cowrock says 3 degrees I believe.
Probably a very old map?
This is the magnetic declination for Suches, Ga. in 1999 http://www.ngdc.noaa.gov/geomagmodels/struts/calcIGRFWMM
This is the magnetic declination for Suches, Ga. Today http://www.ngdc.noaa.gov/geomagmodels/struts/calcIGRFWMM
And Suches is further south than Cowrock (declination increases further north you go).
Also declination changes year from year.
Pedaling Fool
08-26-2009, 15:36
Probably a very old map?
This is the magnetic declination for Suches, Ga. in 1999 http://www.ngdc.noaa.gov/geomagmodels/struts/calcIGRFWMM
This is the magnetic declination for Suches, Ga. Today http://www.ngdc.noaa.gov/geomagmodels/struts/calcIGRFWMM
And Suches is further south than Cowrock (declination increases further north you go).
Also declination changes year from year.
I see the links didn't work, but 1999 had a decl. of - 3 deg 58 min and today it's - 4 deg 49 min
JaxHiker
08-26-2009, 22:43
Hmm, both my Neels and Cowrock maps are dated 1988. Neels says 2.5 deg and Cowrock says 3. I just bought the maps for a hike in Feb. Sounds like the USGS needs to update their maps.
take-a-knee
08-26-2009, 23:29
I see the links didn't work, but 1999 had a decl. of - 3 deg 58 min and today it's - 4 deg 49 min
Grid north is a fixed location, magnetic north is a slowly moving target. Grid north coincides with the earth's rotational axis, magnetic north does not.
atraildreamer
08-29-2009, 10:23
Hmm, both my Neels and Cowrock maps are dated 1988. Neels says 2.5 deg and Cowrock says 3. I just bought the maps for a hike in Feb. Sounds like the USGS needs to update their maps.
The USGS maps usually have a notation that states the expected change in magnetic declination per year. So, by subtracting the issue date year of the map from the current year and multiplying by the correction factor you get a number that is added to the old declination number to get the current declination.
Now does that clear up the question for everybody? :confused:
Don't ask me to go over it again...I still get confused about which way to turn the compass base to set the declination! :-? :banana
Try these sites:
Magnetic Declination Calculator
http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp
Magnetic Field Calculator
http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp