Magnetic Declination

General discussions on hiking in Oregon and the Pacific Northwest
User avatar
Koda
Posts: 3437
Joined: June 5th, 2009, 7:54 am

Magnetic Declination

Post by Koda » December 19th, 2011, 10:41 pm

Some of my maps are getting old and show a different declination value than newer ones. It's my understanding that actual declination slowly moves each year. 3 different maps I have for Hood show 3 different declinations... 16, 17, and 20. I'm tired of buying new maps...

So I asked Google. open the link and click on the map or compute... http://www.ngdc.noaa.gov/geomag/declination.shtml

looks like our correct declination we should be using is about 16 degress for most anywhere in Oregon. I wonder how much it matters in the field if you are off by one degree?

Image
lightweight, cheap, strong... pick 2

Lurch
Posts: 1259
Joined: May 28th, 2008, 10:03 pm
Location: Aurora
Contact:

Re: Magnetic Declination

Post by Lurch » December 19th, 2011, 11:53 pm

1° is ~90' off at 1 mile. I usually round it up to 100' while I'm teaching just to make things easier.

Realistically you have a 2° margin of error with your compass unless you're *really* paying attention to it and being a stickler. It doesn't matter what the map says declination is, since the map is referencing geographic north (or grid north in some cases) so long as you know what the current declination for your area is it will work and be accurate. I believe for the gorge it's currently about 16¼° East.

The bigger problem is when people *don't* declinate, and assume magnetic north is the same thing as true. I can deal with a couple degrees error, but 16° error is too much.. Or worse yet, if they declinate backwards and they're 32° off.

I've attached a map for you for some reference. From Sherrard Point, all arrows are 3 miles long. Bright red is 0° True, the smaller Dark Red arrows are ±2°. Blue to the east is Magnetic North (16.25°T) and the Purple arrow would show someone declinating backwards at 32.5°T
declination.jpg

User avatar
retired jerry
Posts: 12758
Joined: May 28th, 2008, 10:03 pm

Re: Magnetic Declination

Post by retired jerry » December 20th, 2011, 5:48 am

The location of the magnetic North Pole changes over time.

At some point, the magnetic North and South will swap. http://en.wikipedia.org/wiki/Geomagnetic_reversal

During the transition, it's not well known what will happen, maybe there'll be no magnetic field for a while. That will let in a lot of solar radiation. Probably no mass extinction event or anything because it happens every several 100,000 years. The reversal takes 1,000 to 10,000 years. The field strength has decreased 35% in the last 1,000 years. Maybe we're in the beginning of a reversal. Maybe there'll be really good "Northern Lights" except they'll be everywhere.

User avatar
CampinCarl
Posts: 573
Joined: June 17th, 2011, 7:41 am
Location: Salem

Re: Magnetic Declination

Post by CampinCarl » December 20th, 2011, 7:52 am

Koda wrote:
So I asked Google. open the link and click on the map or compute... http://www.ngdc.noaa.gov/geomag/declination.shtml
Love this site for my land navigation maps... it ensures I am providing the most up-to-date declination. Lurch is right on.

User avatar
geographics
Posts: 963
Joined: August 5th, 2010, 2:11 pm

Re: Magnetic Declination

Post by geographics » December 20th, 2011, 9:13 am

.
Last edited by geographics on February 24th, 2012, 12:40 am, edited 1 time in total.

Lurch
Posts: 1259
Joined: May 28th, 2008, 10:03 pm
Location: Aurora
Contact:

Re: Magnetic Declination

Post by Lurch » December 20th, 2011, 9:42 am

geographics wrote:lat/lon 42N 2E
Where did you pull that number from? That'll put you in Spain I believe! There are multiple 0 points where magnetic north happens to line up with true for the time being. Jerry is right the magnetic pole is shifting around a little bit constantly.

Just to reinterate what I said before since it was buried in my paragraph up there. You do *not* need to keep buying new maps because the declination has changed. Maps reference True, and true is static (at least as far as our life times are concerned). The declination reading on the map is simply what it was at the date of publication. Most quads for the gorge will say that dec is like 19-20°E. I busted out the calculator and I was a little off, for multnomah falls area we're 16° 1' E. (that would be 16 degrees 1 minute.)

If you have a good compass that has adjustable declination than you just need to set it and forget it for a couple years. You'll be accurate enough from the coast to eastern oregon.

Now we just need to discuss map datums ;)

User avatar
geographics
Posts: 963
Joined: August 5th, 2010, 2:11 pm

Re: Magnetic Declination

Post by geographics » December 20th, 2011, 9:55 am

.
Last edited by geographics on February 24th, 2012, 1:09 am, edited 1 time in total.

User avatar
Splintercat
Posts: 8195
Joined: May 28th, 2008, 10:03 pm
Location: Portland
Contact:

Re: Magnetic Declination

Post by Splintercat » December 20th, 2011, 3:44 pm

I still remember getting my first Silva compass back in the early 70s, and the declination at that time was 21 degrees east. That seemed to stick for awhile, too.
...there'll be no magnetic field for a while. That will let in a lot of solar radiation. Probably no mass extinction event or anything because it happens every several 100,000 years. The reversal takes 1,000 to 10,000 years. The field strength has decreased 35% in the last 1,000 years. Maybe we're in the beginning of a reversal. Maybe there'll be really good "Northern Lights" except they'll be everywhere.
And hopefully by then, we'll have converted all of our magnetic media to optical formats..!

Tom :shock:

User avatar
retired jerry
Posts: 12758
Joined: May 28th, 2008, 10:03 pm

Re: Magnetic Declination

Post by retired jerry » December 20th, 2011, 3:56 pm

I don't think you have to worry about magnetic media, or magnetic motors or anything else, but you had best switch over from that Silva compass to a GPS :)

User avatar
Grannyhiker
Posts: 4594
Joined: May 28th, 2008, 10:03 pm
Location: Gateway to the Columbia Gorge

Re: Magnetic Declination

Post by Grannyhiker » December 20th, 2011, 4:37 pm

I had a cheap zipper-pull compass reverse polarity on me in Europe--got off the train in Switzerland, started looking for the hostel and realized the sun was setting in the east!

If the pole switch is relatively sudden, we can cope, as I did in Europe. If there are no poles for a while, it's back to the moss on the north side of the trees. Oops, that doesn't work around here; the moss grows on all sides of the trees! :lol:
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view.--E.Abbey

Post Reply