Magnetic Declination - Add or Subtract?

[carload]

Here's a stupid question, so the answer can guide generations of FTA enthusiasts to come.

In the Western US (for example) do you add or subtract the magnetic declination factor from the compass reading?

In particular, from Denver (factor 10 degrees) G10R is 206.1 degrees azimuth. So what compass reading points from my back yard to G10R? 196 or 216?

(And whatever the answer is, you do the opposite in the Eastern US, with the midpoint staying pretty close to the Mississippi River.)

 

[PSB]

http://www.geo-orbit.org/sizepgs/magmapsp.html

 

[carload]

Thanks for that link; it's half of my confusion. It says for west declination, "subtract from compass reading for true azimuth". So Compass 216 - 10 = 206.

Then there's this link: http://www.sadoun.com/Sat/Installation/Satellite-Heading-Calculator.htm It says for west declination, "subtract from calculated azimuth for compass reading". So 206 - 10 = Compass 196.

So who's right?

 

[carload]

Aha! Perhaps I have the answer, courtesy of Dish Network.

From Denver, E*1 (148W) is visible from a true azimuth of 235 degrees. (I checked that figure in a couple of places.) Dish Network has a downloadable program that will produce the apparent azimuth for any Zip Code, "adjusted for magnetic / true north deviations". I enter my Denver Zip in the Dish program and the result is ... 225!

Therefore, Sadoun is correct in saying westerners should subtract the declination from the calculated (actual) azimuth to produce the compass (apparent) reading. So tomorrow morning, I'm going to try to find G10R at Compass 196.

A chorus of corrections should soon follow if I'm wrong. Otherwise, I have to assume that Geo-Orbit's page misspoke.

 

[Mr Tony]

East of the Mississippi river..you add to the azimuth
West of the Mississippi river...subtract to the azimuth
Duluth, MN has a 0

 

[cookie74]

East of the Mississippi river..you add to the azimuth
West of the Mississippi river...subtract to the azimuth
Duluth, MN has a 0

What does the Mississippi have to so with it? All of the United States is WEST declination. It doesn't change to east until you reach half way across the pacific and then back to West off the coast of Africa in the Atlantic. FYI the internet is not limited to the US

 

[Jim S.]

I think you're thinking of LONGITUDE, because with the North Pole being roughly in the north of Quebec, the zero magnetic declination line does in fact run roughly down the longitude of the Mississippi River.

 

[Anole]

Pretty much all modern satellite locator programs give BOTH corrected and uncorrected azimuth, so I never have to think about it.
.
?I can remember which way to twist the dish/LNBF for skew, depending if the bird is east or west of me.
?But the magnetic correction is a big "don't care"!

 

[rv1pop]

In my back yard ----- you can not correct it! As it was explained to me --- 50 years or so ago --- science can change though, Magnetic deviation is LOCAL. areas can be as small as 5 acres. My land says that! North is straight down!

 

[. Raine]

In my back yard ----- you can not correct it! As it was explained to me --- 50 years or so ago --- science can change though, Magnetic deviation is LOCAL. areas can be as small as 5 acres. My land says that! North is straight down!

I have weird magnetic deviations/fluctuations on my land for whatever reasons, ya can never be sure of a compass reading here. I've pretty much mapped out my back yard with points of reference and don't hardly ever bother trying to use a compass here.