Author Topic: Declination  (Read 6827 times)

Grant

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Declination
« on: April 20, 2012, 09:02:15 PM »
Please help me out here, I am having serious trouble getting my head around the adjustable declination scale on the Suunto M3 - just bought one on reading the reviews etc.

When you set the declination scale for your area do I take it that no further maths are needed, even when changing from grid to mag and back again?  Cant quite get my head around this!

Please explain in simple terms, haha

Grant

Brian

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Re: Declination
« Reply #1 on: April 21, 2012, 05:18:53 AM »
Please help me out here, I am having serious trouble getting my head around the adjustable declination scale on the Suunto M3 - just bought one on reading the reviews etc.

When you set the declination scale for your area do I take it that no further maths are needed, even when changing from grid to mag and back again?  Cant quite get my head around this!

Please explain in simple terms, haha

Grant
Hi Grant,

Welcome to the forum!

First of all, declination is simply the difference in degrees between true (polar) north and magnetic north.  If you understand that, and you understand that magnetic north is located somewhat south of true north and that it is moving north (see the graphic here:  http://tinyurl.com/7aqs5om  ), you're well on the way to understanding how declination works.

I think of adjustable declination as a "set-and-forget" feature.  Once you've set your compass (by turning that little screw, or by other means) for the correct declination of your area, you can forget about any math. 

I love this "set-and-forget" feature in compasses, and wouldn't use one that lacks it . . .  unless circumstances absolutely forced me to.  I urge my students to get a compass with set-and-forget declination, and discourage them from getting one without that feature.

My reason is this:  when the chips are down and I'm stressed out of my gourd, the last thing I need to think about is calculating and applying some value necessary to save my life or someone else's.  I have higher priorities than doing arithmetic (which I'm not good at) or remembering and applying some mnemonic. I want everything as simple and foolproof as possible.

Having said this, you should be aware that declination changes according to time and location.

For example, where I am (southern Oregon, USA) declination is presently about 15.5 degrees to the east (  http://tinyurl.com/c4gyor  ).  In my location, that value is decreasing at about 7 minutes per year.  What that means is that any maps of my area whose declination was determined years ago, say in 1980 (as one of my USGS topos is), would be about 3.5 degrees greater than it is today (30 years X 7 minutes per year  ÷ 60 minutes per degree) or roughly 18.5 - 19 degrees.  And I emphasize the word "roughly."  And if armed with such a map, I'd calculate a new declination based on what I know.

Now — were I presently in Indianapolis, IN, my declination would be about 4.5 degrees to the West  ( http://tinyurl.com/c4gyor).   When I lived in Indy some 25 years ago, it was virtually 0 degrees . . . I actually lived on the agonic line, a line in which magnetic and true north were aligned — so it's changed by about 4 degrees 30 minutes in 25 years.

We happen to have a couple of surveyors in our SAR group, and when I asked "Bill" what the declination for our county was, he laughed.  He wanted to know where in the county I meant! 

In fact, he surveyed a point near our SAR barn, and, from it, determined that the azimuth from that point to our SAR flagpole was exactly 265 degrees.  Knowing this, we were instructed to adjust our compass's declination to whatever value necessary to give us a 265 degree bearing to that damned pole.  For me, that turned out to be 19 degrees, not 15.5 degrees.  In other words, we worked back from a known azimuth to establish our declination.  (How weird is that?)

Bill makes the point that declination varies greatly even within the same smallish area.

So what's a poor, bewildered soul to do? 

For SAR missions, I want IC to tell me what they think the declination is.   If I'm not involved in a SAR mission, I simply dial in 15.5 degrees East (or get as close as I can) and call it good, at least for Josephine County.

If I'm going out of county, my first preference is to consult an on-line declination calculator, my fall-back position is to look at the declination printed on my map and the map's date, and calculate what a reasonable declination might be, an educated guesstimate.

Having said all this, keep this in mind.  You can use a simple formula to calculate how far off you'd be if you get your declination wrong, or just shoot a bearing that is "off."  That formula is:

(Number of degrees "off" X distance travelled) ÷ 60.

So . . . if you're 3 degrees off (either because of a declination error or an error in shooting your bearing), and you travel 600 meters, you'd miss your attack point by 30 meters (3 X 600 = 1800.   1800 ÷ 60 = 30).

While this may not be a problem when shooting a straight shot, if you're trying to follow a multi-leg course, error can add to error, and you can end up way off.

Hope I haven't confused you too much!

Grant

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Re: Declination
« Reply #2 on: April 21, 2012, 09:19:31 AM »
Thanks for that Brian, I get the bit about the differences etc, it's just once I have set my declination adjustment - thats it, leave well alone while in that area - just seems too easy !!

Thanks

Grant

p.s. How are things over the pond?

Callum

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Re: Declination
« Reply #3 on: April 21, 2012, 09:49:34 AM »
Yes it is as simple as that Grant :)

Personally I would think about correcting manually for a while because we make corrections for magnetic variation (declination is just another term for the same thing) for two reasons, as Brian says small errors can become cumulative, and secondly when we travel abroad we can face substantial differences between true (polar) north and magnetic north and you need to have this technique as routine – just a thought ;)

Grant

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Re: Declination
« Reply #4 on: April 21, 2012, 11:05:13 AM »
Thanks Callum

So where I am the variation is 1degree 40mins West, so when I turn the compass over would I be right in adjusting the ring towards the E dec as this would be negating the westerly error?

Very sorry to appear so dim but this just confuses the hell out me - perhaps I should stay with the maths !!

Lyle Brotherton

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Re: Declination
« Reply #5 on: April 21, 2012, 11:20:58 AM »
Easy pneunomic for westerly declination Grant: Add for Mag, Rid for Grid (so when transferring bearing from a map to your compass add the variance).

Good point Callum about becoming familiar with it, when I was on Mount Rainier, Washington State, I had to account for a variance of 16° East!
“Opinion is the medium between knowledge and ignorance” - Plato

Brian

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Re: Declination
« Reply #6 on: April 21, 2012, 07:58:19 PM »
Thanks Callum

So where I am the variation is 1degree 40mins West, so when I turn the compass over would I be right in adjusting the ring towards the E dec as this would be negating the westerly error?

Very sorry to appear so dim but this just confuses the hell out me - perhaps I should stay with the maths !!

I don't think anyone thinks you are dim by any stretch, though in this case you are mistaken.  But as someone who confused himself mightily about declination for a long time, I can assure you I've been there, done that and bought the t-shirt!   :D

To me, understanding declination was like learning how to ride a bicycle . . . I can't, I can't, I can't, I can't . . . I can!  The learning was painful, but once I got it, it's permanently there.

So - using the adjustment screw, move the tic mark in the middle of the (black-bordered) south end of the gate about 1.5 degrees onto the scale marked W decl. to correct for a declination of 1.4 degrees West.  Once that's done:

1)  Turn the capsule until the N designation (=360 degrees) is aligned with the direction of travel mark on the baseplate.
2)  Turn the entire compass until the red end of the magnetic needle is centered in the red arrow of the capsule's gate.
3)  Now, your Direction of Travel arrow will be pointing towards True North, and your magnetic needle will be pointing ~1.5 degrees West.

If you look closely, you'll find that the gate is now not quite parallel with the North-South meridians of the capsule's floor:  the gate's red arrow will point slightly (1.5 degrees) to the West.  Where I live, the angle formed by the gate and the capsule's North-South meridians is pretty striking (it's 15.5 degrees). 

Here's a poor quality photo showing how the gate and meridian lines are not parallel when a compass has had its declination set (right compass).  Note that above the left compass, whose declination is not set-and-forget, there is piece of map showing the (East) declination.  Notice also that the set-and-forget compass's gate points towards magnetic north, while its True North (=360 degree mark) indicator points towards the maps's True North  http://tinyurl.com/bnkd93b

Once you've set your declination using that little screw and scale, you can pretty much forget about adjusting the compass again, unless you travel to a region where the declination is different.  It really is that simple.

Callum's point about practicing adjusting for declination manually is a very, very good one, and if you play with this a little,  you'll get a "feel" for how the system behaves.  Well done, Callum!

And things are good on this side of the pond.  Spring is here and I'm recovering from being pepper spray daubed this morning!  Thank you for asking, and I hope I haven't confused you!

Grant

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Re: Declination
« Reply #7 on: April 21, 2012, 09:33:23 PM »
Brian

You are a star!  I fully understood the manual side of it but couldn't believe that it was simply a matter of turn the screw and leave - I think I was looking for something more complicated, typical of me.

You guys on this forum are the biz, thank you all very much, excuse the pun but I am now headed in the right direction, haha.

Thanks to you all

Grant