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!