# Finding declination/variation

Heard people give a couple of strange-sounding numbers for declination for my home, so I surfed the 'net and found the following resource:

http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp

Rounded the figure they calculated to the nearest degree and used that in my practice, and it seems good to go.

published on the chart
If you’re using a nautical chart, you should use the variation printed in the center of the nearest compass rose. I’ve seen charts covering less than 20 miles with quite different variations in each compass rose on one chart.

According to the latest
Issue of Backpacker, if you can find the north star you can use a compass to figure out a rough estimate as according to the article the north star is usually within a degree of true north. By using your compass you can estimate by measuring the difference between line of travel (set to north star) and compass arrow direction.

If you don’t have a chart for your area
the website is a quick way to get something to work with. All I have right now are topos of varying ages.

I “think” all GPS units are …
… plotted to True North (not sure but I think so) , but have internal conversion capability to Mag.North , and you have a select option between TN or MN for the read out .

In air or water , there’s more to consider in the formular than just -E or +W Variation though if running a plotted course by compass Heading .

Ccheck the date on the chart
Declination varies over time. As much as 3/4 degree in ten years.

Check the chart
Like it’s been said earlier, set it to whatever the map or chart you are using has on it, or else get a few for the area and average them. EX in my area it varies between 18 degrees and 19 degrees, i set my compass around 18.5 and i am usually close enough for using a compass.

The declination also wanders as the years go by, and depending on the coordinate system it can very quite a bit.

GPS always points to north as defined by the coordinate system it is using. The most common is usually UTM, but you shouldn’t really notice much of a difference between one or another as long as you’re not actually getting close to the north pole.

Remember Deviation as well…
Kayakers needn’t be so anal as to eastablish a compass card as they can’t steer straight enough to that accuracy, and loads vary, so this is impractical.

So why bring it up you may be wondering? Keep metal stuff away from your compass or you may be significantly deviated.

maps…
…by the cartographers rule book …all maps are laid out to true ( geographic) north… you have to use the declination degrees on the map for accurate navigation, regardless of maps age, because that’s the way that map was drawn up. A very experienced trekker could do the math to accurately use a old map , with present day declination. just as a side note …the NMP is moving towards Russia at a rate of 37 miles per year!

variation varies
Charts show the date of measurement and annual change in the compass rose.

– Last Updated: Apr-29-10 9:29 AM EST –

..... I don't paddle across open bays or in the ocean (canoe fisherman here, lol) , but would imagine kayakers doing such and running a plotted course would need to consider and adjust for it .

I'm just imagining a scenario where a kayaker sets to cover something like a 10 mile crossing to an island or other fix point and visability is (or becomes) low enough that it has become imparitive the course is made good ... low vis. situations , haze , fog , dark , rain .

Dead reconing is a fine thing to use for recalc. along the way , but what if the 10 mile only has one fix at each end , shore and the island .

Seems to me the paddler would be about the most sucseptable to tidal and wind drift .

I can see using estimated (direction and speed) tidal flow and wind info. to make corrections for establishing a mag. heading , but how would one apply that info. in combination ??

Would you calc. each and apply seperately or is there a combination rule method ??

In the air forcast winds aloft are used to make WCA corrections , so I imagine tidal flow and ground winds that would cause drift would be an important factor for a paddler .

Great question
Kayaks are very susceptible to set and drift. I clear conditions establishing a natural range marker and staying lined up is the best way if you have no GPS. If land is too far off and you have no ranges the best option is to calculate any current and account for it with your bearing. It’s a good idea to purposely correct to one side or the other so when you do arrive near shore you’ll know which way you’re off. Ex: trying to hit a narrow lagoon entrance after a long crossing.

Of course GPS accounts for all this, and is super useful in fog.

Kayakers get set all the time because they point their bow at the target and it always looks to be in front of them, yet they are doing a big arc. Establishing ranges is a good habit.

GPS doesn’t really “point” anywhere
Global Positioning just calculates your position (lat/lon) on the earth’s surface. It doesn’t actually point you anywhere. There’s no inherent magnetic or true north direction involved in the raw GPS time offsets that your unit gets from the satellites, since it’s just a position.

Most GPS units nowadays include software that can compute a course between your position, and the position of another place that you enter or select on an e-chart. And the unit will display either magnetic or true bearings, depending on what you choose.

That’s a good practical check

– Last Updated: Apr-29-10 12:13 PM EST –

This from Calvin Rutstrum:

"If there is no compass variation indicated on your map, observe where the compass needle comes to rest in relation to the North Star (Polaris). This will give you a fairly close check on your variation, since the North Star is never more than about 1 degree off true north. To be more exact in your observation, check with the North Star when the star Alkaid, on the end of the handle of the Big Dipper, is either (directly) above or below the North Star, (in the position shown in the illustration). Polaris is then at true north."

He then goes on to suggest:
"You can best do this by driving two sticks into the ground, one short and one long, sharpening them to points with a knife, and aligning them to the North Star. Now, by placing your compass between the two sticks, see where your needle has come to rest. The difference in the stick alignment and the needle's pointing is your compass declination or variation for that area."

PS: Not meaning to be "preachy" or anything... (There are many better at orienteering than I) But it occurred to me that in this age of GPS usage many, but not all, have forgotten how to find the North Star, without which all the preceding Rutstrum stuff is useless. (It can only be hoped that the Big Dipper is still recognized.)
To find the North Star, look for the first bright star pointed to by the two stars that form the "front" of the Dipper's ladle, opposite the "handle." Look to the "open side" of the ladle. That's Polaris, the North Star. Everything in the sky revolves around it and eventually Alkaid will be either directly above or directly below it. That's the time to figure your magnetic variation if you really need that much accuracy.

One degree of error will only put you "off the mark" by 92 feet/mile traveled or ~461 ft/five miles if you paddled your heading accurately. Five miles is a pretty long crossing, at least for me, and normally I doubt anyone would be real upset by being 461 feet off.

Here's to clear nights. Down with light pollution!

Deviation
There’s a Mnemonic for remembering how to use variation and deviation when translating between True direction, Magnetic direction, and Compass direction.

“True Virgins Make Dull Companions. Add Whiskey.”

So if you have a True heading, you first add/subtract the Variation (virgins) (from chart) to get the Magnetic heading (make). Next add/subtract compass Deviation (dull), to get your Compass heading (companions). The “Add Whiskey” phrase indicates that when Var or Dev are Westerly (whiskey), you add them. When Easterly, you subtract them.

(When calculating from Compass to Chart, as when plotting a bearing Line of Position, use “Can Dead Men Vote Twice, At Elections?” (translation: Compass + Deviation = Magnetic, + Variation = True, Add Easterly)

Like Salty said, Kayakers don’t normally swing the compass and create a deviation card, the way larger boats with nice big compasses do, so we mostly ignore that last part, as we just don’t have that kind of accuracy when steering or even reading our small compasses. But it is good to remind yourself that there IS a difference between “Compass North”, and “Magnetic North”.

It does the same thing as “point” though
If you are moving, the GPS uses its position data to calculate the direction you are going, and so in actual practice the GPS gives you the same information as a compasso. If you compare the GPS to your compass while on-the-go, you will easily see the difference between map orientation and magnetic orientation if the difference is significant.

getting picky

– Last Updated: Apr-29-10 12:08 PM EST –

Well, this may sound like it's getting semantic, but there is actually a very important difference between the "COURSE" that a GPS reports, and the "HEADING" that your compass tells you, but that difference is NOT the local magnetic variation. Basically, a compass only tells you which way your bow is pointing (that's your heading). A GPS tells you what direction you're travelling over the ground (that's your course), either in magnetic or true degrees. Without a fluxgate compass (which a few GPS do have) a GPS cannot tell you your true or magnetic HEADING. Similarly, a COMPASS cannot tell you your COURSE. In other words, the compass tells you the direction you're pointed. The GPS tells you the direction you are moving. Those two sound pretty similar, but in a boat they are seldom the same. In fact, the two could be 180 degree opposites (if, for example, you're paddling at 2 knots through the water into a 3 knot contrary current).

Imagine you're in the fog, padding 2 miles from Crud Island to Point Beautiful. You look on a chart and the bearing is True North. Variation is 10 degrees W, so you look at your compass and point the bow to 010 degrees magnetic. You look at your GPS and it also says the bearing from your position to the Point is 010 degrees magnetic. Perfect! After 5 minutes of paddling, and diligently keeping your deck compass pointing at 010M, you glance at the GPS and it says your course is 070 degrees magnetic! (or 060 degrees true).

That 60 degree difference between your compass heading, and the GPS course is NOT the magnetic variation for your area. It's the difference between where your bow is pointed, and where your boat is traveling.

Specifically the difference between heading and course is your SET, DRIFT, LEEWAY, or whatever force you want to blame for your boat not going directly towards where you pointed the bow. You may have paddled north out into a 3 knot Easterly current. Or maybe a west wind is blowing, and you wore the biggest poncho you could find.

Theoretically, if you were on land, you could walk straight in one direction, and compare your compass heading and your GPS course (with GPS set to true) to get an idea of the local variation. Or you could very carefully paddle toward a fixed range of two objects ahead on land. But I'm not sure why that would ever be useful. The only thing variation is needed for is navigating with a chart, and charts always have Variation printed right on them. Is this not always the case with topo maps? Don't they have the "Declination" printed somewhere on every chart?

That’s all clear to me

– Last Updated: Apr-29-10 12:24 PM EST –

Thanks for going to the effort to explain all that. Yes, I did not differentiate between "heading" and "course", because I was over-simplifying and assumed that the person using the GPS and compass were already able to establish their exact direction of travel, so I wasn't assuming the need to actually rely on either one of them to get you where you are going while doing this comparison (for my own purposes, when crossing a large lake and not needed either device to find my way, I like to line-up two landmarks on shore to establish an arc-free course during the crossing, and yes, "heading" and "course" are usually very different). For traveling by boat on water when there are unquanitied effects of crosswind or current, your points are quite important.

hopefully it’s news to someone!
That was a long way to tell you something you already know. Sorry for underestimating your understanding.

That’s why I like the website
I don’t have to check how old these topos are, or do any math.

My land navigation book has instructions for accurately figuring out declination for my location, so there are other ways to skin this horse. But the website is the fastest.