42

4

This question (Calculate true north with a compass and map) got me thinking. I rarely bother adjusting compass bearings for magnetic north. Is this bad? I understand the theory but I just rarely bother. It's never seemed to have caused me any problems.

I mostly navigate in mountainous areas, I wonder if this is more relevant in flat featureless areas? Typically the terrain is just as big a navigational aid as the compass (even in low visibility) and the compass points me at the correct ridge, etc.

1The title has to be more like "I never adjust for true north", hasn't it? Edited it already. Roll back if it's wrong.OddDeer 2015-12-17T09:48:21.713

4It matters if you are travelling long distances on a bearing over featureless terrain, such as sailing a ship across the ocean, or travelling across a desert.Qwerky 2015-12-17T10:30:08.103

@Liam your comment caused me to post How much distance is there between True North and Magnetic North just fair warning as I quoted you, and disagreed a bit.

James Jenkins 2015-12-17T13:35:30.637

1

@Liam True and Magnetic North are far from the same thing! And "Grid North" is something else again... https://en.wikipedia.org/wiki/True_north

Roddy 2015-12-17T13:53:56.503

@Liam Based on the answer posted and some other research it looks like the length of your country is 4 times less than the distance between true and magnetic north. I am happy that you are happy to be so wrong :) This kind of highlights how assumptions carried from one location to another can get us in trouble.James Jenkins 2015-12-17T15:20:40.457

5@JamesJenkins Declination is a strange and wonderful thing. Your compass doesn't actually even point at Magnetic North. It just sort of "points".Roddy 2015-12-17T22:55:40.060

Martin F 2017-09-29T03:28:59.440

51

This map (and the Wikipedia article) will explain why you don't have a problem (in the UK, I'm guessing), but others do, elsewhere.

In the UK, you can ignore it at the moment, but you need to understand WHY you're ignoring it :)

As an aside, compare this with the declination in 1872... Then, the declination error in the UK would have been between 20 and 30 degrees west!

Typically when walking on a bearing you need to allow a margin of error of about 5°. (ie, when 'aiming off', allow at least 5° of error). So the sub-one degree at present in (most of) the UK isn't a big issue.

One thing other haven't mentioned is the precise impact of ignoring the declination. Let's imagine we need to pace 500 metres on a bearing in thick Welsh fog across typical Welsh moorland to reach a bridge on a stream. We know we can't possibly hit the bridge directly, so we need to 'aim off' to a point upstream, and then walk downstream to find the bridge.

You can only follow a compass bearing accurately to within +/-5° (errors both in working the bearing out on the map, and actually following it on the ground. A 5° error over 500 metres would mean we miss our target by about 44 metres (500 * tan(5 degrees)), so we'd want to 'aim off' by that amount as a minimum. Let say we aim off by 50m.

Now, if the declination is 1° and we ignore it, our worst case error angle changes to 6°, meaning we could end up 52m from our aim point - very slightly downstream of the bridge.

But if we ignored a declination of 10°, we'd be up to 134m from our aiming point, and so far from the bridge we'd miss it completely.

Of course, there's one thing worse than ignoring declination: That's adding it when you should be subtracting, or vice-versa. The potential 25° error (10° declination, wrongly applied, plus 5° general error) would put you almost 250m from you intended destination.

nice drawing, but I don't really understand what the lines mean?Liam 2015-12-17T13:23:09.223

5It's a map showing the magnetic declination around the world. If you're on the "-10" line, your compass will point 10 degrees off true north. The UK is close to the zero line, but parts of Canada, for example are over 20 degrees off.Roddy 2015-12-17T13:28:17.430

1Ah ok, very interesting. What's the "point" below Australia?Liam 2015-12-17T13:30:50.680

7I like this, I'll drop you a bounty I think. I've never seen this map beforeLiam 2015-12-17T13:33:21.520

6

Cheers :) It's the South Magnetic Pole. https://en.wikipedia.org/wiki/South_Magnetic_Pole. The North one is off the top of the map somewhere.

Roddy 2015-12-17T13:41:58.477

2While I understand conceptually what these lines mean, I find it really hard to translate it into a more tangible understanding of the real effect. Is there a map with lines drawn that show "from this point, your compass says north is this way"? I feel like that would be much simpler to grok.Dan Henderson 2015-12-17T17:49:44.340

@DanHenderson Interesting. I've never seen a map quite like that. Maybe distory the whole projection so that magnetic north is always 'up'?Roddy 2015-12-17T18:01:35.930

@Roddy I'm thinking more like keep the landmasses and lat/lon lines as normal, but just draw lines similar to the ones in this image, but they are lines depicting the course one would follow if one tried to travel N/S by following an unadjusted compass.Dan Henderson 2015-12-17T18:03:50.530

@DanHenderson What you're thinking of would be something like a vector field, I believe.

JAB 2015-12-17T18:48:20.257

7

@DanHenderson So... like this?

Ordous 2015-12-17T19:04:26.547

@Ordous yes, something like that. Although it would be even better if it were on a Mercator projection, and line-y. :)Dan Henderson 2015-12-17T19:06:34.603

Hah, this map indicates that I correctly remembered the offset for New York from the Boy Scouts.KRyan 2015-12-17T21:02:30.303

3@KRyan You were in the scouts in 1872?Roddy 2015-12-17T21:17:08.523

@Roddy :P Scouting was actually founded in 1907 (and Boy Scouts of America in 1910).KRyan 2015-12-17T21:18:08.233

@DanHenderson I don´t really understand how you would translate a vector field to lines... I think its impossible. The only thing you can do is to connect points with the same error, which is the map in this post, or to show the actual direction, which is the vector field. But how would you connect the lines in the drawing you suggest?Paul Paulsen 2016-05-31T09:31:03.050

30

Where your magnetic compass points can be quite far away from the north shown on your map

)

For example, on the line marked 30, your compass would point 30 degrees away from true north.

3Love the animation :)Roddy 2015-12-19T21:44:00.727

+1 for the great animationPaul Paulsen 2016-05-31T09:32:33.760

28

This depends highly on your location. Contrary to popular belief, the difference between the magnetic pole and the geographic pole is not the only reason for declination. As a matter of fact, the magnetic poles are simply defined as the points where the magnetic field points vertically. This is not the same as the pole of a anyway non-existing earth-magnet. Local variation in the magnetic field results in strongly varying declination all over the globe.

That said, we are not talking kilometers here (at least not in general), but on global scales. So whether you have to care or not depends on whether you travel long distances and if not, where you are. The reason why you do not have to care in the UK and most of Europe is, that the current declination is very close to real north. So for visual navigation with lots of terrain features this is negligible. Other areas (northern part of North America) the declination is much higher and adjustment is really necessary. Some compasses come fixly preadjusted for a certain area (bad idea), some can be adjusted by a screw and some are not adjusted at all. So make sure you know which one you use.

The other matter is, that this is subject to change over time. What is written here for different regions might not be true in some years. So if you pick up your navigation gear after a really long pause, you might have to reconsider you declination settings :)

14

Olin Lathrop's very good answer to the question you reference basically already contains most of the information needed also your gut feeling about the topic is quite right. So let's look at it in a bit more general way:

Keeping your compass declination in mind gets the more important, the

• closer you are to one of the magnetic poles,
• the more you travel sideways with respect to the magnetic poles,
• the less landmarks you have on your route of travel to check and adjust your course, and
• the longer the distance you travel mainly based on compass bearing.

To take a quite extreme example, we consider the following situation: you want to travel by ship from the southernmost tip of South America (Tierra del Fuego) to New Zealand. For whatever reason, you don't intend to approach New Zealand in a direct course but plan to keep basically parallel to the Antarctic coast line (i.e. on an east-west course following a certain latitude) and do a sharp rectangular turn towards the north once you're in the gap between Antarctica and New Zealand. Your only measure of direction is your compass and since you are on the ocean, there are no land marks to correct your course on the way.

According to Wikipedia the magnetic south pole (which is of course also off from the geographic one) is currently somewhere at the Antarctic coast right opposite of Australia, i.e. shifted towards Australia with respect to the geographic south pole. So if you start in Tierra del Fuego at the southern end of South America and head straight east according to your uncorrected compass, you will in fact travel slightly northwards on the globe. This northwards tendency will increase the closer you get towards New Zealand (and hence the magnetic pole) and – it's just en estimate here, I would have to run the numbers for the exact outcome – in best case you would just hit New Zealand without the intended northward turn in the end while in the worst case, you could miss New Zealand by its northern side and end up in Australia or South East Asia instead.

Fortunately, measuring latitude is relatively straightforward.gerrit 2015-12-17T14:41:30.697

2@gerrit Don't disturb my little thought experiment ;-)Benedikt Bauer 2015-12-17T21:06:25.913

8

As others have said the importance depends on your location. In the UK & Europe the correction is quite small, approx 2-3 degrees and can therefore mostly be ignored. Compared to parts of the US where the correction can be at least 15 degrees. This is because the magnetic north pole lies somewhere in northern Canada, so the UK is relatively far away for its inclination.

Given this unless you are traveling long distances over distances in featureless terrain (which is pretty much only going to be at sea), the difference becomes negligible. If you walk 10km in a straight line a 2 degree error only gives about 300m of lateral distance so you don't really need to worry.

I didn't realise the difference was 15 degrees in the US!Liam 2015-12-17T13:06:48.273

7

Not discounting the existing answers which provide great perspectives. You don't really care where the North Pole is. It is not like there is only one or two, nor is like anything is stable. There are half a dozen north poles and most of them move around regularly and they move a lot (10s and 100s even 1,000s of miles/kilometers).

So what do you care about? You care about where you are, where you want to be, and what you will need to overcome between the two. A compass is one of your tools, and is subject to local variations which also must be accounted for.

Your primary tools is the map, your primary goal is to define your current locations, secondary is to define your desired direction of travel. Putting it all together you orient yourself and the map. Depending on where you are on the globe, a compass may be of no added value (i.e. near either pole).

So not is not bad, to never adjust for true north, just be aware of the limitations of all your tools, and make good judgement on based on realistic expectations of the available information.

Is there a physical pole installed somewhere near one of them, like there is for the South Pole?Random832 2015-12-17T15:39:03.143

@Random832 see When is the best time of year to find the North Pole?

James Jenkins 2015-12-17T15:43:28.030

3@Random832 - every attempt to plant one has been thwarted by Santa Claus. Recall that in the early days of exploration many of the explorers simply disappeared, and those that made it back were often deemed "mad". This is because of Santa and his legions of elf-troopers. Either the explorers were killed outright, or the stories they told of being attacked by green-clad pointy-eared micro-humanoids screaming "Jingle bells! JINGLE BELLLLSSS!!!!!" (the elvish war-cry) led to them being thought insane. Nowadays Santa still has an army - but of lawyers, not sword-wielding midgets. Ho-ho-ho... :-)Bob Jarvis 2015-12-19T14:07:22.430

@Random832 I am not sure about this, but I don´t think there is any. I would guess that is the case because the arctic isn´t based on land, but practically a lot of swimming ice, so it would probably move too much. But that´s just my suspicion...Paul Paulsen 2016-05-31T09:38:14.917

"Half a dozen" is a gross exaggeration not supported by that link. There's no suggestion that we're considering the celestial north pole (in the sky) or the pole of inaccessibility, never mind poles of other astronomical bodies! That leaves just three. The rest of your answer makes sense, though - the two which matter to the individual are local magnetic north and grid north.Toby Speight 2017-07-11T08:28:15.573