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Compasses to point true north for first time in 360 years in UK (theguardian.com)
87 points by sohkamyung 53 days ago | hide | past | web | favorite | 46 comments

The concept of a compass not pointing to True North was something I struggled with at first when learning orienteering because I had assumed north meant north. The reality is that you have a Grid North, Magnetic North and True North.

That said, orienteering is one of the most valuable skills I think a person can learn but it can be hard to grasp learning solo. For those not familiar there are really two ways to go about it: - Dead Reckoning - Terrain Association

Dead Reckoning is all about planning waypoints and using your compass and pace count to get you there. For example: I need to walk 5km at 220° then 2km at 90°, etc.

On top of that, it's easy to make simple mistakes when planning your route. Errors when converting from grid to magnetic(and visa versa) can put you off by a few grid squares so it's important to understand terrain association which is a technique of studying the terrain on a map, plan a route and use those visual terrain features to guide you(If the river is supposed to be on your left but it's on your right, something is wrong).

Having both techniques in your back pocket are essential, but like any skill, they're perishable so you have to practice often.

Some key mistakes people tend to make when orienteering: - When you're dead reckoning through the forest, make sure you switch which side you favor. For example, if your compass points directly at a tree do you step to the right of it to go around or to the left? Most people will pick their dominant side and tend to veer to the right over time. This won't be corrected by staying on a specific azimuth. - Night time is challenging when orienteering in a forest with no roads/creeks/etc to handrail. You often can't see more than 10-20 meters and there's no terrain for you to associate with. - After you plan your waypoints out, do it again from scratch. You likely made a mistake. The protractor you used to get your heading represents your grid angle, every map will have a GM Adjustment(grid to magnetic) you'll need to make.

Source: I'm a former US special ops soldier with hundreds of hours using a map and compass humping through the woods.

I really enjoyed reading your comment thanks! I think orientation is very valuable also but we kind of lost it because of GPS and so. I remember maybe 20 years ago, my parents and relatives always had a physical map in their cars. When I started driving, I had one too (even if I used it only one or twice)

Thanks so much!

I'm a sucker to GPS as well!

I didn't know about having to correct for magnetic vs grid north.

Are the maps specialised for orienteering? Since I've never seen that info on a standard street paper map.

Is it something that a digital compass could automatically do? Especially since the article is indicating that magnetic north drifts.

In the UK the three compass points are on OS maps, sometimes with the changes predicted for multiple years.

The older the map the less useful it is. You then have metal in the local rocks which can cause problems.

Terrain recognition is far easier, there is far more error correction built in, but if you are hit by a patch of fog and visibility is down in to 30m (let alone 3m!) you need dead reckoning.

Trouble is even a 1 degree error can turn into significant errors after a very short distance.

Aeronautical charts include this information. Terms are "true north" which is based on lat/long grid, and "magnetic north". An airplane has a magnetic compass as the primary reference, but it's a little wobbly, so on the ground prior to departure, a gyroscopic compass (called a heading indicator, directional gyro) is set to match the magnetic compass. If you do a some steep turns, or many shallower ones, you can get your heading indicator to drift. And if you travel some distance east or west it'll also drift relative to the magnetic compass, so you have to reset them to match periodically.

The magenta dashed line going from top to bottom at a slight angle on the left of this image indicates degrees magnetic declination. https://www.faa.gov/air_traffic/flight_info/aeronav/media/VF...

Great question! I had never thought of whether a smartphone points to grid or magenetic north. Quick google search shows that you can set at iPhone to either[1].

As for specialized maps for orienteering: Orienteering maps are different than normal street maps. They always have gridlines, very precise scales and clear markings of major and minor terrain features[2].

Then, on top of the data presented on a map you have maps of different scales. For example, one inch of a 1:24,000(pronounced - one over twenty four thousand) map represents 24,000 inches on the ground. A 1:250,000 map it very zoomed out relative to a 1:10,000 map.

If you're hiking 500 miles of the PCT you may want a 1:100,000 or 1:250,000 scale map because you don't need to know every twist and turn because you're following a trail.

But if I dropped you off in the middle of nowhere and said walk due west to a specific point 10 kilometers due westyou'd want a map that's 1:10,000-1:24,000 scale because you need to see specific terrain features.

To answer your question more directly: There are what you would "orienteering-specific" maps because they represent some critical data well. For orienteering you use the one that works for you based on what information you need to get you from point A-B

[1] - https://www.idownloadblog.com/2017/04/28/how-to-iphone-compa... [2] - https://pubs.usgs.gov/unnumbered/70039582/report.pdf

Some maps provide this information but as it changes over time it's better to check it online beforehand.

Useful tool: http://www.magnetic-declination.com

Here's a nifty visualization of magnetic pole location and strength from NOAA-


edit: Some more on this stuff here (probably to be taken with a grain of salt, but neat no less)-


I'm usually a Mercator apologist, but this is a really bad map to use that projection for

In this case they want to preserve directions rather than e.g. area or distance, so Mercator is suitable. (You can click the tabs on the right side to see polar projections.)

Yeah that second site is run by people that believe in some pretty fringe stuff (electric universe, etc). Definitely take it with a grain of salt. Just look at their last two "Fast facts"

That is a great map! If you want the magnetic field for a particular location they have a calculator too!


I have been working on a smartphone AR visualizer for the magnetic field. I was awarded a grant from the National Science Foundation to develop this app. One of the surprising things I found was the inclination of the magnetic field vector. You can test the free Android app here:



Please don't do this here.

(un?)fortunately the 'Agonic' is moving Westwards, so we are just aligning with our European brethren.

Judging by https://upload.wikimedia.org/wikipedia/commons/6/68/IGRF_200... this map, it's moved about 6 degrees in the last 18 years.

So far as I understand it the existence of a magnetic "North Pole" is a weird temporary coincidence, like if you discovered a friend's phone number is the same as yours but in the opposite order of digits. It doesn't mean anything and it might change also for no particular reason, so you should probably avoid depending on it if possible.

Magnets are more or less bound to point somewhere on a planet containing a core of molten iron, but my understanding is that it could easily have been towards Australia, or Mexico or anywhere.

No, it's almost always roughly aligned with the axis of Earth's rotation. I believe that's because it's related to the rotation of the internal molten iron core.

Though every few million years, the north and south magnetic poles swap, for unclear reasons.

We are also about 350,000 years overdue for a magnetic pole swap. [1]

Also, it cycles at 300,000-400,000 years not millions.

[1] https://futurism.com/earths-magnetic-poles-overdue-switch

Looking at a map someone else linked to, the pole has shifted drastically in the last 50 years. Could that be an indication that it is in the process of shifting? It's been moving North in a straight line for the past 100 years, and now it's actually heading South towards Siberia.


> Could that be an indication that it is in the process of shifting?

It could be. Since we've never seen a shift, we don't really know how it looks.

And a shift takes a few thousand years, so we can probably be centuries into it before it's clear what's happening.

Ah, too bad, it would have been great drama if it flipped instantly! Or even better, oscillated a few times over the course of a few years before settling in.

I mean, for all we know it might oscillate like that.

I doubt the geological record is fine grained enough to tell fast oscillations apart from a slow drift.

Thanks for the timing correction!

I hoped "every few million years" was vague enough that I didn't have to look it up, but sometimes you gamble and lose :)

Also, the swap apparently takes ages and during that time you have loads of magnetic poles floating about and the ionosphere dips by 80% or so. We currently have a little north pole floating around the south Atlantic. https://en.wikipedia.org/wiki/South_Atlantic_Anomaly

Is it really a "little north pole"? I don't know anything about it beyond what's in the WP article, but that doesn't sound like an accurate description.

I reckon it isn't that inaccurate, especially given the reported behaviour of compasses.

"From these models and extrapolating down into the Earth, it is known that regions of reversed flux at the core-mantle boundary have grown over time. In these regions the compass points in the opposite direction, in or out of the core, compared to that of surrounding areas. It is the growth in area of such a reversed flux patch under the south Atlantic that is primarily responsible for the decay in the main dipolar field. This reverse patch is also responsible for the minimum in field strength called the South Atlantic Anomaly, now centred over south America. In this region energetic particles can approach Earth more closely, causing increased radiation risk to low Earth orbit satellites."


Does anybody have a link about that first part?

"Almost always" is a term of art in probability theory, where it means p = 1 despite the existence of possible outcomes that defy it. For example if you keep tossing a (fair) coin it will /almost always/ eventually come up "Heads". But I guess here you mean something else and I'm not sure what.

It isn't just a theoretical supposition, I believe we measure alignment of magnetic fields in igneous rocks and the 'almost always' comes from that, the current field of the earth being fixed in the rock when it solidifies.

> "Almost always" is a term of art in probability theory, where it means p = 1 despite the existence of possible outcomes that defy it.

That's a somewhat misleading term! I much prefer "probability one".

> But I guess here you mean something else and I'm not sure what.

Read it literally. The everyday definition of "almost". In other words, "the vast majority of the time".

I agree this should be read as “the vast majority of the time”, but if one wants to be precise, it is unavoidable to deviate from everyday definitions (is 99% the vast majority of the time, or does that need to be 99,9%, 99,99%,…?)

“probability one” is misleading, too, as laymen would assume the reverse cannot happen.

The interpretation of “almost always” as ”p = 1 despite the existence of possible outcomes that defy it” likely is based on http://mathworld.wolfram.com/AlmostEverywhere.html (which has “almost surely” as the mathematical expression, but I would immediately read “almost always” with the same meaning)

> “probability one” is misleading, too, as laymen would assume the reverse cannot happen.

In the real world it cannot happen. It can only happen in an unbelievably unlikely mathematical abstraction. So I would call that layman understanding closer to the truth.

I would assume that "almost always" is a common English phrase that has been adopted by probability.

Sure, but the more accurate English phrase for that meaning is more like a simple "always".

"almost always" is more like a 95% threshold, sometimes higher but with an implication that failure is realistic.

Perhaps. However - "the magnetic axis of Uranus, for example, is inclined by as much as 60°"

While that's true, when using something to measure you're not trying to find a universal truth, just something constant and approximate enough for your purposes that you can utilize it.

If magnetic north was located in Mexico it would still be of use, assuming it was relatively constant.

Is it not roughly perpendicular to the plane of rotation because of the relationship between electromagnetism and applied force (see the right-hand-grip-rule we all learnt in highschool physics)?

However it's still currently good enough to get an average person to their destination if their GPS enabled device has run out of battery and they happen to have a map and compass on them.

Unfortunately for most people nowadays, the compass is the same electronic device (their smartphone) that does GPS.

...and the map is also the same device.

Most smartphones that are out of a battery can give you magnetic north if you also have a big rock (smash phone apart), and make a compass leaf out of a piece of metal in the phone, something like a leaf and a patch of still water. The phone most likely has a magnet (e.g. in the speaker) to magnetize the metal.

I can't think of any ferrous parts in a modern phone.

Enclosures are all either plastic, aluminum or magnesium. Circuit board will be copper. Maybe some internal structural elements or RF shield cans are made of steel?

Microphones are MEMS these days, so no magnets there. Speaker magnets are tiny and also tend to have weird magnetization patterns that seem useless for making a compass (e.g. disc with a circular channel for the coil where N-S are inside of the channel)

Note that PCBs are made of dielectric (non-conductive) composite materials such as fiberclass-resin laminate. The conductive copper film covering the substrate is on the order of 100 micrometers thick.

I believe most if not all smartphones also have a magnetometer in them, apart from the GPS or the metal components in general.

True but that's going to be a MEMS[1] device, so not much use without the rest of the phone operating.

[1]: https://en.wikipedia.org/wiki/MEMS_magnetic_field_sensor

I doubt this is true for most people who can and might realistically need to be able to navigate with a paper map and analog compass.

In other words, the skill is about as rare or rarer than the equipment.

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