There’s a blurb from the UK Ordinance Survey about a unique alignment of ‘North’ lines occurring in southern England:
I get that Ordinance Survey maps will have a unique Grid North line for Great Britain (other nation’s maps will have different Grid North lines, but they don’t mention that), and that for every point along the OS Grid North line, True North is the same direction as Grid North. Move east or west of the Grid North line, and True North is off to one side or another due to 3D->2D map projection difficulties.
I don’t get why they have a single line for Magnetic North, however. Every point along that Grid North line should have its own line pointing to Magnetic North, so why single out the line that briefly passes through Langton Matravers? I’m guessing that there must be some other reference point that they are using but not mentioning. My guess is that it’s the shortest or longest great circle segment between North and South magnetic poles (since they aren’t perfectly antipodal to one another). Does that seem right? Any other ideas?
I would imagine, though I can’t prove it, that other countries with national survey services that lie along that particular magnetic north line also have their own points where grid, true, and magnetic north currently align; they just don’t have an Ordinance Survey office to publicize the fact.
And while I’m on the subject, do compasses point toward geomagnetic north or magnetic north, and which might the OS be using?
I don’t really understand what you’re saying - the article is saying that for the next few years there will be a single point on the 2 degree west line, which is also the 400km east line of the GB* National Grid, where compasses point exactly to True North and Grid North. Move north or south and compasses will point very slightly east or west. Or to move to a nearby place where compasses also point to True North you will have to leave the 2 degree west line. At present that point on the 2 degree line west line is somewhere in the Isle of Purbeck in Dorset, but it will continue to move northwards over the next few years.
*The whole of Ireland, including the bit of the UK in the north, has its own grid.
They might do, but the point is that when navigating in Great Britain, for years we’ve had to remember that Magnetic North was, for example, 11 degrees west of True North. It’s exciting that at the moment that’s not really necessary.
I had a whole response typed, but I need to think about this more. Why oh why isn’t there a Great Circle mapping app on the Internet somewhere?
ISTM now that the orange line sweeping across GB in the video in the link represents the line of 0º declination - at any point along that line, magnetic north and true north are in agreement. That line represents the great circle connecting magnetic north and the North Pole, and sweeps across GB as the magnetic pole drifts.
What I still don’t get is how this line can cross the True North/Grid North agreement line at just a single point. As the 0º declination line sweeps across GB, doesn’t it have to coincide with the TN/GN line along its entire length? I think this might be an issue of map projection that I’m not understanding.
No, it’s nothing to do with map projection. The point is that the line of places in Great Britain where compasses point to True North at that moment in time, is not itself aligned with True North - why would it be? As you can see in that video it’s aligned a few degrees west of True North.
I now understand what you’re saying, and I now realise I don’t right now know the answer. My working wild ass guess is that the orange line is actually curved, not straight.
Isn’t the grid north line dependent upon the specific gridded projection you are referring to? In other words, how can there be a single “grid north” line for all of England or Great Britain, because you could create an infinite number of maps including that geographic area with different grid north lines?
There isn’t - Ordnance Survey maps give the difference between grid north and true north at each corner of the map. We can actually forget about Grid North for the purposes of this question.
The OP is asking: given that the Magnetic North Pole is on the far side of the actual North Pole from the point of view of the UK, if a compass somewhere in the UK points exactly true north, why does it no longer point exactly true north if one moves a few miles exactly true north?
Well, the OS link sure seems to suggest that there is One Actual Grid North line, that runs 2 degrees west of Greenwich. I would suspect that that is for some particular Official Map, since yes, any particular map down to street level can have its own unique Grid North. It’s kind of important vis-a-vis the ‘conjunction’ that True and Grid north are only ever the same along that line 2º west of Greenwich.
“The OP is asking: given that the Magnetic North Pole is on the far side of the actual North Pole from the point of view of the UK, if a compass somewhere in the UK points exactly true north, why does it no longer point exactly true north if one moves a few miles exactly true north?”
Yes, that. It seems like compass north and true north should align everywhere along that True North line, or not at all. Unless that orange line isn’t really passing through the North Pole after all.
" My working wild ass guess is that the orange line is actually curved, not straight."
On further investigation, that sounds like a very good guess. I’m used to the declination in North America, which seems pretty regular. But that isn’t the case everywhere, given the relatively low latitude of the South Pole. I’m now more confused than ever as to what that orange line is.
It’ll be something to do with the magnetic field of the Earth - it’s beyond my understanding, but I don’t think the magnetic field has many, or any, straight lines.
Yes - this is a decision made by the UK government at some point in the 20th century. All other countries can choose their own grid norths, and as I mentioned the UK has another one in Northern Ireland.
The mapping agency pretends that the area in question is actually flat, and accepts that there will be errors in the region of (I think) the centimetre range.
Magnetic field lines are not perfectly straight. Or even perfectly great circles. The dynamo that generates them has all sorts of little complexities, and the lines will be further distorted by the distributions of various minerals in the Earth.
So the article is explicitly talking about the current Ordnance Survey grid, which is defined with its grid North running along 2 degrees of longitude. The only places that the triple alignment can occur are on this line.
The Earth’s magnetic field is a mess. Even if it behaved nicely it would still be a mess. The horizontal component of the field along the surface of the Earth is never straight.
All that seems to be being said is that in amongst all the wobbly mess of the field, that there is a region where the horizontal component points due north that is currently moving over the UK, and the location where that region intersects the 2 degree common grid line will start at the southern end of that line on the OS grid and slowly move north and exit at the top.
There isn’t a field line or horizontal component matching a great circle linking this region with the magnetic pole. Well maybe by sheer coincidence, but in general no.
The ubiquity of GPS and use of lat long on the WGS84 geoid avoids a lot of faffing about and the notion of grid north. But you can’t have a triple alignment so it is a bit dull.
However if you want to measure a rectilinear grid on the ground, locally defined projections remain important. But there is always an accumulating scale error which can become significant at the edges. Enough that property boundaries could be totally out of whack. So you still end up with local grids.
OK, so I can now see how if the field lines are all bent by geological factors then the line(s) of zero declination would also be curved, and would not even necessarily pass through the North Pole at all.
The line of zero declination is more like a connect-the-dots of the points on the surface where the tangent to each field line happens to point toward the North Pole, even if it will point east or west of the NP as you move along the field line toward the MP. I’ve worded that badly, but it seems to make sense of that orange line in the video. It’s the curving field lines that do it.
Still, it would be fun to see a competing claim from anywhere else in the world with a bit of zero declination line and a local Grid North.
But why do you see this as a competition at all? You sound like you feel the Ordnance Survey has made some sort of unreasonably jingoistic brag.
That’s not what the article is doing though. It is an interesting time for people that go walking in the hills in Britain or do other outdoor activities with maps. For years we’ve had to adjust compasses to allow for magnetic north being quite a bit west of grid north when orientating the map. We don’t need to worry about that so much at the moment, and in the future we will have to adjust for MN being to the E of grid north.
It just adds a little extra interest to note that the change is currently happening where grid north and true north are the same. As one of the experts says in the article, “This is a once in a lifetime occurrence.”
I don’t think of the OS announcement as jingoistic - a bit of local boosterism at worst, and not described very thoroughly, but certainly not offensive. It is true that this is the first such alignment for Great Britain since the establishment of the current OS map standard. It’s once in a lifetime for residents of Great Britain, but it probably already happened for the folks in Ireland if there’s a OS Grid North line in Northern Ireland.
It is also possible that it is the only such current alignment anywhere on Earth, but I wouldn’t bet on it. Given that the line(s) of zero magnetic declination pass(es) over quite a lot of landmass, I’d predict that it crosses the official Grid North of at least one other country, somewhere. Maybe not, though. Call it a ‘parallel claim’ rather than a ‘competing claim’ if you like.
Here’s a bit of complication, though: this report from 2017 indicates that the agonic line (zero magnetic declination) has been sweeping into Great Britain from the East, not the West. So maybe the orange line is the line of zero magnetic grid angle instead, which describes the angle between magnetic north and grid north rather than magnetic north and true north… This link from 2014 has this comment:
"Declination is the angle between true north and magnetic north. Grid magnetic angle is the angle between grid north and magnetic north. In the UK we are not expecting to see easterly declination till about 2018. However, because of the relatively large convergence angles in the UK (the angle between true north and grid north), the arrival of easterly grid magnetic angle precedes that of easterly declination. It starts in 2014 in the SW and will take about 20 years for the whole country to be affected. Easterly grid magnetic angle is likely to be around in the UK for more than a century, but it’s hard to say exactly how long. "
So maybe the line to be looking for is the ‘zero magnetic grid angle (ZMGA)’ for each country instead, and checking to see if that crosses the Grid North line. It would seem that the agonic line and ZMGA have to cross the Grid North line at the same point: no difference between magnetic north and True North along the agonic, and also no difference between magnetic north and Grid North along the ZMGA line, and thirdly no difference between Grid North and True North anywhere along that grid line, all in one place.
But all of the Ordnance Survey’s customers and all of the intended audience for the article are in Great Britain. Northern Ireland has a completely separate Ordnance Survey of Northern Ireland. The article doesn’t make any claim for exceptionalism. I’m puzzled that you seem to have such a bee in your bonnet about this.