UKC

Not sure, but I think this might be an online OS map nav question that hasn't been asked on here before..

On a paper OS map there's an arrow across the margins to show you where True North is relative to Grid North on that particular sheet, and the same info also appears in the legend.

Looking at the OS map online though (and using the OS website/app to print my own), I can't find that information anywhere.

Googling is no help, because I just find lots of explanations that Grid North, True North and Magnetic North are different (yeah, thanks, I know) - but nothing that tells me what exactly the difference is in any specific place*.  Either that or various places I can look up the current magnetic declination in a specific place, which is great, but it's little help relating True North and Magnetic North to each other if I'm looking at an OS map without knowing how either of them relates to the grid.

This is more of an academic question really in my case, as it's only a couple of degrees or so which generally just gets lost in the 'noise' of my sloppy navigation - but still, I'd like to know.

Can anyone point me to a website or something that'll tell me what direction Grid North on the OS map is pointing in a specific spot?  Or failing that, perhaps a formula that relates it to longitude or something?  Ta.

* - I'm sure we all know this, but to be clear..  because of the projection of the grid, the difference between Grid and True North varies across the country, and from sheet to sheet of the paper OS maps.  Grid North is West of True North in the West of the country, and East of True North in the East.

Edit to add: pic.

This will probably help:

https://www.ordnancesurvey.co.uk/documents/resources/guide-coordinate-syste...

True North is a bit academic. That's why it doesn't really feature in mapping tools.

Ah yes, I'd forgotten about that document ta. It doesn't seem obvious whether that helps or not, and I notice that you're not committing yourself one way or the other either. 

Off out now - I'll take some time later on to sift through it and try to figure out whether or not my answer is there in a form that I, with my flaccid little brain, can successfully extract and apply.

Maybe so but Magnetic North is not at all academic, if we assume for a moment that I can take and follow a bearing accurately to within a degree or two and that I'm using my home-printed OS map to navigate with a compass.

It's dead easy to look up the current magnetic declination for any particular place online, so I know how Magnetic North relates to True North.  But without the information that's already printed on an OS map I might have bought from a shop I can't relate either to the grid on the map that I have in my hand.

And neither is Grid North. 

Grid and Magnetic Norths are the ones you need for navigating with map and compass. True North is irrelevant.

What you seem to be after is the local Grid Magnetic Angle. BGS have a calculator:

https://geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html

I can't think of a situation where you'd need to know true north so I'm interested to know how you're using it?

As far as I'm aware Harveys maps don't have information about true north on any of their sheets either.

Bit patronising.  What I'm after, as I've said very clearly, is the direction of True North as indicated on a printed OS map for use with a map that I've printed myself off the OS website.

The link is handy though, thanks.

The reason I want it specifically right now is that I'm printing some maps and charts for a sea kayak tidal navigation exercise.  Marine charts and OS maps are often used together for those (in the UK) - the OS map tends to have more useful and detailed information about the coastline (and the land obvs), but you need the marine chart for buoyage, lights, tidal diamonds etc.

Forgetting about magnetic North for a mo - if you want to draw vectors and bearings on OS maps and marine charts and relate them to each other, it's obviously going to be very handy to know how the relationship between the two grids to within, say, about half a degree over a relatively small area.  The correction between True North and grid North, as indicated on an OS map, is good enough to make the bearings compatible with those drawn or measured on an Admiralty Chart using a Portland plotter or similar.  (Because the 'grid' on the marine chart consists of lines of longitude and latitude - so 'grid North' and 'true North' on the chart are effectively the same.)

I've never bought a printed one.  But a printed OS map has it both in the legend and drawn in the top and bottom margins right across the middle of the sheet.

Wasn't meant to be. I said 'seem to be', because I couldn't understand why you were interested in grid convergence, other than to calculate GMA.

You said you had found various places that give the magnetic declination (the NOAA has a good one, IIRC). This only gives you part of the GMA, since you also need the grid convergence, which is static, but not so easy to find.

Since I could think of no practical use of knowing the direction of True North relative to Grid North (the convergence), other than to combine with magnetic declination, to give GMA with which to correct between magnetic and grid north, I assumed you were trying to find GMA. Hence the link to the BGS GMA calculator.

If what you're after is the grid convergence, so you know where True North is, what use are you going to make of that? 

[edit: I see from your intermediate post that you are trying to relate two grid systems: OS and maritime. If you'd said that in your OP, it would have helped people understand what you were trying to do, and were generously trying to help...]

A recently purchased OS Explorer seems to lack the grid vs. magnetic symbol that's usually at the top, probably because at the moment that deviation is absolutely tiny - under 1 degree - most people can't navigate to that sort of precision anyway.

Googling 'OS grid convergence' finds this:

https://www.wilmslowastro.com/stuff/psion/OSGB.html

Maybe I'll dig out my old Psion.

Fortunately, the next hit is:

https://www.ordnancesurvey.co.uk/business-government/tools-support/os-net/c...

It includes a full set of Transverse Mercator projection functions, so you can easily:

Project latitude and longitude to grid eastings and northings for any Transverse Mercator map projection, including the Ordnance Survey National Grid – and vice versa.

Compute local scale factor and grid convergence at any point on the grid, and compute t-T and true azimuth between any pair of points on the grid.

I know it's not so easy to find - that's why I'm asking on here if anyone knows where I can find it!!

See above - I want to relate grid North on an OS map to grid North/true North (which are the same) on a marine chart of UK coastal waters.

An easy, but clunky, workaround I guess would be to convert two appropriate sets of lat/long coordinates (with the same longitude) to OS grid coordinates and use those to draw a 'True North' arrow on my OS map.  That'll do in the absence of a more elegant solution.

Edit to add:

Aha!  That looks like it'll do it, if I can figure out how to use it, and loads more things besides.  Ta.

Maybe so.  I haven't bought one for a while, but can see I'm going to have to visit a shop and have a sneaky look at a few.

Though I'd have thought there might be some info in the legend so you can deal with the magnetic variation in a few years when it becomes significant again. Is True North on there?  Is it a map for the middle of England somewhere, where grid and true north are also about the same? (See the pic I included in the OP - they line up exactly where the longitude is 2 degrees West.) 

There's quite a significant difference between East and West.  The legend of the Landranger I'm looking at here gives figures separately for each corner of the sheet.  (As well has having the arrow in the top margin.)

Yeah; all you needed was the correct search term.

Sadly the spreadsheet doesn't seem to quite work.  I don't have Excel so maybe that's because I'm trying to use it in Libreoffice, or something to do with the macros, dunno.

It seems it'll give me a convergence for a set of lat/long coordinates, but not for a grid reference.  Nor does it seem able to convert between lat/long and easting/northing.

I think I'm just going to go back to my clunky workaround - putting a dot at the bottom of my map, converting its grid reference to lat/long, adding a bit to the latitude and converting it back to get the grid reference of another dot due North of the first one.
(One minute added to the latitude is a nautical mile, so 5.4' or 0.09° is 10km.)

The coordinate conversion is really easy here:
http://movable-type.co.uk/scripts/latlong-os-gridref.html

So I just checked and it appears Explorer (and indeed Outdoor Leisure) maps have never had the magnetic deviation/grid deviation at the top but it's in the key instead.  That seems to be the case for all of the ones I have - I guess it's just Landrangers that have it at the top.  Shows how often I've looked at it!

Learn something new every day I guess.