UKC

I'm considering some walking in the Mourne mountains, I have a reasonably dated map (2006), but wandering either what the current magnetic variation is, or where I can find out?

https://www.magnetic-declination.com/

First hit on this thing I have installed called Google.

Google magnetic declination maps. Most of the UK is quite close to 0 degrees at the moment. Looks as though the Mourne mountains are at a declination of about 4 degrees west. Hardly enough to bother with.

And how do I calculate from this declination to a bearing correction?

Do I just add the declination to the difference between grid and magnetic North?

Number I get for declination at Newcastle is 1°46' , and difference between grid and magnetic about 1°36', giving 3°22' if I add both together. Is this the correct number to use for correcting a bearing - 3° (ie add to a bearing taken off the map to use on the ground)?

GMA (Grid Magnetic Angle) is the combination of magnetic declination and grid to True North angle (caused by the pincushion effect of the spheroid to flat map projection).

You need to consider the E/W value of these two values, and add/subtract as necessary. That will give you a GMA that is E or W (i.e. is Grid N E or W of Mag N0.

In use for navigation, think of where Grid N is relative to Mag N. Don't bother reading bearings; just turn the compass capsule between the two North (Grid & Mag), depending on whether your are taking a sighting from the ground. or reading from the map.

But is that website showing the declination between Magnetic North and True North or between Grid North and Magnetic North. Neither are huge numbers about 1 and about 3 (ish). I think from the original post he needs Magnetic/Grid declination

GerM: When taking a bearing off the map, add 3 degrees to your compass. When sighting from the compass subtract 3 from that if you want to plot that onto a map. Unless you have a prismatic compass and very practised skills or the need to land a mortar round on a location (in which case we'd be talking 55 Mils not degrees), you can probably just ignore 3 or 4 degrees and all this correction stuff   

If I am in indistinct terrain in poor weather, I'll often add/subtract a relatively big chunk, like 20 degrees or something to a bearing anyway ("bearing off")  to purposely arrive too far to one side or other on a feature. For example if I want to arrive at a stile on a wall, I might "bear off" too far left and when I arrive at the wall I know I need to turn right and walk along that way it to find a stile (or whatever) - otherwise if you take the "correct" bearing and arrive at the wall and can't see the stile in the mist - how do you know which way to turn to find it?

This site gives a pretty clear calc between grid and magnetic north for a given location.  

http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html

As a few folk have said, it's so low ATM it's largely not worth getting to stressed about for 99% of nav.

Cheers!

Stu 

Note the caveat:

"Please note that it only outputs the correct grid magnetic angle for locations in Great Britain. Ireland uses the Irish National Grid."

https://en.wikipedia.org/wiki/Irish_grid_reference_system

The grid datum is different to OSGB, so the grid:true convergence is different.

Depends where you are though. This used to be the case across the whole of UK and Ireland, but not any more. The line is moving East, the whole of Ireland now would be taking off degrees when going from map to ground.

(Whether its an amount that currently needs bothering with is debatable)

https://www.ordnancesurvey.co.uk/blog/magnetic-north-continues-its-march-to...

Thanks Jez, I stand corrected!  Depending on which maps, it's either 0 or 1 degrees. So you're right it's definitely not worth trying to add or subtract that!

Area around Slieve Donard 

Based on OS area NW
"...Magnetic north is estimated to be 1 deg 23 min EAST of grid north (British National Grid) at this location in July 2022..." decreasing about 13 seconds a year so pretty close to 1 degree now

Based on Ordnance Survey Ireland area J
in 2020 the declination between OSI grid north and magnetic North was 0 degrees 49 seconds decreasing roughly 13 seconds a year, which means give or take a whisker the declination between grid and magnetic is 0 degrees now

So Ger.. don't forget to add or subtract a Zero

As much as the Mag to Grid vice versa question has been answered, I have been walking the Mournes now for every weekend for the last 5 weeks. If you need any help or route ideas, give me a shout. It's a brilliant place to walk and escape the world. Although I am yet to have a summit to myself. The locals seem lazy and the crowds arrive around 10:30/11:00 ish but there is always those dedicated ones up and early. 

A bit of local knowledge always a bonus, beats messing around with maps and compasses and such nonsense any day of the week!

Thanks. Sounds like your last paragraph there is the information that I was looking for. Is this just lifted from a recent map? Or is this information also available online if I look in the right place?

I just seem to remember from last time I was over that the grid was aligned differently in Ireland, and at the time meant a significantly greater correction, so wondered whether this was still the case.

Kind of glad I asked as it seems the range of answers so far are:

Google it you f'in idiot

add 3°

add 4°

as near to 0° as dammit is to swearing

I'd be mostly inclined to do the correction if it was 4°, maybe not so much if it were 2° or less, but that's just me.

Interested in how to figure out how to get to this from the declination now, but that will require taking a moment to think it through to get my head round it I think.

Always got the fall back position of finding the massive wall along the tops of the mountains till it feels like a good idea to head downhill.

What are these massive walls along the tops of mountains you speak of? Do you mean summit ridges that have cliffs on one side, or the occasional hill that has a dry stone wall along the crest?

The data for the location in relation to ordnance survey GB grid was calculated by a util from following a link on the GB Ordnance survey website and putting in their 8 fig grid ref for the summit trig point 

The one for OS of Ireland was reading a pic from the legend of that area map I found online, but *** if I can find it again and it was on work computer so can't check my history at this point.

My first "it's 3 degrees" answer was woefully out of date sorry (but thank Jez from prompting me to dig deeper). The "Google it" answer given above wasn't so helpful especially as the values on that site are based on true north versus magnetic North, not grid North and magnetic North you'd need. Although as it turns out, all 3 are for practical purposes the same, currently in that location.

This is the one I meant:

https://en.m.wikipedia.org/wiki/Mourne_Wall

I have recollections of basing routes to get to the wall, and just follow from one mountain to the next just for a bit of shelter on some ridiculously windy days.

Do you need a compass? Just follow the wall  🙂

A route I must do soon.

Can Google help with that? Possibly the OP might feel the need after reading some of the responses