UKC

...no longer get rid?

Am I right in thinking that current magnetic declination of anything to the west of Birmingham is currently near as damn it zero (in fact slightly positive, i.e. mag to grid, add)? I was looking forward to teaching the Scout troop the joys of setting a map, but looks like (at least for the next few years...) it's not relevant?

I haven't bothered adjusting for years now.

snap

If it's <5 degrees wither way, I'd wager you (you plural) aren't good enough at following a bearing anyway for it to matter.

At the time of reply, the magnetic declination at Wrexham (west of Birmingham) is -0.236 - I'd never be able to adjust a compass bezel to reflect this so, I would treat declination as 0.

You can calculate the declination at your location at https://geomag.bgs.ac.uk/data_service/models_compass/wmm_calc.html

Thats true,  but not a reason not to adjust for it if it were around 5.....which it isn't!

It was always a stupid mnemonic to teach, as it is not universal.

Much better is to think about where grid N is relative to magnetic N, and turn the compass capsule N is the appropriate direction, by the required amount, whether taking bearings from the ground, or setting the compass from the map.

It's easier to remember, once explained, and applies anywhere in the world. But it doesn't have a catchy, if jibberish, 'mnemonic'.

Magnetic declination is only one aspect of Grid Magnetic Angle (or variation); you also need grid convergence, and that depends on your local projection system.

Yes.

I stopped bothering about the whole thing years ago when it dropped below 2 degrees. Doubt I'll need to worry about it again for a good few years as I only ever really teach map reading in the UK.

why?

in the uk anyway, at it's worse its only 4 degrees, that's if you are navigating on Barra beach, where it would be about 30m over 500m, I doubt if you are holding a bearing for that length, and if you are you are aiming for a point off a map that is about 1mm on a 1:25k map.

if you are doing that I take my hat off, that's gnarly navigating

Good luck teaching that to a bunch of scouts, Dude!

Back in the good old days, we used the declination model on the map to make an (appropriate) adjustment to the compass bearing - I never worried about grid convergence, altitude, or anything else!

On long bearings (legs), I'd aim off.

In bad weather I'd leapfrog.

In good weather, I'd simply orient the map and look for features, leaving the compass in the bag.

These simple practises usually got me to where I needed to go. 

https://geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html is probably more useful. 

I agree with the suggestion that it's a stupid mnemonic, since it isn't even universal. Much better to explain - and understand - the underlying principle, and then simply apply it.

Obviously none of this matters in the UK right now. But down in Tasmania, say, the current declination is something like 14 degrees east. (And some of those scouts you're teaching might venture down there one day ...)

It's all bundled in to the little annotation on the map; OS maps shows GMA (declination and grid convergence), not declination.

I doubt if you ever used magnetic declination.

Or New Zealand (19-28 degrees W)

For explanation of what correction you actually need. Magnetic declination does not include grid convergence. My point was addressing a link to a site showing just magnetic declination.

If you're going to correct, you might as well do it correctly. OS maps (and others) show GMA correction, not magnetic declination.

East, I think.

d'oh! brain fart.

https://www.ncei.noaa.gov/maps/historical_declination/

You don't need a declination adjustment to teach the joy of setting the map. And any nav training is better in the real world than the village hall. Get them out on interesting terrain. Teach setting the map, and locating based on last known position, general direction and observables.

If they are Scout scouts rather than Cub Scout or Explorer Scouts, then the hill walking badge looks fun.

Written for a more northern audience but I'm sure you'll get the idea.

L.A.R.S.
Left Add, Right subtract. This refers to the North Arrow Diagram shown in most maps. If you are moving left on that diagram, then add. If you are moving right on that diagram, then subract. In the NW Highlands during 2024, the North Arrow Diagram is in the order GMT (Grid- Mag-True) from left to right. Applying LARS, Grid (map) to Magnetic (compass) is Right, so Subtract.

mAp & compaSs
This makes use of the widespread convention of East being positive and West being negative, and then taking the second last letter as your clue, the following works.

compass to mAp = Add

map to compaSs = Subtract

TERMINOLOGY
In practice, we deal with the differences between grid and magnetic North on maps with a grid system such as OS Grid, UTM, Irish grid or Swiss Grid. We might refer to that as magnetic deviation, magnetic variation or some similar term. A truer term for this is Grid Magnetic Angle which is defined as the difference between Magnetic North and Grid North (East Angle relative to Grid is positive).

Similar terms are often used with maps without such a grid that use geodetic co-ordinates (lat/long) where the correct term is Magnetic Declination which is defined as the difference between Magnetic North and True North (East Declination relative to True is positive).

Grid Convergence is the difference between Grid North and True North for OS Grid, UTM and so on (East Convergence relative to True is positive).

Grid Convergence + Grid Magnetic Angle = Magnetic Declination

DATA SOURCES
Grid Magnetic Angle and Magnetic Declination on the British Geological Survey website.

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

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

If you remember the relative positions of grid and magnetic N, you don't need to do any arithmetic; just rotate compass capsule N between the two Norths.

The actual numeric bearing (be it mag or grid) is only required for communication to someone else, not for navigation; you only need to set the compass, and follow it, or transfer a bearing back to a map, using the edge of the compass.

A lot of the obsession with reading numeric bearings comes from military navigation, or the need to do rather academic map reading exercises in a classroom, and create route cards with bearings on them.

There are rare occasions where it might be useful to record such route details in advance (e.g. descent fron Ben Nevis). But that is a form of communication.