## / Maps and variation (Nevis)

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Looking at an old map of Nevis taking into account the est annual change between magnetic north and grid north.

worked out to be approx 1 degree west from magnetic..

can anyone give me a rough idea on the difference on a current map from grid and magnetic (my map 2002 😬)

If it's an OS map it tells you at the top.

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

You don't need to work it out manually - the BGS website has a GMA calculator (difference between grid and mag) you can use.

its about 0"12' west  , which is fairly negligible over short distances

Do you mean 0°12'00" (as in 0 degrees, 12 minutes, and for the sake of argument 0 seconds)?

And by short distances, you mean any distance you're ever likely to walk a bearing over?

Pedantry aside, it raises a number of interesting questions:

How long before it's ever worth making the correction again on the UK mainland? (I.e. gets anywhere close to being worth adjusting on a compass, maybe half a degree if you're very dextrous, and with good eyesight)

How many of us will ever use it again on home turf given advances in technology and/or time it takes to be significant again?

Will all those old mnemonics go out the window in future? (I.e. will the direction, and hence the correction reverse?)

How will it be different in different places, e.g. Ireland, Scottish Islands, and maybe the far north of Scotland?

I'm off to do a little googling...

For most of the UK the difference is within 1 degree, and the max (except in somewhere like Norwich) is about 2 degrees.

So if your navigation was 1 degree out, you'd be about 1.75m out after going 100m. I think you'd be doing pretty well to navigate on foot to that accuracy on rough ground in non-perfect conditions (actually you'd be doing ok on smooth ground in perfect conditions unless you rigorously used a sighting compass onto a discernible feature).

To all intents and purposes, I think the occasions where you might currently need to adjust for magnetic-grid variation are going to be pretty rare, if ever.

Obviously, if you were flying a plane, you might want to still include it (1.75 miles out after 100 might be more important ).

Aye, interesting.

Most of the navigation I do is at sea traveling longer distances , where the variation is magnetic to true , then add or take deviation , so with the variation from mag to grid over shorter distances isn't that great  ..   the given web site gave it at 12 minuets west . .

@GerM  yes 0"12'  .. equates to zero degrees twelve minuets  ..  I purposely omitted seconds for the sake of simplicity ...

Can't seem to get my head properly round this one at this time of the evening, it seems an interesting time for magnetic variation. Looks like it will be fairly small, but not completely insignificant for a few years, but from what I can see what exactly will happen in future is not fully quantifiable, as predictions are relatively short term, depending on current trends. So I'll just enjoy the minimal variation in Wales at the moment, read a bit more about it in the meantime, and keep a careful eye on the variations on those maps when further afield.

Makes sense, it's just I thought the symbol for degrees was ° and " was the symbol for seconds. Or maybe my eyes are getting tired too and I can't tell the difference.

My keyboard doesn't have a symbol for degrees on it , and I cant be asked to look up the short cut ... so it was much easier to write as I did above for that sake of time saving and simplicity , but I should have known that someone who couldn't help themselves would make such a comment regardless ..

Out of interest do you correct grammar too ?

Sorry, was aiming for clarity, but instead I've managed to come across as tetchy and nit picky. Thought it was worth pointing out because of seconds and the " symbol being another smaller unit of angle measurement. Hard to keep it light on here isn't it? Even now I'm wondering if this post might sound argumentative, thought it  might be better to attempt some sort of answer rather than leave it hanging though.

In terms of the original topic, I have had a little look at a few maps, and it seems that the information given on declination (is that the right term?), is only a good approximation for a relatively short period of time (for about 5 years or so?), and so is well worth checking the information given on older maps (which is kind of the whole point of the thread I guess). By this I mean that not only the current variation given dates since the variation changes, but also the predicted future changes also become outdated too. I was always aware of magnetic variation changing with time, but not really fully considered the non linear and slightly unpredictable nature of these changes and the implications.

You should have converted to radians to eliminate any confusion 😃😃

Always important to get your units right:

Alt 249 😁

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The link above to the BGS shows that for some areas of the UK the difference between grid and magnetic north is so small that, at present, it can be ignored.

How many folks have compasses that are accurate to less than one degree and even if they are how many people can walk on a bearing to that level of accuracy?

Dave

I have been ignoring the magnetic variation since it got down to about 3 degrees and it's not got me lost so far. It only makes a significant difference on legs that are too long to keep to accurately anyway.

> I have been ignoring the magnetic variation since it got down to about 3 degrees and it's not got me lost so far. It only makes a significant difference on legs that are too long to keep to accurately anyway.

Yes, agree.

Dave

I agree with that. I stopped using it years ago and have never had an issue due to it. I think that in terms of practical navigation it's a complete red herring.

That is not an unreasonable strategy at the moment in the UK, given the current situation, especially in the more popular hilly and remote areas. Personally I think it is still well worth knowing the effect the magnetic variation will have on bearings, even if it is small enough not to be worth correcting for in practical situations, as it could subtly change navigational strategies in certain situations. It would be unwise to discount accounting for it entirely, as it could easily cause issues on trips a little further afield, to Na Cruacha Dubha in county Kerry for example (not sure if I've got this right, but I make it around 5° west there at the moment?).

The other reason to still consider magnetic variation, even if it is small enough to discount at the moment, is because it will change in the future. The current coincidence of such a small magnetic variation is a special circumstance, though I can't fully work out if this is just luck, or some very canny bit of foresight from whoever originally set the orientation of the grid used by the Ordinance Survey. This is why I was interested in how it might change in future, but from what I have managed to find out I understand that future predictions are based only on current trends, and no one really knows exactly what will happen. I think some things I read were suggesting that the rate of change is reducing. I would be interested if anyone could point me in the direction of  information on the current rate of change of the magnetic variation, the only place I have seen any information on this is on the paper maps that I own, which gets out of date quite quick.

If anyone knows anymore about how these things are changing I would be very happy to be corrected on what I have said above, and very interested to learn more about how the magnetic variation is likely to change in future.

Post edited at 19:51

> I think some things I read were suggesting that the rate of change is reducing. I would be interested if anyone could point me in the direction of  information on the current rate of change of the magnetic variation, the only place I have seen any information on this is on the paper maps that I own, which gets out of date quite quick.

> If anyone knows anymore about how these things are changing I would be very happy to be corrected on what I have said above, and very interested to learn more about how the magnetic variation is likely to change in future.

In the age of the Internet this information can't be hard to find online.  You could start here:

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

Post edited at 20:32

Maybe I'm not looking in the right places, I've found some interesting related information, just nothing on current trends in changes of magnetic variation in the UK, this being a combination of the movement of the earth's magnetic field, as well as it's complex geometry in relation to the UK Ordnance Survey Grid.

Latest interesting stuff I've come across:

Historical changes in the earth's magnetism:

https://maps.ngdc.noaa.gov/viewers/historical_declination/

Some general information on changes in the earth's magnetic field: