Magnetic Variation & Navigation?

[Matthew.Passmore]

Not quite sure where to put this so Mod's feel free to move.....

The Current Magnetic variation for the UK (Sheffield at least) is 3 Degrees 11min(3 11/60 degrees) West of grid North. (as of July 2009 British Geological Survey)

So it's 3 Degrees as far as we need navigation wise.

so my question is as compass bearings are marked in increments of 2 on the compass Bezel would you round up to 4 degrees or round down to 2 degrees or position it in the middle of 2 increments?

I would round down and add 2 degrees.


hope you understand what I mean

 

[fred gordon]

I would put it in the middle. However, remember that with the Silva type, or whatever, of compass you would be doing very well to have a navigation accuracy greater than 2 deg. anyway.

 

[Matthew.Passmore]

cheers, it's something I've always been abit unsure about.

I think I will put it in the middle from now on.

 

[Tor helge]

I`ve always wondered why people care about such small differences in magnetic variations.
Are you all walking on large flat surfaces without landmarks or big flat forrests?
With 3 degrees "wrong" direction you will be only 180 meters off the target after 3 kilometres.
I`m just curious since no one here seems to give a dam* about it. The declination here is about 3 degrees.

That said I don`t even use a compass. I just move from landmark to landmark. Therefore the map is used often.

Tor

 

[Peterlee_Paddler]

On the whole I would agree with Tor, BUT, there are times (like in poor visibility, eg.) when a compass is a vital aid to map reading.
To answer the original question, on some outings I have been in areas of over 4/under 5 degrees declination and have tried to adjust for 4.5 !
Futile, I know, but I'm a pedant

 

[forestwalker]

I`ve always wondered why people care about such small differences in magnetic variations.

For Scandinavia it certainly makes no difference for most of us, but I first learnt to pay attention when directing artillery fire (say 20+ km range means you do end up quite a bit off if you ignore a whole degree). And in other parts of the world it can be big enough to make a difference in real life.

 

[Scots_Charles_River]

At the bottom of the OS map it tells you for that map.

Good clear example here
http://www.ordnancesurvey.co.uk/oswebsite/aboutus/reports/misc/north.html

Nick

 

[Kerne]

Not quite sure where to put this so Mod's feel free to move.....

The Current Magnetic variation for the UK (Sheffield at least) is 3 Degrees 11min(3 11/60 degrees) West of grid North. (as of July 2009 British Geological Survey)

So it's 3 Degrees as far as we need navigation wise.

so my question is as compass bearings are marked in increments of 2 on the compass Bezel would you round up to 4 degrees or round down to 2 degrees or position it in the middle of 2 increments?

I would round down and add 2 degrees.


hope you understand what I mean

1. I would go halfway - just a habit but it's worked so far.

2. remember - what you need to know most of the time is the magnetic variation from GRID north, not from TRUE north. In fact, you can pretty much ignore true north if you're walking.

 

[welshwhit]

Magnetic variation can be critical, if you have an older map and you are forced to use bearings from it.

I can think of times in real bad weather and visability that even 10-20metres off target could have been interesting!

But on the whole I'd say the ground features should be giving you more info in this country most of the time.

But when you do have to use it, you want to strive to be as accurate as you can!

Drew

 

[Sainty]

A (good) few years ago, a pal and me were walking on Dartmoor when the weather came down really badly. We needed to get off the moor but couldn't see more than 50 meters. We knew precisely where we were and looked for a river some 1000 meters away as our escape route. By 'piggy backing' on a precise bearing, we hit the river head spot on. If we had not taken an accurate bearing, including for magnetic variation, we may have missed the head of the river and walked into real danger.

Agreed that for good conditions and picking out landmarks, magnetic variation is somewhat pedantic but it's important to understand the principle and when and how to make allowance for it when it counts.

Martin

 

[Kerne]

A (good) few years ago, a pal and me were walking on Dartmoor when the weather came down really badly. We needed to get off the moor but couldn't see more than 50 meters. We knew precisely where we were and looked for a river some 1000 meters away as our escape route. By 'piggy backing' on a precise bearing, we hit the river head spot on. If we had not taken an accurate bearing, including for magnetic variation, we may have missed the head of the river and walked into real danger.

Agreed that for good conditions and picking out landmarks, magnetic variation is somewhat pedantic but it's important to understand the principle and when and how to make allowance for it when it counts.

Martin

Another place where accuracy can count is in the forest - where we spend a lot of our time! I often try to find odd little bits and bobs of features on the map by walking on a bearing through the forest. This is great for simulating bad visibility since you can't see the destination for the trees but it is also good because you have to, effectively, practise piggy backing as it is impossible to walk in a straight line and you need to find a tree on the route, walk to it any which way, find another tree etc. You have to concentrate as well - one tree doesn't half look like another...

 

[Mirius]

When working with old maps you need to be aware that the marked variation per year can be wrong. Take an old map, use the notes to update the magnetic variation and compare to a newer map - often you'll find it's not the same, so generally don't use an old map if accuracy of navigation by bearings could be critical.

It's generally also accepted that when trying to hit a feature such as a river, its best to deliberately aim slightly off (left or right) then when you hit the feature to follow it along until you hit where you wanted. If you follow an exact bearing and then don't hit exactly where you expected (which is very normal considering that limited visibility, slopes and other hazards can hinder your accuracy), you don't know which way to walk to hit the right point - it could be left or right.

 

[bearbait]

Compass bearing accuracy:
1) Assumes you can walk or hike on a compass bearing accurate to a half or one degree. You need a proper sighting compass for this - and a mark on the ground some distance off to help keep you on track;
2) You have no metal objects about your person that will affect the compass;
3) There are no local magnetic anomalies.

If you are striving for this level of accuracy you also need accurate pace counting/distance measuring. Your pace length will vary according to terrain (up, down, steep, rocky, boggy, snow, chest deep bracken, wooded, tree roots, etc.), ambient light, your fitness and your tiredness, the load you're carrying, and who you're hiking with. Again if you need this level of accuracy you need to be aware of, or use, search patterns such as the expanding box search to find your target when you know or realise you've missed it.

To add a little less accuracy to the debate: I have sailed thousands of miles at sea in small boats. In a gentle seaway it is difficult to keep the boat pointed to an accuracy of even five degrees; in a large seaway ten to twenty degrees would not be unusual yet I still managed to make successful landfalls, including taking into account compass deviation (varies according to heading), leeway (approximated), tides and ocean currents (also approximated), and this before the advent of GPS. Even a position line from a sextant on a small boat can be several miles out.

Navigation is an art, not a science: the art of navigation is "Knowing you are not where you think you are". To this end you use all available tools: map/chart, compass, sextant/theodolite, GPS, and especially the Mark I Eyeball. Knowing what to expect (stream, ridge, fence) and when (five minutes, one hour) and ticking off the features as they appear is the way, as welshwhit and Tor helge suggest above. After all, you can see where you are. (I'm here!) A map shows much of what is around you - both visible and invisible, assuming you know where you are on the map.

It is also worth bearing in mind that a decent modern GPS, particularly with WAAS/EGNOS, will give you a position that is more accurate than than the map you are using, that is the terrain was surveyed at a lower level of accuracy some (many) years ago. OSGB produces some very reliable maps but even they 'fess up that "individual features on the map may only have been surveyed to a local accuracy of 7m (for 1:25000 scale maps)".

Consider also the scale of the map: 1mm on a 1:25k map is 25m; on 1:50k map it's 50m!

I've just returned from a trip which included hiking in temperate rainforest where the trail changed direction 10 times every 30 metres, and the trail would swing through up to 180 degrees as I traversed slopes via a switchback. It's not even worth trying to keep a DR plot going. I just kept note of time on the trail (well marked or blazed trails). Where there was doubt I left markers to indicate the route back if I needed to retrace my steps. I knew where I was - on the trail, about x hours in - and I had a vague idea where I was in relation to the surrounding topography. It was good enough.

It's important to be pragmatic with navigational accuracy.

IMHO.

 

[John Fenna]

I find "aiming off" and "handrailing" and use of "attack points" very useful techniques that avoid you having to be overly concerned with 0.5 degree accuracy for most of the time - they allow you the luxury of apreciating your suroundings/wildlife/views etc while potentially allowing for faster movement through rough terrain as well.
Ask any Orienteer!

 

[timboggle]

I find "aiming off" and "handrailing" and use of "attack points" very useful techniques that avoid you having to be overly concerned with 0.5 degree accuracy for most of the time - they allow you the luxury of apreciating your suroundings/wildlife/views etc while potentially allowing for faster movement through rough terrain as well.
Ask any Orienteer!

Absolutely spot on mate, couldn't have said it better, this came up on a similiar thread here recently. I will say that if you try to hit a small feature dead on with a bearing, you'll most likely fall victim of 'Parrallel error', as John says, aim off and target approach using handrails, you are then in control of the 'parallel error'

 

[Matthew.Passmore]

I find "aiming off" and "handrailing" and use of "attack points" very useful techniques that avoid you having to be overly concerned with 0.5 degree accuracy for most of the time - they allow you the luxury of apreciating your suroundings/wildlife/views etc while potentially allowing for faster movement through rough terrain as well.
Ask any Orienteer!

They are techniques I use often, it's very rare that I use bearings or even a compass at all.

as a few people have mentioned the variation or even compasses are not at all important, and I agree that this is the case in bushcraft and hunting most of the time.

I was asking the question from a more Mountaineering point of view, I have had to navigate down off mountains with large Vertical cliffs in white-outs before where a few degrees can be critical and can mean the difference of falling to your death and not

of course you are never going to be 100% accurate, and even if somehow you were things like gradients and weather(wind especially) will push you off the bearing, but being as accurate as possible when setting the bearing at the start at least minimises the mechanical error.

If you think I won't bother adding that extra degree or 2 because you think it doesn't matter, when coupled with human error i.e walking slightly off the bearing because of a slope or obstacles etc. means that when you have paced your calculated distance or reached what ever marker your using i.e. a river you could be 10, 20m or more out than you could have been just for the sake of of adding 2 degrees.

Thank for all your replies, it's interesting to see the different views.