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Compass questions...it's gone mad....

Compass questions...it's gone mad....


Postby MusicalHiker » Wed Jul 14, 2021 9:00 am

Hi,

I've had a little Silva compass since the 1990s. (Type 7NL) Up until now it's always been fine.... but recently I took it out to check something in the middle of a Corbett hike and it most definitely wasn't pointing North! Now, it's variable, but never pointing North - it's not really pointing South either - I did wonder if the poles had reversed, but it's changing slightly every time I pick it up, and so I will purchase a new one - any recommendations for the most accurate? And any ideas what's happened to it? Cheers!
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Re: Compass questions...it's gone mad....

Postby Caberfeidh » Wed Jul 14, 2021 9:25 am

Maybe it was stored beside something magnetic or electrical for too long? As ever, a choice depends on how much moola you are willing to part with... Silva are good, I'd just get a new one of what you were used to.
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Re: Compass questions...it's gone mad....

Postby MusicalHiker » Wed Jul 14, 2021 9:44 am

Caberfeidh wrote:Maybe it was stored beside something magnetic or electrical for too long? As ever, a choice depends on how much moola you are willing to part with... Silva are good, I'd just get a new one of what you were used to.


Ha that's funny - yes I think I will just replace it, after all, it has served me well since I was a teenager! It lives in my backpack which to my knowledge hasn't been next to any weird magnetic forces but I guess it's possible....
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Re: Compass questions...it's gone mad....

Postby Mal Grey » Wed Jul 14, 2021 10:08 am

Maybe its become demagnetised at some point, maybe a mobile phone.

I'd stick with Silva or Suunto. Almost any base plate compass from their ranges will be more than adequate.

The NL on your 7 meant "non-luminous" if I remember correctly. Whilst I've almost never used my compass in the dark, I do like the reassurance of luminous dots, which most do have, so you can keep the needle lined up in the dark more easily without losing night vision from turning on a torch.

The Silva Ranger would be a good mid-level choice, the slightly longer base plate than on the 7 can make it easier to take bearings.
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Re: Compass questions...it's gone mad....

Postby St Pedro » Wed Jul 14, 2021 3:31 pm

Just been on an Mountaineering Scotland navigation course in Ochils a couple of weeks ago

They recommend the Silva expedition 4, and one they ensured everyone was using on the day

it has a bigger base plate with route line for taking bearings and includes the 1:25, 1:40, 1:50 romer scales for OS and Harvey maps

and magnifying section for detail on maps

can be purchased for under £30, money well spent
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Re: Compass questions...it's gone mad....

Postby davekeiller » Wed Jul 14, 2021 7:36 pm

Sounds like the needle has become demagnetised. Storing it close to a mobile phone will do this, as could a knife, GPS, or anything else that's either made of steel or has an electrical current running through it.

Silva are probably the best brand on the market. Expedition 4 is generally the recommended one.
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Re: Compass questions...it's gone mad....

Postby Tringa » Wed Jul 14, 2021 10:41 pm

I think this has been mentioned before, but along with compass needles becoming demagnetised by a close magnetic source, the accuracy of bearings taken by any compass can be affected by, for example, a mobile phone.

There was a report a couple of years ago of a group who took the wrong route in bad weather because they happened to have a mobile in a pocket close the compass while they were taking a bearing.

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Re: Compass questions...it's gone mad....

Postby rockhopper » Wed Jul 14, 2021 10:59 pm

Another vote for a Silva Expedition 4 which I've used for years along with carrying a Silva Ranger as a backup.
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Re: Compass questions...it's gone mad....

Postby MusicalHiker » Thu Jul 15, 2021 7:58 am

Thanks everyone- I’m definitely going to get the recommended Silva, but meantime I got a magnet and fixed it- it’s now pointing North again! 8)

Will keep it away from the knife and phone…. It may have been with my knife at some point, yes.
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Re: Compass questions...it's gone mad....

Postby boriselbrus » Tue Jul 27, 2021 4:22 pm

But, but maps and compasses are infallible apparently whereas GPS units fail or run out of batteries about every ten minutes.

That is slightly tongue in cheek of course. I always have a map and compass as well as my GPS. But over the last 40 years I've occasionally had compasses lose their magnetism or even reverse polarity despite being kept well away from any known magnets or metal. I've had maps disintegrate when the waterproof map case fails, and even blow away. My GPS however has (touch wood) never failed.

Not really sure where I'm going with this except that it sometimes pays to have more than one method of navigation and cross check between them.
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Re: Compass questions...it's gone mad....

Postby Tringa » Tue Jul 27, 2021 7:57 pm

Slightly off the main angle of this thread but it is worth noting at present the usual adjustments from grid to mag or vice versa are not needed.

The difference between the two norths in the east of the UK is a little over two degrees and in the NW it is well under one degree. I doubt anyone needing to walk a bearing could manage that level of accuracy.

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Re: Compass questions...it's gone mad....

Postby Robert Haynes » Tue Aug 03, 2021 1:24 pm

Tringa wrote:The difference between the two norths in the east of the UK is a little over two degrees and in the NW it is well under one degree. I doubt anyone needing to walk a bearing could manage that level of accuracy.

One of the more useful navigation tips I've picked up over the years is the '1 in 60 rule' - a bearing error of 1 degree equates to a lateral position error of 1 metre for every 60 metres travelled. It's not precise, but is close enough for small errors and short distances. You'd need to walk 6km (just shy of 4 miles) to pick up a 100m error, and in that distance you'd usually expect to find some kind of terrain feature that would keep you on track - at least in Scotland, anyway!

It's more use, in fact, to know how far to aim off in bad visibility in order to know that when you hit a burn, the bridge you want is somewhere to your left.

The real issue with walking on a bearing, of course, is that it's all too easy to stick merrily to 250 degrees on the compass, but all the while slip gently to the side down a hill or get blown sideways. At least we don't have to deal with the hills moving around underneath us, like sailors do.
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Re: Compass questions...it's gone mad....

Postby Tringa » Wed Aug 04, 2021 1:38 pm

Not heard that 'rule' before Robert, thanks for posting it. Agree, aiming off is very useful and also about setting a bearing and almost immediately diverging from it - been there and done it.

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Re: Compass questions...it's gone mad....

Postby davekeiller » Wed Aug 04, 2021 8:09 pm

The 1 in 60 rule is a good approximation.
If you're at an angle X to the bearing you want, then after travelling Y metres, the distance you are off course is Y*sin(x)
For small angles (in radians) sin(x) ~ x
seeing as 360 degrees is 2*pi radians (pi = 3.141592...), that makes 1 radian ~60 degrees, or 1 degree about 1/60 radians.

Putting this together gives the distance off course as approximately Y*x/60 where x is the angle in degrees.
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Re: Compass questions...it's gone mad....

Postby Robert Haynes » Thu Aug 05, 2021 7:05 am

Interestingly, because it slightly underestimates the sine at low angles, the 1-in-60 rule is surprisingly accurate at larger bearing errors.
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