# Reading a Compass



## darklord (Jan 15, 2015)

Forgive this improbably stupid question, but I'm not very electricabubble!.....

When mapping underground I have to read a compass. This puts the headlamp unit (LED emitter assembly) directly above the compass which is held to the eye...with the obvious risk of magnetic deflection through the proximity of an electrical field - thus giving incorrect readings.

My question is: is it better to use a headlamp with the batteries on the back of the head? Or does it not matter because the same electrical field is going to be passing through the LED unit anyway? I'm just wondering if the field is stronger around the batteries?


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## more_vampires (Jan 15, 2015)

If you're concerned of it, proof is in the pudding. You could see how much deflection you get on the needle by waving your headlamp around compass? Magnetic fields fall off rapidly with a little bit of distance. (Inverse square) Wouldn't think it would take much separation.


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## D6859 (Jan 15, 2015)

Tested with my phone app and Tiara A1. There was no change in the magnetic field meter that the app has when changing modes in the headlamp. Tiara has a strong removable magnet at its end, but it didn't seem to change the reading when looking at the phone from arm's distance.


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## StarHalo (Jan 15, 2015)

Better to use a tritium compass.


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## darklord (Jan 21, 2015)

Hi guys, sorry been away for a few days - thanks for the answers. You're right, the thing is to do some testing! Unfortunately I've not got the kind of compass we use to hand at this minute - http://www.suunto.com/en-GB/Products/Compasses/Suunto-KB-14/Suunto-KB-14360R1/ - but it has been reported that strong headlamps cause a deflection. Sadly you couldn't apply a 'standard deviation' because the deviation might vary with even a small change of head or helmet position.

D6859.... we have to read compasses at eye level..which is just below forehead/headlamp position ! I'm sure at arm's distance there is not nearly so much, if any, effect. But up close (less than 10cms?) may well be different.


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## 5S8Zh5 (Jan 21, 2015)

Suggest a Cammenga 3H Tritium Military Compass. ....


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## darklord (Jan 21, 2015)

5S8Zh5 said:


> Suggest a Cammenga 3H Tritium Military Compass. ....



Yes, but the design is totally unsuitable for cave surveying, I'm afraid. Many years experience make the Suunto the only tool for the job.


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## dss_777 (Jan 21, 2015)

No direct experience with your uses, but I would think it's the metallic components of the headlamp that cause the deviation, not the electrical field from the LED. If true, then putting the mass of the batteries at the back of the helmet (or on the belt) would be better. as would any headlamp with fewer non-ferrous materials in it. You could try this theory out with any compass and any LED headlamp just by putting them together and gauging the effect.

I've only ever seen LED lights interfering with wireless connections (on a bike computer), but YMMV. The effect was related directly to the bike computer's proximity to the LED emitter. The battery pack was in a different location.


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## uk_caver (Jan 22, 2015)

Unless there's meaningful steel in the lamp (or its mount), cells/batteries are probably the most likely thing to affect a compass, and for most proper caving lights, they'll be at the back.

With a sighting compass, it should be fairly easy to check when you do get your hands on one, taking sightings on the surface with lamp on and off and with and without helmet (and if you use a second light to light the dial, with and without that).

Personally, while I appreciate people who do survey precisely, in reality minor errors aren't generally earth-shattering.

When we finally closed a >2km loop in North Wales after a *lot* of digging (including a 90m long excavation in the top of a completely filled passage) the total error was something like a metre, due to there being some very good and experienced surveyors, (not including me).
In some ways, knowing the surveying was likely to be very good was a bit soul-destroying when the final dig had only just started.

On the other hand, where I go in Slovenia in the summer, surveying is done by many people including some complete beginners, often in difficult conditions, and when we have had loops of similar orders of length, errors have been much larger, yet still close enough to point us fairly clearly to where we needed to look for connections.


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## darklord (Jan 23, 2015)

Thanks for the further input, guys. I think it's fair to say that some testing might sort this question out - at the time I just thought maybe someone had a de facto answer, but naturally it's not a a very 'mainstream' application we're talking here!

uk-caver - yep, I'd be happy with a metre after a 2k loop! (where is that, btw?..or is it top secret?!) But if your passage is 3k long and not part of a loop, you could get significant error in your final position. So say your passage is heading for the hillside/cliff.... in say 60 legs with a 2 degree (same direction) error in each leg...well, you could end up rather far from where you should be looking on the surface for an entrance! :thinking:

I suppose the Disto-X obviates this question, anyway....just that we haven't all yet moved (or can afford to) from the good ol' Suunto compass/clino routine.


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## crybaby (Jan 27, 2015)

many yrs. ago I notice my watch affects my compass so I use the other hand


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## uk_caver (Feb 3, 2015)

darklord said:


> uk-caver - yep, I'd be happy with a metre after a 2k loop! (where is that, btw?..or is it top secret?!)


Not top secret - it's the Ogof Llyn Ddu II - Ogof Dydd Byraf connection.



darklord said:


> But if your passage is 3k long and not part of a loop, you could get significant error in your final position. So say your passage is heading for the hillside/cliff.... in say 60 legs with a 2 degree (same direction) error in each leg...well, you could end up rather far from where you should be looking on the surface for an entrance!


Absolutely, though I guess unless your passage is ramrod-straight, you might not expect it to punch right through the surface carrying on from its last direction, and if you thought you were really close from the underground side, that might be a time to try and sweet-talk your nearest radiolocation guru.

Also if you have a 3k section surveyed in typical expedition conditions, there's a meaningful chance there's a typo somewhere in the data recording or transcribing anyway...


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## TEEJ (Feb 3, 2015)

If you have two compasses, or other way to take two readings....you can see if there is a difference between them when one is read with your suspect condition and the other is read w/o the headlamp (A flashlight at greater distance, a glow in the dark stick or tritium fob, etc.)

IE: Reading #1 with headlamp, vs reading #2 with glow stick, etc.


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## TX_caver (Feb 20, 2015)

Your head lamp will certainly deflect your compass. Even something as small as the screws in eyeglasses will deflect your reading some. When it really counts, keep anything metal or electrical away from the compass


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## uk_caver (Feb 20, 2015)

I just played with a Disto X, one of three I've been converting for a friend*.

Seems like the answers may be 'it depends', and 'distance is a huge factor'.

Leaving the Disto stationary, I can get a reading change of up to two degrees holding my metal-framed glasses flat against the case top or bottom surface, but that effect drops to nothing if they're more then ~30mm away from it, and if they're facing the same way the laser points, there's much less effect, - more like +/- 0.2 degrees.

My caving headlamp is a conversion in an Oldham mining lamp headset with stainless steel mounting clip and helmet bracket, and lit or unlit, that seems to have no real effect in any plausible orientations and proximities, likely due to a fair extent to the plastic and non-ferrous metal parts making it hard to get the steel close enough to have a meaningful effect.

By contrast, my Casio watch (resin strap) has much more effect - both the watch itself and the small stainless buckle on the [non-original] strap can give a deflection up to 10 degrees if held right against the case, but over 50mm away, and effects fall off to pretty much nothing.

(*)In the Disto conversions, the first one I did, I used some JB Kwik to hold the USB charging board in place in the battery compartment, because it's a nice epoxy to work with. When I did the next two, I remembered that JB Kwik is metal-filled, so I used standard epoxy on them. When it came to calibration, the first one of the three gave consistently unacceptable calibration error numbers, while the other two seemed to calibrate easily. Having now fettled the first one, it'll be interesting to see if the first one now calibrates easily.


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## more_vampires (Feb 20, 2015)

Wow, UK_CAVER! That's one heck of a post!  Awesome! :thumbsup:


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## uk_caver (Feb 20, 2015)

Thanks, though it's really down to having something to hand I could put on a desk that gave a continuous reading to 0.1 degree.
With that, it's easy to do a fair bit of experimentation in a few minutes.

I just had a quick try with my large Swiss Army knife, and that was affecting it from much further away, 5 degrees or more from 150mm.


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## more_vampires (Feb 20, 2015)

Many variants of stainless steel have less of an EM signature than carbon steel. Mass of item can make up for it. Basically, if your item attracts a magnet then it is a candidate for affecting a compass.

I used to carry a compass on the tank bag of my motorcycle until I realised it was always wrong. Being on a magnetic mount tank bag on a 100% magnetic no-plastic bike will do that.

I was just wondering the margin of error. That's pretty darn huge!


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## darklord (Feb 25, 2015)

uk_caver said:


> Not top secret - it's the Ogof Llyn Ddu II - Ogof Dydd Byraf connection.



Not familiar with it - I must pay more attention to what's happening outside the Dales 




> Absolutely, though I guess unless your passage is ramrod-straight, you might not expect it to punch right through the surface carrying on from its last direction, and if you thought you were really close from the underground side, that might be a time to try and sweet-talk your nearest radiolocation guru.
> 
> Also if you have a 3k section surveyed in typical expedition conditions, there's a meaningful chance there's a typo somewhere in the data recording or transcribing anyway...



Accept your first comment there, although in some situations radiolocation can be logistically rather difficult...(think 10,000 miles away!)

There can be error in anything you do, I'm just thinking about minimising it where possible.

ps. re converting Distos - have PM'd you


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## datagutten (Mar 1, 2015)

I would not think that a battery powered light would generate a significant magnetic field.
You usually need AC to generate a magnetic field, but batteries deliver DC.


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## uk_caver (Mar 2, 2015)

DC will generate constant magnetic fields.

What AC is needed for is generating varying magnetic fields, which are needed for the operation of devices like transformers, and for the generation of electromagnetic radiation.


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