Author Topic: Numbering systems on the outside of a compass bezel  (Read 1630 times)

Rob Goes Walking

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Re: Numbering systems on the outside of a compass bezel
« Reply #15 on: 22:08:37, 25/04/19 »
.... No need to worry about magnetic declination either in this country.

It did look like a small error but I wasn't sure how much error it would bring in practice. I think I can calculate using mils! So (6400 / 360) * 2 = ~35.5 metres error every kilometre, is that right? Or does it change depending on the angle?

Edit: No I'm fairly sure it shouldn't change with the angle.
« Last Edit: 22:21:23, 25/04/19 by Rob Goes Walking »

archaeoroutes

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Re: Numbering systems on the outside of a compass bezel
« Reply #16 on: 22:28:14, 25/04/19 »
Bear in mind that its not possible to set a compass better than a couple of degrees anyway (just like you can't read a millimetre ruler to better than 1mm). Then remember that following a bearing to degree accuracy is really unlikely to happen. So a degree or so variation isn't going to be significant.


I get what you mean about needing to be happy with your kit.
I've got five different compasses, each with their own pros and cons. I use them all at various times as each has a situation where they excel.
I have also been through many iterations of some kinds of kit because they just weren't perfect.
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sussamb

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Re: Numbering systems on the outside of a compass bezel
« Reply #17 on: 22:37:56, 25/04/19 »
My experience of the "dropshorts" when training in Canada was that despite sending them an eight figure grid reference and accurate adjustments in 100 mil increments they still managed to lob their 105mm bullets somewhere between me and my HQ troop.

Even worse when they're US troops, nearly killed twice by US fire supposedly in support  ;D
Where there's a will ...

Innominate Man

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Re: Numbering systems on the outside of a compass bezel
« Reply #18 on: 22:44:11, 25/04/19 »
Not the best tutorials but 'certainly worth knowing' are the series made by Lyle Brotherton. He could be the author of the manual you bought.
https://www.youtube.com/watch?v=BADUq3Maqbo
Also, and perhaps better are similar tutorial youtube vids by Glenmore lodge.
All of these tend to give you that familiarity that you'd gain from being shown by somebody rather than simply reading a manual.
Check them out, if nothing else they may confirm that your understanding of the manual is correct.


Watch out for Lyle's cat  ;D
« Last Edit: 22:48:15, 25/04/19 by Innominate Man »
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ninthace

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Re: Numbering systems on the outside of a compass bezel
« Reply #19 on: 23:02:27, 25/04/19 »
Even worse when they're US troops, nearly killed twice by US fire supposedly in support  ;D
Watch out for RAF Regt (Rockapes) too.  A Rockape boss of mine told me a tale that happened when he was in a sandy place long before the recent unpleasantnesses - Oman I think.  As a Junior Officer he was forward, conducting a mortar shoot.  He gave the fire order and asked for "one for effect".  There was muffled crump from somewhere but no sign of the explosion so he ordered "up 100" or whatever it is pongos say.  At this point his NCO drew his attention to a newly formed hole in the desert about 100m behind them and suggested he didn't ask for another. one, or words to that effect.
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Mel

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Re: Numbering systems on the outside of a compass bezel
« Reply #20 on: 23:07:49, 25/04/19 »

It did look like a small error but I wasn't sure how much error it would bring in practice. I think I can calculate using mils! So (6400 / 360) * 2 = ~35.5 metres error every kilometre, is that right? Or does it change depending on the angle?Edit: No I'm fairly sure it shouldn't change with the angle.
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I've no idea what you're on about. 


I've never adjusted my compass readings for magnetic variation.  It's only about 2 degrees in this country anyway and for the (relatively) short distances of travel before changing direction, I wouldn't say it's that important.


Walking on a bearing just means you've pointed your compass in the direction you want to travel, twiddled the compass housing so the arrow shape is pointing in the same direction as the red bit of the compass needle and whatever the number says (be that in mils, degrees, or whatever) is your "bearing".  But in reality, all you're doing when walking on a bearing is making sure you keep the red needle in that arrow shaped bit whilst walking in your "direction of travel arrow" direction.  The numbers are irrelevant.


The only time a bearing (number) might be relevant is if written walk instructions say something like "from point A walk for 400 metres on a bearing of 270 degrees to point B" (or mils, or whatever).  In reality, all that means is you turn your compass housing twizzly bit to 270 degrees, line your arrows up by (this time) turning the whole compass (not the twizzly housing bit) and off you go along your direction of travel arrow.


If I was to walk in a straight line from my house to the church, I would be walking on a bearing of 260 degrees.  For the short distance that would be (half a mile) making any magnetic variation adjustments would make no difference at all to where I would end up. It's not unreasonable to walk that sort of short distance before needing to change direction (and, thus, your bearing).



I can't believe I just dug my map and compass out to do that  ;D 

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pdstsp

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Re: Numbering systems on the outside of a compass bezel
« Reply #21 on: 23:15:38, 25/04/19 »
Mel's answer above just about covers it, in my experience.

Mel

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Re: Numbering systems on the outside of a compass bezel
« Reply #22 on: 23:27:01, 25/04/19 »
My cat is asleep 6 feet away from me on a bearing of 291 degrees.
 
My kettle is 10 feet away from me on a bearing of 160 degrees but to get to it I actually need to walk 2 paces at 249 degrees, 3 paces at 149 degrees, then 2 paces at 72 degrees.
 
Oh god.  Look what you’ve set me off doing.  Indoors nav practice  :-[
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Rob Goes Walking

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Re: Numbering systems on the outside of a compass bezel
« Reply #23 on: 23:34:04, 25/04/19 »
Yes IM he is the author. His descriptions were very clear I did understand them when I read them though not all the methods of procedure stuck, that will require practice.

I know know of enough techniques to be able to navigate around with a good idea of my current position, various ways to confirm my position and ways to relocate myself, using only 1 feature if necessary. What I don't yet know is how good I'll be at doing it but practice makes perfect (or hopefully good enough in this case). Route stories was the only thing from outside the book I required to make sense of it.

I'll save the videos until after I've practiced the techniques which will take several days if I follow Lyle's advice. Don't want to overload myself with information. I'll watch them before setting out on a real route instead of playing in the park.

Plus I want to know if I can learn navigation just from this book, route stories notwithstanding.

Or maybe curiosity will get the better of me and I'll watch them sooner. Probably.

Thanks for the links.

Mel it's how many metres west you will drift every kilometre you walk if you don't adjust for magnetic declination although as pointed out, it won't be quite that precise as the adjustment won't be 100% correct. As an average though it should be roughly correct. Dr Ninthace, was my sum correct? Seeing as you don't walk in perfectly straight lines (drift) and may not 100% accurately know your position (for resection) I'm sure this error is trivial but I'm still going to correct for it, trivial or not.

 ;D at indoor nav. I was practicing keeping my map orientated indoors earlier!

Mel

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Re: Numbering systems on the outside of a compass bezel
« Reply #24 on: 23:46:57, 25/04/19 »
The westerly drift difference would be about a couple of metres (two stride lengths, or thereabouts) over a kilometre.  Big wow.  By all means spend time learning it but, in practice, it's simply not needed in this country (IMO). 


Anyway, I'm navigating myself to bed .... on a bearing of 200 degrees and 20 foot of ascent  ;D






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Rob Goes Walking

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Re: Numbering systems on the outside of a compass bezel
« Reply #25 on: 23:49:21, 25/04/19 »
The westerly drift difference would be about a couple of metres (two stride lengths, or thereabouts) over a kilometre.  Big wow.  By all means spend time learning it but, in practice, it's simply not needed in this country (IMO). 


Anyway, I'm navigating myself to bed .... on a bearing of 200 degrees and 20 foot of ascent  ;D

So you're saying over 1 kilometre, 1 degree causes 1 metre of drift, not 1 mil as Alan said? Maybe Alan meant something to do with the vertical properties of the artillery fire not metres of horizontal distance like I took it - I can't remember exactly what he said nor did I understand it completely when I looked up the word he used in a dictionary. I could always plot 2 degrees on my map and measure the distance over 1km I suppose.

I want to learn to do it properly even if it's not needed.

Good night! Sleep well.
« Last Edit: 00:15:27, 26/04/19 by Rob Goes Walking »

Innominate Man

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Re: Numbering systems on the outside of a compass bezel
« Reply #26 on: 23:56:38, 25/04/19 »
Not entirely relevant to this debate but related and interesting (well, to me anyhow) is how some aspects of measurement & positioning are very precise.
Many years ago, as part of my vocation training I had need to understand surveying techniques, which I found really interesting and totally practical:- you knew if you were right because certain things were proven.
Anyhow, what I am getting to is that within a short while after the basics you learn how to make certain corrections within your calculations and one of these is an allowance for the curvature of the earth.
You may think that on a local level it isn't discernible, but I can assure you it is.
I know of an instance where a major survey at an airport for a runway extension had been undertaken and they were having all sorts of problems with the results. A friend of mine, of the old school, was sent to troubleshoot.
Despite protestations to the contrary 'that of course they had allowed a curvature correction' the autopsy proved otherwise !  My friend was vindicated as it was his first hunch that the amount of error indicated no such allowance had been included.
By the way, the learning of this was the manual way and didn't include any GPS technology (mainly because it didn't exist at the time !!)
So whether it is a building, a football pitch an airfield or the Everest range - it is 'certainly something worth knowing'.


But, despite professional accuracy - I agree with Mel, keep it simple. You don't need to know how to perform a lobotomy to cure a headache  ;D [size=78%] [/size]

« Last Edit: 00:01:28, 26/04/19 by Innominate Man »
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Rob Goes Walking

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Re: Numbering systems on the outside of a compass bezel
« Reply #27 on: 00:19:33, 26/04/19 »
IM, I've been accused of being excessively precise before. I just like knowing the numbers...

Rob Goes Walking

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Re: Numbering systems on the outside of a compass bezel
« Reply #28 on: 00:31:22, 26/04/19 »
According to this page:

https://whitehatcrew.com/blog/a-mere-one-degree-difference/

My math wasn't far out. It's close to 35 metres of westerly drift, the half metre error comes from rounding in the mils system, there are 6283 mil-radians in a circle not 6400.
« Last Edit: 00:43:52, 26/04/19 by Rob Goes Walking »

ninthace

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Re: Numbering systems on the outside of a compass bezel
« Reply #29 on: 00:44:17, 26/04/19 »
Mel it's how many metres west you will drift every kilometre you walk if you don't adjust for magnetic declination although as pointed out, it won't be quite that precise as the adjustment won't be 100% correct. As an average though it should be roughly correct. Dr Ninthace, was my sum correct? Seeing as you don't walk in perfectly straight lines (drift) and may not 100% accurately know your position (for resection) I'm sure this error is trivial but I'm still going to correct for it, trivial or not.
It has been a while so I am open to correction by one of the cognoscenti but IIRC if you walk 1km on a magnetic bearing your drift from the true bearing is the sine of the declination times the distance gone. Say the declination is 2 degrees and the distance gone is 1km. Sin(02) is 0.0349 so the distance off track will be 1000x0.0349= 34.9m. We never had declination when I was alive, we had variation and deviation.
However, it will be a clever person that can walk 1km in a straight line across the countryside armed only with a compass and arrive within 35m of where they are supposed to be so to all intents, you can ignore variation in the UK given its current low value. Mind too, I could get pretty close with a Garmin.
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