Author Topic: Practicalities of compass and map navigation  (Read 2578 times)

metanome

  • New Member
  • *
  • Posts: 16
Re: Practicalities of compass and map navigation
« Reply #15 on: 20:10:59, 26/08/20 »
Returned from the New Forest. Compass reading a bit of a mixed bag to be honest, but mostly self-inflicted because we confined ourselves largely to the forest tracks and we missed a day out because of particularly atrocious weather. It was nevertheless highly refreshing to use a physical map for the first time in ages, and certainly gratifying to be able to follow a basic route on the map and find real world features (more or less) where I expected them to be - high point was locating a spring and accompanying stream in thick woods based on the map, when I couldn't locate it by sight until I was a couple of feet away from it.

I think next steps is to bed in a little with compass usage a little closer to home, so will get myself a map for my local area. I also picked up a copy of Navigation for Walkers (Julian Tippett) for next to nothing - could have sworn I got the recommendation from this thread, but apparently it came from somewhere else. Has already proved very useful.

I was a little confused on my OS map about magnetic declination (tried to post a link to a photo of the relevant map section on WeTransfer, but not allowed). For navigation, what's the significance of the bit about where true north is located at the centre of the East and West sheets? If Magnetic north (and its variance from Grid north) is what is used for orienteering, I would have thought that would be the leading sentence of the 'North Points' section? Also, does the fact that the representations of the different Norths are 'Diagrammatic only' mean the image is just a placeholder? That I can't actually use the image to measure the difference between Grid, True and Magnetic north? If so, what's the point of including it?

And finally (just to be sure), if Magnetic north is 2 west of Grid north, that means I have to point the N of my compass bezel 2 anti-clockwise to compensate (not the other way around)? Would another way of saying this be that Magnetic north is -2 of Grid north?

Ridge

  • Veteran Member
  • *****
  • Posts: 8272
Re: Practicalities of compass and map navigation
« Reply #16 on: 20:33:20, 26/08/20 »
Real world features more or less where you expected them . That is as good as navigation gets.
So much navigation is talked about as if you are in a desert but most of the time walking north or 2 degrees away from north you are still going to get to the feature you are heading for.

April

  • Veteran Member
  • *****
  • Posts: 8291
"Who would've thought...... you are light and darkness coming through" words by Tim Armstrong

metanome

  • New Member
  • *
  • Posts: 16
Re: Practicalities of compass and map navigation
« Reply #18 on: 21:27:47, 26/08/20 »
Thanks yes, happy with fact that it changes! Less clear on the significance of the explanation on the OS map. Any guidance appreciated.

ninthace

  • Veteran Member
  • *****
  • Posts: 7238
Re: Practicalities of compass and map navigation
« Reply #19 on: 21:28:34, 26/08/20 »
Map and compass work does does not have to complicated in much of our countryside.  Correcting for magnetic variation is more of a nicety rather than a necessity in the UK.  In normal walking as there are usually plenty of features to navigate by and your compass is only really needed to help you face in the right direction and perhaps help you tell one feature from another.  The only time you need to invoke the dark arts is crossing featureless country particularly in poor visibility.  That can come later.
Solvitur Ambulando

metanome

  • New Member
  • *
  • Posts: 16
Re: Practicalities of compass and map navigation
« Reply #20 on: 21:36:55, 26/08/20 »
Still, would be helpful to have a broad understanding from the get go. Don't want to develop my navigation solely by reference to UK idiosyncrasies and then get snookered if I want to do some hiking elsewhere in the world.

GoneWest

  • Full Member
  • ***
  • Posts: 122
Re: Practicalities of compass and map navigation
« Reply #21 on: 21:40:53, 26/08/20 »
If you are using just a magnetic compass and an OS map, the two norths you need to be concerned about are grid north and magnetic north. The difference between them is properly known as the Grid Magnetic Angle.

The diagram on the map is meant to show only the E-W relationships of the three norths. The figures given are what really matter. All flat maps have to cheat in order to represent the curved surface of the Earth. This is why the difference between grid north and true north varies slightly around and between map sheets. The discepancies are tiny over the area covered by an Explorer map and can be ignored; if you ever do need to consider true north, just use the figure for the centre of the map. If I remember rightly, the Landranger maps, covering four times the area, give a figure for each corner.

Why is true north mentioned at all on the map? Because some navigator might be using a gyro compass or resorting to a bit of astronav, or because a surveyor or cartographer might have some more esoteric interest in it.

The answer to your last question is "it depends"! If the GMA is 2 degrees west and you have just worked out a heading from the map then, yes, you rotate the bezel 2 degrees anticlockwise before attempting to follow that heading over the ground. If, however, you have just sighted a landmark, you need to rotate the bezel 2 degrees clockwise before plotting the landmark's bearing on the map. Some compasses allow you to preset the GMA to save you all that fiddling about.

April

  • Veteran Member
  • *****
  • Posts: 8291
Re: Practicalities of compass and map navigation
« Reply #22 on: 22:45:06, 26/08/20 »
Thanks yes, happy with fact that it changes!


I meant that Magnetic north in the UK has changed a lot in the years since I learnt to use a compass at school, so there is much less variation at the moment. If I remember correctly the magnetic variation may have been something like 7 or 8 degrees west when I was at school.  In some parts of the UK it is now 0 or only 1 degree west. I rarely add the variation these days but I did do it religiously at one time. You do just turn the compass 1 degree (if the variation is 1) anti clockwise eg 120 becomes 121 if the variation is west. In a few years the variation may be east and you would subtract 1 degree so 120 becomes 119 and you would turn the compass clockwise.

See the link below, some places are now positive east and not negative west in the UK!
www.magnetic-declination.com/Great%20Britain%20(UK)/Greenwich/901867.html


It is a handy website, you can check the magnetic variation for other countries on it too.

"Who would've thought...... you are light and darkness coming through" words by Tim Armstrong

sussamb

  • Veteran Member
  • *****
  • Posts: 7462
Re: Practicalities of compass and map navigation
« Reply #23 on: 07:05:22, 27/08/20 »
In practical terms magnetic variation can be ignored in uk, it's impossible to navigate so accurately using a hand held compass that a few degrees makes a significant difference.
Where there's a will ...

richardh1905

  • Veteran Member
  • *****
  • Posts: 5494
Re: Practicalities of compass and map navigation
« Reply #24 on: 07:12:21, 27/08/20 »
I certainly don't worry about magnetic variation at the moment as it is so small. Back when I was taught map reading in the school cadet force, it was about 7 degrees, more significant.

I still remember

Map to Grid - Get Rid
Grid to Map - Add
WildAboutWalking - Join me on my walks through the wilder parts of Britain

richardh1905

  • Veteran Member
  • *****
  • Posts: 5494
Re: Practicalities of compass and map navigation
« Reply #25 on: 07:19:17, 27/08/20 »
In fact down near the New Forest, magnetic declination is practically zero.



Latitude: 50 50' 13.8" N
Longitude: 0 47' 50.5" W
FISHBOURNE
Magnetic Declination: +0 0'
Declination is POSITIVE (EAST)
Inclination: 65 54'
Magnetic field strength: 48693.6 nT


https://www.magnetic-declination.com/
WildAboutWalking - Join me on my walks through the wilder parts of Britain

metanome

  • New Member
  • *
  • Posts: 16
Re: Practicalities of compass and map navigation
« Reply #26 on: 09:16:01, 27/08/20 »
Excellent, appreciate all the helpful replies (this is a much more engaged forum than some I have used in the past). Thanks Gone West for the explanation of the North points, that makes perfect sense.[/size]

[/size]
So with the variation of Magnetic north, and in the current case of the UK I assume there must basically be a meridian (not sure if I'm using that word correctly) where, depending on whether you are East or West of it, the declination is either positive or negative?[/size]

[/size]
Also a side question - how can I contact the site admins, I couldn't find any contact details anywhere? I have an issue with my account I need to discuss.[/size]

GoneWest

  • Full Member
  • ***
  • Posts: 122
Re: Practicalities of compass and map navigation
« Reply #27 on: 09:52:10, 27/08/20 »
In fact down near the New Forest, magnetic declination is practically zero.



Latitude: 50 50' 13.8" N
Longitude: 0 47' 50.5" W
FISHBOURNE
Magnetic Declination: +0 0'
Declination is POSITIVE (EAST)
Inclination: 65 54'
Magnetic field strength: 48693.6 nT


https://www.magnetic-declination.com/


There seems to be some confusion about declination, variation, etc. Sites like the one above, when they quote declination, are quoting what sailors call variation, which is the difference between magnetic north (at a given place and time) and true north. Magnetic north is variable with respect to place and time but true north, by definition, points along the local meridian towards the geographic north pole and is essentially invariant. Accordingly, true north is the north which all navigators, surveyors, cartographers and earth scientists can agree about, world wide. It is, in that sense, the gold standard, provided the underlying ellipsoid (datum) is also agreed. The modern gold standard for that is WGS84. The OSGB36 datum on which our OS maps are based is different but the difference is of little relevance to a walker.

Notwithstanding the above, when we use OS maps we use grid north, not true north. These are the same only on the meridian at 2 degrees west, or 400km easting, in OS terms. Because of the way the grid is defined, these two norths are different everywhere else. Where I am, true north is 2 degrees east of grid north. I am lucky because magnetic north here is currently 1.28 degrees west of true north, which means my compass points to grid north within about three quarters of a degree, which I can live with!

Someone walking near Cranbrook, in Kent, is not so lucky. There, true north will be 2.5 degrees west of grid north and the local variation is 0.43 degrees east. The local grid magnetic angle (GMA) there is therefore about 2 degrees west. Some may still regard this as negligible but I don't. It's comparable with the reading error of a mirror compass and greater than that of a prismatic sighting compass. More importantly, it's a systematic error which will never be "averaged out" no matter how many compass readings are taken. It's enough of a GMA to be taken properly into account, in my view.

« Last Edit: 11:43:41, 27/08/20 by GoneWest »

sussamb

  • Veteran Member
  • *****
  • Posts: 7462
Re: Practicalities of compass and map navigation
« Reply #28 on: 12:35:09, 27/08/20 »
If you can navigate with a hand held compass to an accuracy of less than 5 degrees I'd be amazed. In my youth I was a national level orienteer, and did lots of detailed navigation in the Army, 5 degrees was always the quoted accuracy variation.


I'm not sure what others were talking about but the variation I was referring to was between grid north and magnetic north, true north is never considered when map reading.
Where there's a will ...

ninthace

  • Veteran Member
  • *****
  • Posts: 7238
Re: Practicalities of compass and map navigation
« Reply #29 on: 13:17:18, 27/08/20 »
Variation and deviation, true N, grid N and Mag N are subtleties that matter to aviators and mariners (or at least they did once upon a time).  As Sussamb says, you would have be good to able to walk with such a degree of accuracy.  Just do a little trigonometry.  If you could walk a mile in straight line and failed to correct for a 2 degree difference between grid and magnetic, you would still only be around 60 yds away from where you expected to be.
Solvitur Ambulando