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Brunton 7DNL Compass Review


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Home Forums Campfire Editor’s Roundtable Brunton 7DNL Compass Review

Viewing 13 posts - 51 through 63 (of 63 total)
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  • #1618187
    Roger Caffin
    BPL Member

    @rcaffin

    Locale: Wollemi & Kosciusko NPs, Europe

    Hi guys

    I had to stop and think about this issue of the separate declination adjust by screwdriver bit. Why have we never been worried about not having it?

    Then the answer came to me: because it is completely unnecessary in the field. That's right, you don't need it at all (at least when walking on land). I had better explain why.

    When you are navigating in the field the only thing you have for getting a bearing is magnetic north. 'True North' is an artifact of no relevance at all to practical navigation EXCEPT when you orient your map. That's ALL it is useful for.

    Some might argue that without True North you won't have a grid to get grid references off. That is not true either. The grid is related to various survey and geographical things, but principally it too is an artifact of the map makers. In fact, on many maps you find that there are not two but three arrows in a group at the bottom of the map: True North, Grid North and Magnetic North. When this happens you don't care at all about True North: you have to use Grid North. But you won't find grid lines on the ground either.

    So, step 1: Orient the map allowing for magnetic declination. You can do this by setting the bezel for the declination, or you can do it using the arrows at the bottom (side?) of the map, or you can just estimate it. Once you have oriented the map, don't move (rotate) it any more!

    Step 2: Work out what bearing you want to go on in terms of magnetic north. You might do this for instance by lining up the baseplate between where you are now and where you want to end up. Alter the rotating bezel so the lines on it line up with the needle.

    Step 3: Travel keeping the baseplate aligned with the needle.

    If you want to set a back-bearing, do it directly: point the base plate and rotate the bezel. You are going to navigate off the local magnetic field after all.

    Yes, this means I USE the rotating bezel. It's not just for setting a declination: it's also for setting a bearing. All you have to do is to stop worrying about True North and Grid North, and use Magnetic North. After all: it's all you have really.

    I don't know of anyone walker in Oz who has a compass with a lockable declination bezel – in fact I don't think I have ever seen one in the local shops. Too heavy, too expensive. Go light-weight in grams and dollars.

    Cheers

    #1618203
    Robert Blean
    BPL Member

    @blean

    Locale: San Jose -- too far from Sierras

    Roger,

    Perhaps some find it either easier or more precise (even if not essential) to use a settable declination. Once the declination is set, the latter part of your Step 2 can be changed to read "Alter the rotating bezel so the lines on it line up with the top of the map (or grid lines, if available)."

    Doing it this way would mean there is no need to orient the map to find a bearing from it.

    –MV

    #1618218
    D G
    Spectator

    @dang

    Locale: Pacific Northwet

    One easy thing to do is to draw in on your map (use the small arrow if it's there) a few grid lines aligned with magnetic North and use those grid lines to orient the map to magnetic north instead of true north. You really only need to draw in one or two lines. Then you don't even have to worry about magnetic declination when orienting your map.

    And of course, if you are using the compass to take a bearing to follow, magnetic declination is irrelevant. It's only when the map comes into play that you need it.

    #1618220
    Chris Townsend
    BPL Member

    @christownsend

    Locale: Cairngorms National Park

    I do the same as Roger except that I adjust for declination after taking the magnetic bearing and with no reference to the map. This has always worked well for me and only takes a few seconds.

    #1618276
    Bob Gross
    BPL Member

    @b-g-2-2

    Locale: Silicon Valley

    The worst part of this is that some beginners are reading this and won't figure it out. You notice that several experienced navigators each have different methods for adapting to declination, and they probably all get good results. This multiplicity of methods is so confusing to the beginners. Unfortunately, when a beginner takes a Land Nav class, he will be taught only the method that the instructor likes, and that may or may not be the easiest to learn.

    I work differently. I think I have a piece of lodestone in my head.

    –B.G.–

    #1618283
    Chris Townsend
    BPL Member

    @christownsend

    Locale: Cairngorms National Park

    Lacking a lodestone in my head I use the Silva 1-2-3 system, which I learnt sometime in the 1970s. It still works!

    http://www.silva.se/upload/Catalogues/123_eng.pdf

    There are other map and compass techniques of course but this is the basic. The difference between Roger and me is only to do with when to adjust for declination. Taking a bearing from the map is the same.

    I've taught navigation (and written about it several times in books and magazine articles) and always use this as the starting point when introducing compass use. (I actually start with just map use).

    #1618312
    Tohru Ohnuki
    Member

    @erdferkel

    Locale: S. California

    I don't think I'm in disagreement with Roger and certainly there are many ways to do this: hike your own hike. But I suppose the issue is whether one considers triangulating your position as a navigation task or surveying task.

    ROger is correct that magnetic north is just as good a reference if what you are doing is transferring a bearing to navigate to a destination, in fact, air navigation charts have their compass roses set to magnetic. And as Daniel has pointed out (and i've read this in some books on navigation) you can easily mark your map with mag dec parallels as a reference.

    But most maps are aligned to true north and to transfer bearings one either has to add/subtract a mag dec correction or use a compass which does it for you. When you are taking several of them to plot a fix, the math can get tedious.

    I agree that I've never had to get a fix in normal hiking, usually the terrain features and a rough estimate of north is good enough. But I have practiced getting a fix by triangulation and for that and adjustable dec compass makes it a lot easier. IIRC, triangulation is a requirement for Sierra Club leader checkout…

    Actually, the real convenience here is that a baseplate (or mirror sighting) compass is really two instruments in one: a compass and a protractor (fulfilling lightweight backpacking principles!). Having a separate capsule and baseplate which rotates around it is the trick. Without it, you would have to measure the bearing angle on the map with a protractor, add/subtract the mag dec, then modulo 360 to get the number the compass needle has to be pointing at for the 0 mark to be your bearing. Reverse this procedure and repeat for each bearing for your fix!

    #1618437
    Anonymous
    Inactive

    "One easy thing to do is to draw in on your map (use the small arrow if it's there) a few grid lines aligned with magnetic North and use those grid lines to orient the map to magnetic north instead of true north. You really only need to draw in one or two lines. Then you don't even have to worry about magnetic declination when orienting your map."

    If you choose to do this it might be good to do it with an eraseable pencil, otherwise you'll end up with either a lot of lines on your map or an inaccurate declination over time. Magnetic declination is not static. It changes from year to year.

    #1618500
    Roger Caffin
    BPL Member

    @rcaffin

    Locale: Wollemi & Kosciusko NPs, Europe

    Hi Tohru

    > But most maps are aligned to true north
    Sorry to have to disagree, but in fact very few maps are really aligned to 'True North'. Really only happens for the maps very near the national Datum.

    Have a close look at any map handy and you are likely to find that the map has arrows for True North and Magnetic North beside the Grid North arrow.

    > you would have to measure the bearing angle on the map with a protractor,
    ????
    I don't even own a protractor (or if I do I don't know where it is).
    I use the map and compass as I described. The compass has its own protractor built-in.

    Cheers

    #1618535
    Tohru Ohnuki
    Member

    @erdferkel

    Locale: S. California

    >> But most maps are aligned to true north
    >Sorry to have to disagree, but in fact very few maps are >really aligned to 'True North'. Really only happens for >the maps very near the national Datum.

    I'm referring to the map, not the grid. Maybe this is a regional difference, here the US Geological Survey maps are aligned with latitude and longitude, thus true north. The USGS 7 1/2 minute series are just that: 7 1/2 minutes of angle in both lat and lon.So lines drawn parallel to the sides are aligned to true north.

    If you are using the grid on your map, for us the UTM grid, then it is true that the grid will line up with true north in some spot in the zone, but be slightly off everywhere else.

    #1618608
    D G
    Spectator

    @dang

    Locale: Pacific Northwet

    "If you choose to do this it might be good to do it with an eraseable pencil, otherwise you'll end up with either a lot of lines on your map or an inaccurate declination over time. Magnetic declination is not static. It changes from year to year."

    True that magnetic deviation is not static but I wear out a map long before there's a need to change any magnetic north gridlines. :)

    #1619690
    Paul Davis
    BPL Member

    @pdavis

    Locale: Yukon, 60N 135W

    All: I should have pointed out that we like compasses with built-in declination, as all Canadian topo maps are printed with a 1000 Metre Universal Transverse Mercator Grid, and a handy arc showing how far off this is from Magnetic North, and True North.

    So, no pencil lines in Canada! Use the UTM grid! Mind you, the place was only completely mapped in the mid 1980's, and many of our 'maps' North of 60 are 'photo maps', meaning top-line versions of aerial photos from the 1940's!As well, nothing more detailled than 1:50,000 for the Can. national topo system (a few mine areas=1:25,000!)—I envy European and orienteering maps at 1:10,000!

    Always having UTM grid available on a map means pretty much always using the Silva 1,2,3 system to get around.

    You do need to note which UTM Zone you are in, as the UTM grid numbers do repeat—we use Lat. + Long. for setting up stuff with aviators…

    I used the Suunto today for an insurance bearing when coming down from above treeline via a braided horse-foot-ATV series of trails, worked like a charm. Didn't even have to stop walking!

    #3706276
    Jim Morrison
    Spectator

    @pliny

    Locale: Pacific Northwest

    I was looking for a light weight replacement for my Silva Starter that I used for years.  I thought this might be the one and wanted some opinions.  Thanks all for your thoughts. I believe it is just what I wanted.   I have heavier compasses with mirrors.  It will be fun to compare the bearings I take with this and my other compasses.

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