Brunton 7DNL Compass Review
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Rog Tallbloke
(tallbloke) - F

Locale: DON'T LOOK DOWN!!
Re: Re: Re: Another compass type on 06/01/2010 01:51:33 MDT Print View

>I guess it depends whether you regard a westerly deviation as positive or negative number.

>Does it also depend on what the local deviation is? Positive or negative?

Well it's either going to be westerly or easterly. Unless it's zero.

I find it easiest to visualize the problem rather than rely on mnemonics myself. If the deviation is westerly, and I'm heading north, then if I set the compass to the map gridlines, I'll end up walking west of north. Therefore, I need to add the deviation to the bearing to end up going north.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Compass Considerations for the Canadian North... on 06/01/2010 01:58:41 MDT Print View

Hi paul

Thanks for the comments. Just one thing:

> dialing it in on a fixed scale with the metal-tooth-pick-style declination screwdriver
I'm not familiar with this at all, at least on baseplate compasses. Hum?

With the Brunton I set the declination using the red ring and the gradation under it, and then line up the needle with the lines/arrows on the rotating bit; then the base plate is True North.

Cheers

Tohru Ohnuki
(erdferkel) - F

Locale: S. California
adjustable vs fixed dec compasses on 06/01/2010 09:31:52 MDT Print View

These two compasses are doing the same thing, albeit in slightly different ways:
two compasses

The one on the left is a fixed dec, the one on the right an adjustable dec. The point is that you preset the mag dec on the adjustable beforehand and then line up the needle to the adjusted marks, in this case, the yellow fluorescent lines. On the fixed dec, you have to line up the needle to the dec scale each time.

The sharp eyed may notice a few other things:
1) The compasses are doing the same thing as the north mark on the map, this is a quick check that your compass is set up correctly for your region.
2) The bubble is deflecting the left compass's needle slightly and the right compass's marks don't line up due to the camera not being exactly above it (a problem that is solved when using the sighting mirror.
3) You can't get two compasses this close without them interfering with each other, what you don't see in the photo is the 5 minutes it took lining them up.

Edited by erdferkel on 06/01/2010 09:43:22 MDT.

Rod Lawlor
(Rod_Lawlor) - MLife

Locale: Australia
Suunto MC-2G Global Compass on sale on 06/01/2010 15:16:58 MDT Print View

Suunto MC-2G Global Compass is currently on sale at theclymb.com for $40, until Fri 9am I've never used the site, but am registered there. It's members only, but you can use this link

http://www.theclymb.com/invite-from/RodLawlor

to skip past the registration process. (Be aware that this will credit me with $10 for anyone who makes a purchase. I'm not sure how this will work, since they only ship within the US)

Roger, I realise this should be in Gear Deals, but it seemed relevant here. I'm happy to remove it when I get home tonight if you'd prefer, or for you do so.

Rod
------
Relevant.
Cheers

Edited by rcaffin on 06/01/2010 16:28:53 MDT.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: adjustable vs fixed dec compasses on 06/01/2010 16:40:57 MDT Print View

Hi Tohru

The compass on the left is pretty much a generic design - available under most brands with minor variations in colour and trim. The bezel on it CAN rotate and be set for declination. There is a white line under the N symbol which serves as the marker when you rotate the dial.

The Brunton 7DNL is essentially the same. The bezel rotates to set the declination. The blue line points to the marker. You line up the needle with the lines on the rotating bezel.
Brunton 7DNL compass with declination marker 8546

If the bezel on such a compass does not rotate, it is because it is jammed. Sand, dirt etc.

Cheers

Edited by rcaffin on 06/01/2010 16:42:33 MDT.

Tohru Ohnuki
(erdferkel) - F

Locale: S. California
Re: Re: adjustable vs fixed dec compasses on 06/01/2010 16:51:45 MDT Print View

RIght, the baseplate can be set such that you are compensating for mag dec as in your picture. But this means that the baseplate is set to true north only. This is fine if all you want is to orient your map to true north.

The purpose of the dec scale on your fixed dec compass or the adjustable dec is so that the _capsule_ of the compass is set to true north and the _baseplate_ can be set to an arbitrary bearing.

This is so that if you see a mountain off in the distance you line up the baseplate to the mountain, and the capsule to north to get the bearing, which is indicated by the white line you pointed out. Then you can transfer that to your map. Or vice versa, see:
link to Silva 123

Edited by erdferkel on 06/02/2010 11:08:47 MDT.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Re: Re: adjustable vs fixed dec compasses on 06/04/2010 16:54:19 MDT Print View

Hi Tohru

> all you want is to orient your map to true north.
Describes me perfectly. In fact, my wife insists that we do that FIRST, every time.

Then we sight over the map to any peak. Works great for us.

Cheers

Kenneth Linden
(kennethlinden) - F
Declination change with time on 06/05/2010 00:42:59 MDT Print View

Hi Tohru,
Being a southern California hiker, I couldn’t help noticing that the map you set the two compasses on in the picture above is out of date with respect to the magnetic declination shown on the map. For the San Gabriel Mountains, the magnetic declination is now 12.5 degrees, rather than 13.3 degrees as listed on the map, a difference of almost a degree http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination. The declination changes a little year over year.
Having said this, I realize from the above Forum posts, that one group, the baseplate compass group, will be gnashing their teeth at the over-exactingness of it, while the other group, the sighting mirror camp, will take it as a matter of course that one would correct for the current declination adjustment.
Personally, hiking in the San Gabriels and other major mountain ranges in So. Cal., I don’t believe I have ever needed to take my compass out of its case to navigate. The land forms are distinctive enough that I just use the topo map and compare it to the landforms, with the sun as compass enough. The only times I have used the compass is to identify some peak on the distant horizon for interest’s sake by later at home using the bearing and Google Earth (most of these peaks of interest would be off the edges of my topo map). For the record, I use a sighting mirror compass, the Suunto MC-2. I like the clear baseplate, the ability to adjust the declination, the clinometer for avalanche considerations, and I like the idea of carrying a mirror anyway for the reasons mentioned above.

Tohru Ohnuki
(erdferkel) - F

Locale: S. California
Re: Declination change with time on 06/05/2010 01:40:53 MDT Print View

Hi Kenneth, yeah, it's a Tom Harrison map of the Angeles and you can't easily see it but it does say 2007 mean declination. In point of fact, these compasses aren't better than a couple of degrees accuracy anyway. To do better you would need to break out the surveying tools or a Brunton Pocket Transit, and that's not exactly ultralight. Not to mention having to worry about local magnetic anomalies.

I've never needed to know more than vaguely where north is while hiking here locally either, the sun and the time does a reasonable job at that. If you're paying attention to where you're going and where that is on the map, then like Roger said, you only really need the compass to orient the map.

But in other terrain that's featureless or where landmarks can become obscured by fog, sand or trees, a compass would be much more useful. It's good to occasionally do the exercise of getting a fix on your location by triangulation. For that, an adjustable dec makes it easier.

Paul Davis
(pdavis) - M

Locale: Yukon, 60N 135W
Suunto MC-2G or Ranger Decl., why we carry compasses... on 06/08/2010 23:28:45 MDT Print View

Roger: I think the declination thing was answered with the excellent photo above---thanks all---both the Suunto MC-2G and the Silva or Brunton Type 15 (Ranger) compasses have a separate clear baseplate which can be swung around by means of a tiny rack and pinion gear, accessed by a tooth-pick style screwdriver on the lanyard, such that even the terminally-tired cannot screw up the declination...works like a charm!

My delay in responding is partially because I have just carpooled 500km N. of here at 64N Tombstone Terretorial Park on a wildlife-watching tour.

Anyways, not being the trip leader, and being sleepy, I left my survival kit in my tent, and did not (gulp!) bring the Suunto with me. +5C rain, fog, 2000M above sea level, lowering clouds.

Group leaders wanted to go up, did not perceive the visibility lowering, though I pointed it out.

Ended up lingering for lunch lower down with a botanist, saw the hikers higher up get swallowed by the clouds, so laid out the bright yellow foam sitz-pad, blew the whistle, and they vectored themselves in by sound.

None had a compass, nor a GPS, none had taken a back-up bearing on the micro-wave relay tower road which was our jumping-off point. This is a trail-less, featureless tundre landscape.

I had a GPS and an Iridium Satphone, so I used the GPS to generate a back-up bearing to get us back to the tower, once they got back to me, as I was below the cloud level.

Memo to self: 'No group hikes without carrying Suunto'!! Even if I am not leading!

Yes, this is why we carry them: dial the declination in, take an insurance bearing with the mirror on your start point, close the lid, and if the fog comes down, open the lid, put the N needle in the S end to create a reciprocal (go home) bearing, use the mirror to choose a bearing marker, even use a hiker 'no, bit to the left, good, stop!', then hike to them, repeat as neccessary...

Get out on the land!

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
re Adjustable declination on 06/09/2010 01:58:58 MDT Print View

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

Robert Blean
(blean) - MLife

Locale: San Jose -- too far from Sierras
Re: re Adjustable declination on 06/09/2010 06:30:33 MDT Print View

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

Edited by blean on 06/09/2010 06:33:46 MDT.

Daniel Goldenberg
(dag4643)

Locale: Pacific Northwet
Re: re Adjustable declination on 06/09/2010 08:12:06 MDT Print View

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.

Edited by dag4643 on 06/09/2010 08:13:59 MDT.

Chris Townsend
(Christownsend) - MLife

Locale: Cairngorms National Park
Adjustable Declination on 06/09/2010 08:27:26 MDT Print View

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.

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
The worst part of this on 06/09/2010 10:55:26 MDT Print View

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.--

Chris Townsend
(Christownsend) - MLife

Locale: Cairngorms National Park
Compass Use on 06/09/2010 11:27:06 MDT Print View

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).

Tohru Ohnuki
(erdferkel) - F

Locale: S. California
Navigating vs surveying on 06/09/2010 13:11:38 MDT Print View

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!

Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
Re: Re: re Adjustable declination on 06/09/2010 18:08:35 MDT Print View

"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.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Navigating vs surveying on 06/09/2010 21:09:34 MDT Print View

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

Tohru Ohnuki
(erdferkel) - F

Locale: S. California
Re: Re: Navigating vs surveying on 06/09/2010 23:37:34 MDT Print View

>> 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.