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 Post subject: Silva 70 un compass
PostPosted: Mon Aug 30, 2010 9:59 pm 
forum fan

Joined: Sun Jun 21, 2009 6:31 pm
Posts: 19
Location: Guaratuba, Brazil
I'm about to purchase the above item from the UK. It will be a perfect fit on my Kleppers.
I have read that some compasses need adjustment to work properly in different parts of the world. I would be using this in the US, UK and soon in the south of Brazil. Can anyone shed light on this adjustability question? And has anyone got experience with this compass, I think it used to be available as an option with Klepper Germany?

Best Regards,

John

Klepper A2 frame with Polish skin
1975 Klepper A2
2002 Klepper A1


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 Post subject: Re: Silva 70 un compass
PostPosted: Mon Aug 30, 2010 11:57 pm 
john wrote:
I have read that some compasses need adjustment to work properly in different parts of the world. I would be using this in the US, UK and soon in the south of Brazil. Can anyone shed light on this adjustability question?

It's called magnetic declination http://en.wikipedia.org/wiki/Magnetic_declination - to put it simply, compass needle deviates from where it should point. The error can be up to 20 degrees. Any magnetic compass is prone to this error. Many compasses include an adjustment ring to compensate this error, but you should know the amount of this adjustment for your area of paddling.

The bottom line is - map is usually right. Compass might or might not be.

Silva website suggests Expedition 15 TDCL or the Voyager series as models with this compensation.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 5:56 am 
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john wrote:
I have read that some compasses need adjustment to work properly in different parts of the world. I would be using this in the US, UK and soon in the south of Brazil.
It is not differences in declination you need to worry about, it is the differences in inclination: http://www.unc.edu/depts/oceanweb/turtles/geomag.html

Most compasses manufactured to produce a level compass card in the northern hemisphere will not give a level card in the southern hemisphere because the inclination is reversed below the equator, put simply. Go here for more details on that: http://www.wide-screen.com/support/FAQsuunto.shtml

See next message for continuation.

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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 5:56 am 
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As you might expect, some compasses are more successful than others in dealing with this. Go here for Suunto's MC-2G Global Navigator, an "orienteering" compass which gets around the problem: http://www.wide-screen.com/Suunto/global.shtml

The Silva 70 is a rugged unit. Can't tell from any of the links I looked at whether it is designed to keep the card level at varying inclinations.

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Folbot Kodiak, Cooper, and Edisto; three hardshells; Mothership: Surf Scoter the Bartender; dinghy Little Blue Duck.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 1:16 pm 
krudave wrote:
As you might expect, some compasses are more successful than others in dealing with this. Go here for Suunto's MC-2G Global Navigator, an "orienteering" compass which gets around the problem: http://www.wide-screen.com/Suunto/global.shtml

The Silva 70 is a rugged unit. Can't tell from any of the links I looked at whether it is designed to keep the card level at varying inclinations.


I have the Suunto MC-2G. It solves the problem by having a non-magnetic needle and a separate gimballed magnet attached to the needle, and hence is not latitude specific and can be used globally . It is far superior than trying to use a similar, but northern-hemisphere compass, in the southern hemisphere.

Having said that the Silva 70 being asked about is a very different style of compass, it doesn't use a needle but the whole ball moves. Even if this is not corrected for global use it will be less of a problem than with the needle type compasses for two reasons. i) The 'horizon' on the compass will simply dip to the south a bit, and who has a level horizon on a kayak all the time anyway. ii) the increased swinging mass will probably have a stronger levelling moment and so be less susceptible to changes in inclination anyway.

My thought is that for a kayak deck compass, if you can live with the horizon not being quite level, you'd be fine with one that is not latitude specific to your location, but for a handheld sighting compass you really want to find one that is designed for your latitude.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 1:57 pm 
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Suunto sells some binnacle-style compasses which solve this problem completely. Places to look: the D-135. F-135, and D-165 units are the ones, if you want to spend the money; it would be cheaper to have two inexpensive kayak deck compasses, one for a northern hemisphere zone and one for Brazil. Links to ponder: http://www.tacktick.com/catalog/29
http://www.suunto.com/en-us/Support/FAQ ... Compasses/

The first one should lead to dealers and to more product detail.

As to use of a ball-style compass set for a northern hemisphere zone, in a southern hemisphere zone, good luck with that. When the dip goes from +20 degrees to -20 degrees, that is a 40 degree difference. The compass ring may be nearly invisible, and the jeweled pivot likely will stick.

YMMV

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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 3:34 pm 
krudave wrote:
When the dip goes from +20 degrees to -20 degrees, that is a 40 degree difference.


If you're getting that 40deg number from the Suunto MC-2G spec, that number is the difference across the whole globe from northern Canada to southern New-Zealand, and is specific to that one model of compass. I can confirm that using a zone-1 MC-2 compass in zone 4 was horrible, hence my purchase of the global MC-2G.

But he's not asking about that compass, and he's only going as far south as Brazil - it will be a lot less than 40 degrees. The tilt will probably be quite visible, but since a deck compass is designed to be used while not being held truely horizontal, I doubt it will stick.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 6:02 pm 
henry wrote:
...using a zone-1 MC-2 compass in zone 4 was horrible, hence my purchase of the global MC-2G.

But he's not asking about that compass, and he's only going as far south as Brazil - it will be a lot less than 40 degrees. The tilt will probably be quite visible, but since a deck compass is designed to be used while not being held truely horizontal, I doubt it will stick.

He is asking about error, which is both magnetic declination caused by magnetic deposits in particular area, and magnetic inclination (which is hemisphere-specific). Assuming a northern-specific ball compass will still be visible in Brazil - I don't know whether this Silva 70un has any adjustments of the bezel to compensate for magnetic declination. $15 baseplate compass for hiking does have it, and this is what I use ;-) ...


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 9:04 pm 
Alm wrote:
magnetic declination caused by magnetic deposits in particular area

actually magnetic declination is 99% caused by the magnetic axis not being in the same place as the geographic pole. local anomalies are a pretty minor component.

All compasses can be used anywhere whatever the magnetic declination, you just have to know how to use it. Either mentally add/subtract the declination, twist the little bezel, or take a magnetic bearing directly off your chart.

The point that Dave was quite correctly making is that despite this, MANY compasses won't work at all if taken to the wrong latitude.

The real question is whether the OP should spend extra cash on a second compass for Brazil. My guess is that this compass will still be usable, albeit less than perfect, and if it's a one-off trip that is not going to be dependent on accurately paddling a compass bearing, then he should spend the money sampling more of the delights of Brazil instead.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 10:24 pm 
henry wrote:
if it's a one-off trip that is not going to be dependent on accurately paddling a compass bearing, then he should spend the money sampling more of the delights of Brazil instead.

Yes, and it won't hurt to add (to $90 ball compass) a cheap baseplate compass that can be adjusted permanently (for the duration of the trip) to magnetic declination of Brazil. Global compass is expensive. If it's for one and only trip to southern hemisphere, for that price one could probably hire a guide with a motorboat (and a compass on it ;-) )

Quote:
Either mentally add/subtract the declination, twist the little bezel, or take a magnetic bearing directly off your chart.

The bearing taken directly off the chart will be the angle between your planned course and true north. But the compass doesn't point to the true north - instead it points to magnetic north which is few degrees error from the true north. And then, yes - you either have to do the math mentally, or adjust the bezel to proper magnetic declination once and forever (to the end of the trip) and free your brains for more useful or pleasurable tasks.


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 Post subject: Re: Silva 70 un compass
PostPosted: Tue Aug 31, 2010 11:33 pm 
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Alex,

You usually pick up stuff a lot quicker than this.

Henry and I keep saying it is the inclination of the magnetic field which may make a Zone 1 compass unusuable (or, not very usable at all) in Zone 4. But, you keep coming back with issues relating to declination.

What gives?

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Astoria, OR
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Folbot Kodiak, Cooper, and Edisto; three hardshells; Mothership: Surf Scoter the Bartender; dinghy Little Blue Duck.


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 Post subject: Re: Silva 70 un compass
PostPosted: Wed Sep 01, 2010 2:18 pm 
forum fan

Joined: Sun Jun 21, 2009 6:31 pm
Posts: 19
Location: Guaratuba, Brazil
Many thanks to you all for the information, needless to say the Silva is on hold. This is a much deeper subject than I first thought, so your discussions are very interesting, entertaining & sometimes baffling. In three months I am retiring to the south of Brazil so I am now looking for a specific compass for this area and at a later date one for the northern hemisphere.
Thanks again.
John


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 Post subject: Re: Silva 70 un compass
PostPosted: Wed Sep 01, 2010 2:43 pm 
lord high faltbotmeister

Joined: Thu Sep 06, 2007 8:05 am
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Location: atlanta, georgia
Inclination and declination are often used interchageably, but they are not the same. Declination relates to the difference between true and magnetic north. Inclination relates to the angle with which the magnetic field intersects the earth. For the latter I don't think there is any error: the angle is zero at the poles and ninety degrees at the equator. I don't even know what inclination is useful for, but I'm sure somebody out there knows.

Best,
g

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 Post subject: Re: Silva 70 un compass
PostPosted: Wed Sep 01, 2010 4:38 pm 
Quote:
I don't even know what inclination is useful for, but I'm sure somebody out there knows.

As Dave and Henry described this, the result is that a ball compass made for northern hemisphere will be difficult to see (and possibly - even use) far south of equator. At the latitude in the middle of Brazil this problem might will not be too severe yet. It is also my understanding that a baseplate military compass with deep well design can be used globally and at the same time cheaper than global ball compass.


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 Post subject: Re: Silva 70 un compass
PostPosted: Wed Sep 01, 2010 4:58 pm 
John,

Silva is one of the biggest manufactures of small compasses in the world - not just for the northern hemisphere. I would suggest that you contact them directly to see if they make this model for Zone-3/Zone ME, which is the appropriate zone for Brazil (although southern Brazil is on the edge of the zone, so you might want to get Zone-4/Zone SME if your extended paddling area is more likely to go south to Patagonia rather than north to the Amazon).

Then ask them where to find a dealer in South America that stocks them. Since you're going to be living there you might as well get the right one.


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