Tuesday, Feb 17, 2009 at 00:54
Thanks - I do try!
For Kiwi Kia's benefit.....I prefer not to debate those who disagree with me - thats your prerogative - eveyone should be discernig of what they read on the internet IMHO and try to conform the info from independent sources if they are at all doubtfull.
[quote]An interesting detail is that there are northern- and southern-hemisphere compasses. This has to do with the fact that the magnetic field lines, to which a compass needle aligns, point into the earth at the north and south magnetic poles. In the northern hemisphere the north end of the needle is pulled downwards, and the south end is counterweighted to balance the needle. When you use a northern hemisphere compass in, say, Australia, the south end of the magnet is pulled downwards by the magnetic field, and is also heavier than the north end - resulting in a needle that catches and drags on the bottom of the compass housing when the compass is held horizontal.
[/quote]
Source: -
http://bushwalk-tasmania.com/forum/viewtopic.php?f=15&t=1258
[Quote]
Compass Zones
In order to get accurate readings from a compass, the needle must be balanced in the vial so that it does not drag. Due to the components of the Earth’s magnetic field, a compass that works in the United States will drag or stick in Brazil. To remedy this problem, the compass industry has developed five compass zones (see zone map below). Here is a list of the major countries in each zone.
Zone 2: Mexico, Central America, Panama, Columbia, Venezuela, Northern Africa
Zone 3: Peru, Bolivia, Brazil, Central Africa
Zone 4: Paraguay, Uruguay, Southern Argentina, New Guinea, Southern Africa
Zone 5: Australia, Antarctica, New Zealand
[/quote]
Source:
http://www.labsafety.com/refinfo/ezfacts/ezf313.htm
And so it goes!
As it happens the lass from the USA who had the zone 2 silva left it with me when she left to go
home, so I now have the pair to compare (in storeage at the moment) so for anyone in WA who wants to physically "compare the two side bye side, I'll do my best to get them both out of storeage, next time i'm down in Mandurah at the storeage shed.
I seem to recall the Bream fishing commuity went thru a series of problems with auto pilot electric outboards of the Monn Kota variety - when they First were mported into Australia!
Being made in Japan for the American bass fishing tournament fishers - it came with a zone 2 compass card, which here in Oz = would "stick/rub on the backing" - and simply couldn't drive a straight line as a result!
Many were sent back under warranty before "someone at Minn Kota in Japan "finally twigged" that the built in compass in tyhe unit was at fault.
Once fixed the Minn Kotas now are favourites with a Lot of Aussie bass fishers!
Theres a LOT more to compasses than many realise....
That was the point of my post!
Re holding course and ignoring devation and only applying variation -
well lets just say i wouldn't have passed navigation doing that, - maybe in a 4x4 fair enough but not at sea!
Yes you also have to correct for set and drift at sea as
well in offshore nav, but then you would also be using a sextant to get your declination etc, and thats a whole lot harder nav than coastal with just variation and deviation to contend with over the shorter distances involved .
The "test" chart for Nav exam is the whitsundays off Queensland - because there are so many magnetic anomalies to contend with as
well as a LOT of islands and shoals etc, which will REALLY sort out the wheat from the chaff in manual navigation, due to the complexity.
That one chart has 3 different compass roses printed with 3 different variations all within some 30 odd nautical
miles or so - due to the influence of such things as magnetic island etc!
I couldn't in all conscience suggest anyone ignores magnetic deviation and just alows for variation, UNLESS - the compass deviation chart has very minimal values for that specific compass.
There are some that are quite accurate - my own compass in my vessel is ONE of the few that seems to not require any correction the last time the compass adjuster tested it - I got a "nil" deviation result in 8 directions which s quite unusual according to the adjuster.
In a scenario like that -
well of course one can ignore deviation.
It's less likely to get that in a 4x4 because theres so much MORE ferrous metal (my boats aluminium hence non ferrous) and electronics and theres less deliberate design to keep such sources away from compass mounting positions.
On land - if your good at your land marks and positioning - then deviation mightn't be of much conecern - whereas at sea there are few if any visible landmarks on occasion, to continually correct
course, so over longer distances and a course madeup of several different legs - the error is compounded in magnitude with increasing distance.
In plotting such courses usually a clearance of 1 Nm is allowed to major obstacles......now it doesnt take a LOT of compounded position errors thru ignoring deviation on a compass that has large deviation in some directions to end up on rocks or worse at night.
On land the same might be true for example in a long distance desert crossing where all dunes begin to look alike...
It's horses for courses.
If for example you don't understand deviation or variation and are contemplating a long remote dessert crossing, relying on just GPS?....... maybe a couple days tuition in nav mightn't go astray if things turn bad and the GPS is on the fritz for whatever reason.
Going back to long distances at sea and set and drift correction.
These corrections can be large, in areas with large tidal movements, strong winds, and large sea state condtions (sea & swell).
One trip 54 Nm across to the Houtman Abrolhos off WA's mid west -I plotted a manual course out of
Geraldton Harbour - aiming to go about a mile north of
Beacon island at the Wallaby group and into little pigeon Island. After about 50
miles - using the manual course, (and watchng the chartplotter) the conditions were so bad we were 6 Nm south of Goss passage - instead of 1 Mile north of
Beacon Island - due to set & drift, requiring reverting to the second course I'd plotted up thru Goss Passage and around to Little Pigeon!
Of course it's easey with a GPS - but doing it the old fashioned way by compass and time / speed, takes a LOT of skill and practice & attention to detail.
It's best to stay familiar with the manual method and use the GOS for convenience - asstated beforetheGPS is really only to conform what you already know it SHOULDN'T be your primary means of navigation.
Letting my number 2 son take the helm (to avoid him getting seasick - it works) was in part responsible for our large set and drift error, but at least I KNEW thathe wasn;t following the plotted course close enough - had a backup course plotted in case - and then still had the GPS chartplotter as the final
check to get us there safe n sound which I did!
Its does show you tho - just how important it is to get it right at sea, - land nav tends to have a LOT more land marks along the way to confirm your position a lot more frequently and correct any cross track error (set & drift) as it occurs!.
Bad steering alignment, a dragging brake pad etc (dunes at a tangent to the desired direction of travel) all might tend to effect the ability of a 4wd operator to steer a straight course in offroad conditions in the dessert.
Any practice you can get is worthwhile IMHO as ot much beats practical experience.
Hitting an offshore island the size of a needle in a haystack, 50+
miles offshore is hard enough even with modern GPS asistance.
Finding a fuel dump in the middle of the dessert with no roads to guide you might be almost as hard!
Now surveyors - they are a breed apart, likely they will get you there to within a centimeter or two - just that it will take all bloody week for them to do it right!. LOL
Cheers
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