NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Magnetic Variation. (has been: Magnetic Declination)
From: Doug Royer
Date: 2004 Feb 13, 10:57 -0800
From: Doug Royer
Date: 2004 Feb 13, 10:57 -0800
Thanks for the well thought out response George.Yes,I see my thinking was narrow as I thought only of what I am used to useing or doing. However,a few questions/comments are still in order. Keiran's statement below still has me interested.I will wait for him to reply and explain what he was trying to relate. >Keiran stated he has a way to find local variation useing " a >sextant,compass and amplitude". Your points below are noted. In such a vessel, measuring the total compass error is equivalent to measuring the local magnetic variation, if the deviation can be neglected. The same applies to Kieran Kelly, travelling overland in Australia with a (non-magnetic) camel, though he needs to look out for the knife, gun or billy-can that he keeps in his tucker-bag down by the billabong. Yes,differant cargoes will effect the compass.So that is why some cargoes are stowed for transit in differant sections of the hold or on deck.Or the same cargoes are split up and stowed in differant areas. The bigger vessels I'm familiar with have 1 magnetic compass,1 main gyro compass and many gyro repeaters placed throughout the vessel. And the deviation can change, for other reasons than a change of dip. A lightning-strike can have an instant effect. Some cargoes can disrupt the deviation: the most notorious being railway-lines. What effect does a full load of containers have on the compass, I wonder; or a loading of tanks, on a landing-craft? Some idiot may have placed a loudspeaker near the master compass. The initial magnetisation of a new vessel can diminish in its first months. The below is correct. When a navigator today measures total compass error, variation plus deviation, from (say) a Sun amplitude, he usually has a good local value for variation from his chart. What he is trying to obtain is a good figure for the deviation of his compass, on a particular course, to compare with his deviation card or to confirm the accuracy of compass correction. If he makes a habit of doing this, on various courses, it will (or perhaps won't) give him confidence in the accuracy of his compass courses, or warn him it's time to call in a compass adjuster, or perhaps find that bit of steel that some fool has left in the binnacle. Yes sir,the below is true to a point.If the measurement is taken with a hand held compass at differant points of the vessel the deviation will change.That is why,I believe,we were taught to not take into consideration the deviation of the hand held magnetic compass(to a point,as you stated the dev. can be large).I've never taken an amplitude or azimuth useing a hand held compass while on board.Always done with the either of the wing or midship gyro repeater. Doug asks whether the compass error should be measured with a hand bearing-compass or with the ship's standard compass. On his steel ship, my guess is that there's nowhere on board that's immune from the magnetic distortions that its steel causes, so there's nowhere to take a hand bearing-compass where it would give sensible readings. In his case the only useful comparison is of the standard compass with astronomical azimuths, or with well-known transits of landmarks on his chart. I always have questions! I hope this has answeed Doug's question, but if not, I hope he will ask further. George. ================================================================ contact George Huxtable by email at george@huxtable.u-net.com, by phone at 01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. ================================================================