Old as Dirt!
Join Date: Nov 2008
Location: Tampa Bay Area
Thanked 150 Times in 142 Posts
Rep Power: 9
Re: Magnetic Variance
Don't make things unnecessarily complicated for yourself.
In general, magnetic variation arises because the position of the magnetic north pole of the earth does not correspond with the geographic north pole, which is centered at the axis of the earth's rotation. Hence, only on rare occasions will one's compass actually point "North" toward geographic north. Unfortunately, the variation of the alignment of the magnetic field at any given position on the surface of the earth is also influenced by local anomalies that arise due to the fact that the earth really isn't a smooth homogenous sphere but a rather a kind of lumpy round semi-sphere, squashed at the top and bottom, bulging in the middle; and, of inconsistent density. Factors that influence the shape of the magnetic field that surrounds the earth that your compass relies on.
Disregarding the foregoing, if you are located in a position where your chart shows a westerly magnetic variation, when your compass is pointing "North", it is pointing west of True North by the amount of variation to the west. Likewise, if you are in an area with an easterly variation, you are heading east of true north by the amount of the easterly variation.
For example, with westerly variation of 7º, if your compass is pointing "North" (0º or 360º), it also pointing 353º "True", relative to the grid on your chart (= 360º-7º). Likewise, with easterly variation, if your compass is pointing "North" (again 0º or 360º), it is really pointing 7º (= 0º+7º) "True". So, one "adds" east and "deducts" west.
Of course, one also has to account for "Deviation", the effect of one's ship on headings displayed on one's compass, which is unique to each boat and can only be obtained by "swinging" one's boat in a position with known true headings and making up a deviation table. With the deviation table, one adds or subtracts the known deviation on any given heading from the compass reading, then adds or subtracts the known variation, to get to one's true heading.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
Last edited by svHyLyte; 01-30-2014 at 02:04 PM.
Reason: correct typo