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Got back from a business trip to the torrent of e-mails on variation,
amongst which Andrew Denman had written:

> so regardless of what convention is used (ie Easterly variation + or -) when
> Easterly variation is applied to true, the resultant magnetic heading will
> be less than true.  Is this the case?

Since that seems to have been lost amidst all the rest:


Andrew,

It isn't a matter of conventions, save for the convention of measuring
headings and bearings clockwise from some reference meridian.

Though hard to prove in an e-mail format, if you try sketching the
various possibilities on a sheet of paper, it is easy to see that if
magnetic north lies to the east of true north, the magnetic bearing MUST
be numerically smaller than the true one (unless it is smaller than the
true bearing plus 360 degrees, the latter being added to avoid negative
angles). Likewise, if a particular compass' north lies to the west of
local magnetic north, then a vessel's heading by that compass will be
numerically larger than its magnetic heading.

Re-label the variation and deviation how you wish, those relationships
must remain correct.


What would change them would be a reversion to the older way of
expressing angles in terms of quadrants. If the true course were not
"105T" but "South 75 East", the variation 16 degrees east and the
deviation 2 degrees west, how many among us could swiftly determine that
the compass course is South 89 East without first converting everything
to 360-degree notation? I could not. [And for those who can do such
calculations in their heads, try a true course of east by south a
quarter south, easterly variation of one and a half points and westerly
variation of a quarter point -- without either converting to degrees or
counting the quarter points around the rim of a compass card!]


Trevor Kenchington


--
Trevor J. Kenchington PhD                         Gadus@iStar.ca
Gadus Associates,                                 Office(902) 889-9250
R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555

                    Science Serving the Fisheries
                     http://home.istar.ca/~gadus

   

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