|
|
Subject: Re: A different variation tidbit
From: George Huxtable (george@XXX.XXX)
Date: Wed Feb 13 2002 - 17:02:04 EST
Yves Arrouye said-
>Still involving French and Bowditch...
>
>Bowditch (1995) says:
>
>Deviation: Angle between the magnetic meridian and the axis of a compass
>card, [...]
>Variation: Angle between the magnetic and geographic meridians at any place.
>
>And my French text book (Cours des Glenans, 1990) say:
>
>Declination: Angle between the magnetic and geographic meridians at any
>place.
>Deviation: Angle between the magnetic meridian and the axis of a compass
>card, [...]
>Variation: Sum of the declination and the deviation
>
>Interesting difference, using the same "variation" word, isn't it?
>http://www.geolab.nrcan.gc.ca/geomag/e_magdec.html mentions that marines use
>(magnetic) variation instead of magnetic declination.
>
>YA
=================
Also, Dave Weilacher still shows an interest in this topic.
Reply from George Huxtable-
I have done a bit more research. I think it's rather more complicated than
Yves has suggested.
I have many French landmaps at 1:25000 (TOP 25 series), on each of which is
shown a small diagram showing geographic North (N.G.) and magnetic North
(N.M.), and the angle between them. The angle is also given in numerical
form as (in one example) 4 deg 46 min, and, being French, also in grads.
The direction of magnetic North is clearly shown on that diagram as being
to the West of geographic North, (which of course it is, throughout
France). But the numerical value of the difference between them, 4deg
46min, has no label (W) or sign (+ or -) attached to it to define its
sense. And the quantity is referred to as "la declination magnetique".
In the French-designed MLR SP24 GPS receiver in question, it's possible to
change the language between English and French (and there are other choices
too, but not American). In English, the quantity referred to is labelled
"compass var.". That is the quantity for which, if you want a magnetic
course to be given correctly, and if your chart states that the variation
is 10 deg West, Andrew Denman has discovered you actually have to input "10
E" and vice versa. And the manual gives not the slightest clue about this.
However, if the instrument's chosen language is changed to French, that
same label changes from "compass var." to "deviation compas", though the
quantity and its label (E or W) remain the same.
It seems that in French, not only is there a different word (declination)
for what we call in English "variation", but also in French the word
"deviation" has quite a different meaning from our own. In English, the
deviation is associated with a vessel, changes with a vessel's heading, and
could not be used as a constant compass-correction. The French use of
"deviation" in this context is incompatible with the definition quoted by
Yves-
>Deviation: Angle between the magnetic meridian and the axis of a compass
>card, [...]
So when the French label this quantity "deviation", and not declination, I
speculate that it might be what we call the "variation" and they call the
"declination", but IN THE OPPOSITE SENSE, West exchanged for East. If so,
that would explain a lot. But it wouldn't excuse the folly of using the
English label "compass var." for that quantity in the Anglophone world.
Anyone know enough French to resolve this puzzle?
George Huxtable.
------------------------------
george@XXX.XXX
George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
Tel. 01865 820222 or (int.) +44 1865 820222.
------------------------------
|