Re: Maps! (let's have a seperate thread, instead of using roll call)


<steviephilips@xxxxxxxxxxx> wrote in message
news:1149892510.312470.261940@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Hang-on a minute!

Is all this precise magnetic variation really nessessary?

Depends on what you're doing and where you are.

If you're using an orienteering map then it's totally unnecessary!

I've been telling my scouts "4 divisions anti-clockwise" for 30 years or
more.

Well, 30 years ago the deviation was around 8 degrees, or "4 divisions"
since most compasses are 2 degrees per division.

Interesting that above you called it variation and now you've called it
deviation! In certain circles (e.g. nautical) these can mean different
things.


These days the deviation is around 2 degrees.

Tut, tut, tut.......

That is not quite correct.

(Think: is your statement complete!)


So come-on you experts....
If I set a compass at 4 div ( 8 deg ) anti clockwise.
How far out would I be from where I should be after walking 1 mile?

As most maps (for leisure activities) are now drawn to metric scales you
really shouldn't be using imperial measures! :-)


About 185 yards.

But if you did....this would be correct!

Want to know how he did this Eddie? Simple trigonometry. Use of Cosine Rule.
Think triangle with 2 sides of unit length (miles, kilometres,
bananas...whatever!) with included angle as the variant angle. The length of
the side opposite to the included angle (y), i.e. the distance (x) between
the end of one unit length and the end of the other unit length is given by:
x = sqrt(2(1-cos(y)))

(A simplification of: x^2 = 1^2 + 1^2 - 2 (1)(1) cos(y) )

For 1 degree variation/drift/error then x = 0.0175 units (where units are
whatever you've chosen them to be, i.e 0.0175 km (or 17.5 metres) or 0.0175
miles (or 31 yards) or 0.0175 bananas - about a very tiny nibble!)

For 2 degrees then x = 0.0349 units (i.e. 0.0349 km or 35 metres using
metric units or 0.0349 miles or 0.0349 x 1760 = 61 yards)

For 6 degrees then x = 0.1047 units (i.e. 0.1047 km or 105 metres using
metric units or 0.1047 miles or 0.1047 x 1760 = 184 yards)

For 8 degrees then x = 0.1395 units (i.e. 0.1395 km or 139.5 metres using
metric units or 0.1395 miles or 0.1395 x 1760 = 245 yards)

And so on...


I would be more than happy, indeed extatic, if my scouts ( and Explorers)
used their compasses to check the direction of ROADS; to turn left or
right
and more seriously to decide on the correct fork of paths in a forest.
Should I really care tuppence about magnetic variation?

Depends on what you're doing!

In most scenarios as you describe above...no! If you come to a T-junction
and want to know whether to turn left of right then you don't really need a
compass, just make sure that you're map is oriented (set) - and as far as a
T-junction goes you really only need to make sure that your map is the right
way up!

In most practical map-reading situations - certainly in Britain - the most
common route choice decision will be one where the direction change is
generally a lot more than the magnetic variation. Furthermore, for the
compasses and the length of route legs that you're likely to be using the
current mean magnetic variation for most of Britain is less than the
practical compass bearing accuracy you're likely to achieve.


In lowland areas really accurate compass work is not really required,
although it should be encouraged.

I'd go further and say that in most areas - even a lot of upland areas -
it's not really needed. The primary aid to navigation is the map; the
compass is only a secondary aid. In a feature rich landscape there should be
little difficulty in correctly and sufficiently orienting (setting) a map by
observation of the landscape and features. Plateau, feature-less moor or
poor visibility is when the compass really becomes a major aid to
navigation.

This does not mean that compass work should be ignored or given low
priority!


As you suggest, in these areas your using the compass to identify the
right path etc, and a few degrees out is of little consequence.

Correct. Keep the compass available though as even here it's a valuable
check.


However, in upland mountain areas and in bad visibility an error of 185
yards per mile could be a problem.

Absolutely!

GAGS


.

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