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

<steviephilips@xxxxxxxxxxx> wrote in message
news:1149924948.345726.19540@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

There is no need for calculator that does trig to work out the
approximate error.

Of course one could use simple approximation.

Consider the 1 mile as the radius of circle, the circumfrance of which
will be 6.28miles.

That's okay. For small angles the arc length is not that much bigger than
the chord length.

If you're going to do that then as an approximate circumference 6 miles will
do.

If your 1 degree out the error is (1/360)*6.28=0.0174mile or 30.7yds.

Whoa there! Approximate! Most people will need a calculator for 1/360 *
6.28. Try calling it 6/360 = 1/60. Near enough.

Then its 1 mile = 1760 yards, but lets call it 1800 yards. Then 1800 * 1/60
= 1800/60 = 180/6 = 30 yards.

Thats close enough for small deviations.

And my very rough approximation didn't need a calculator at all and was only
0.7 yards different from yours.

The errors in this approximation as well as using arc length to approximate
lateral (linear) distance increase as the angle increases. (And also as the
distance travelled increases.)

So, I need to teach 2 deg now?

Its currently (from the data on the 2004 map) 2 deg 10 sec in the
centre of the central beacons sheet.

So yes, if you're using this map sheet!

Using 2 degrees is close enough for all practical purposes.

The 2 deg W magnetic variation varies across the British Isles. However, as
you say for most practical purposes, in most areas of Britain, a correction
of +2 deg to a grid bearing will suffice. In fact as most compasses in
common use have an accuracy at best of 2 deg, applying magnetic variation is
neither here or there and can be almost ignored these days.

However, in teaching map reading and compass use I would recommend that
leaders do not ignore it! Furthermore, rather than using an 'average across
Britain value' of 2 deg W, I would recommend that leaders get yp to adopt
the habit of looking in the key on the map for the magnetic variation given
for that particular map, apply any minor time correction, approximate to the
nearest degree, and then use that value for all bearings taken from that
map.

Good habits is what you want! What you don't want is for yp to get into the
habit of thinking that 2 deg W is the variation they should use all of the
time. (Just like Eddie using 8 deg W all the time!) A hike on an
international trip somewhere else in the world could be embarrassing.

As to using compasses in what you call 'lowland areas', I would say it
is
just as important as anywhere.

Yes. But not as important as being able to read and set a map.

Yes, what I really meant is that a serious navigation error in an
upland\mountain area could have life threatening consequences.

A serious navigation error even in an 'lowland' area could have life
threatening consequences!

I recall many years ago traversing the Crib Goch arete heading upwards in
summer conditions. There was a small group of teenagers and their leaders in
front of us. They stopped a few yards along the ridge to locate their
position. (Fine.) As I passed I heard the leader say how important it was
for them to now use their compasses to check their bearings and route along
the ridge. A little over-kill at that time methinks! A quick glance at the
map, a look around, the fact that they've just come up by way of the path
behind them, they can't go left or right, so that leaves....?

Upland/moorland (with fewer features) rather than hills or mountains (which
have quite a lot of landscape features) tends to be the terrain where more
navigational errors occur. In poor visibility I've always found it harder
navigating across the Kinder plateau compared to navigating round the
Snowdon Horseshoe.

The most 'serious' navigation error I recall making was many years ago on
the ODP between Monmouth and Pandy (Black Mountains) where I got 'lost' for
3 hours in terrain that was a confusing patchwork of fields, lanes, and
small paths with many changes of direction. I left behind a couple of pairs
of trousers that (very hot) day crossing barbed wire fences. Boy was I glad
to bump into a mole-catcher at about 9pm that June day who put me right
(just catching last orders at the pub where I was camping the night!)
Getting hung up or slashing myself on barbed wire that day could have been
serious! I now map read 'complex terrain' in areas such as agricultural
lowlands to the nm and concentrate all the time!

I walk quite a bit in lowland areas myself, I particullaly like coastal
paths. But navigational errors on a coastal path are unlikely to have
any serious consequences.

No, there is likely to be less chance of a navigational error on a coastal
path, but errors still can have serious consequences. You wouldn't want to
be walking along the cliff at say Beachy Head not knowing which is left or
right or north or south! There are a number of 'false trails' heading off
These un-made paths can be seriously 'fragile' near cliff edges.

GAGS

.

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