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


"Stephen Rainsbury" <stephen@xxxxxxxxxxxxxxxxxxxxxx -> wrote in message
news:lv%ig.84454$wl.21698@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I recently read DofE report that a good navigator was doing well to be
less
than 10 degrees off course after 10km over open countryside.

10 deg off a straight line course is an awful lot of error. If my math is
right it's about 1.75 km from where one should be. I think a good navigator
would be pretty horrified to be so far off course! 5 deg off is still about
0.9 km off and even that would give a good navigator some concern.

I don't doubt the statement, however, as you've quoted it above it's largely
nothing more than probably someone playing around with numbers and is imo a
little stretched from reality.

It's well known that people don't walk in a straight line even without the
'help' of a few tinnies; we all tend to favour veering off to one side, mine
happens to be to my right. And that's on top of compass error! If I was
travelling across some featureless sand sea of a desert or some huge flat
as-far-as-the-eye-can-see salt pan, then I'm willing to believe the sort of
error or drift that you quote. However, I wouldn't be doing this sort of
trek with a Silva 3 compass! I'd more likely be using GPS for navigation
(and with that system you can forget error due to magnetic variation!)
Besides, I don't know of many DofE expeditions where a large proportion
(12.5% say) is done in one leg. Just not going to happen!

In reality most expeditions are done in countryside that is not featureless
and legs of the journey don't extend to 10 km, i.e one might cover 30 km in
a day, but this will be split into at least 5 or 6 legs of 5-6 km each or
even less. Even on each of those legs the party will most likely be
correcting for drift, if any, at least every km or so or when they 'collect'
passing features.

I can recall when I was a scout of 14 going on a night hike/compass exercise
across an area of the Denbigh moors in N Wales. It was pitch black (time of
new moon), the landscape really is devoid of good navigation features, most
of the near 10 mile route was cross country, not on paths, and it required a
lot of skill with the compass, a lot of fine contour interpretation, and
great concentration. We didn't travel very quickly; about 2 mph if IIRC. We
ended up about 1/4 to 1/2 mile off our objective. (We were also aiming-off
to ensure that we knew which way to turn when we reached the road.) I still
today consider ourselves to be a little lucky in getting that close and
maybe our error could have been as large as what you quoted, however, it was
exceptionally difficult terrain conditions and at the dead of night!

I have no idea
if this included deviation by 4degress is quit a significant lump of that.

We don't know, do we?


If its a big issue you could probably work it out yourself by finding a
decent length linear feature like a straight road or railway track.

Eh?????

(I think with all this early posting your brain hasn't warmed up yet!)

Let's say, for your convenience, I tested this out by walking from Chatham
to Sittingbourne or Faversham along a nice straight stretch (old Roman
road!) of the A2. I reckon I could get to the end with zero error. How? Just
follow the bloody road!! You can't test the theory by walking along a linear
feature because the linear feature will keep you on course. Why the 'eck
would you want to veer off course!? Bugger what the compass says! You can
really only test it by going out and finding a large enough piece of
featureless landscape. And that's not easy in Britain! You can't do the test
along a linear feature because you'll have so many factors affecting your
judgement causing your brain to sub-consciously correct for drift it just
won't be a fair test. People don't zig-zag their way down a road; certainly
not at this time of the night anyway!

The problem with the bare statement is that it doesn't tell us anything
about how 'good' the navigator is and what sort of equipment, if any, s/he
is carrying/using and how well they're using it.

Yes, I would agree that what this statement is really trying to say is that
dead-reckoning using a bog standard compass without any correction or
reference to landscape features is not easy and that you could well end up
with a course error of 10 deg or more in some circumstances and over a
distance this could be significant. I always instruct my yp that when
dead-reckoning with a compass they should not expect pin-point accuracy - in
fact to think so is stupid! Depending on the context, terrain, etc, a drift
of at least 5 deg or more could well be likely even for a good navigator.

The point is hammered home when one tries to use a compass on a
dead-reckoning to find a point feature in say a forest. (A common occurence
if one does a lot of orienteering!) We don't take a bearing dead-on the
point feature and set off! No, we look for an attack feature, best of all a
handrail (a linear feature) and aim off to one side such that when we reach
the attack feature we know which way to go to reach the point feature. Even
better, and in addition, we also look for a collecting feature beyond the
point feature such that if we reach it then we know we've gone too far. We
know that there's going to be drift when crossing open country so that's why
we always look for attack points, helpful handrails, collecting features and
use aiming off techniques. And when we set off on the leg we'll
traffic-light our way to the objective (fast start - green - slow down as we
get near - amber - stop (almost) - when we reckon we're close).

Of course one could just say 'bugger the drift' and open a few more cans and
wait until someone finds you! :-)

GAGS



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