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


"Stephen Rainsbury" <stephen@xxxxxxxxxxxxxxxxxxxxxx -> wrote in message
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"GAGS" <absolutecrapgags.nw@xxxxxxxxxxxxxxx> wrote in message
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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!)

If you can see a nice long length of railway track on the map and you know
excatly where you are then put your compass next to it (but not too close)
and take a bearing.

Now measure teh angle on teh map and the difference is the error..

Well that's one way you can sort of do it. It's not the most accurate or
best way though! Doing it this way the best you can probably hope for using
a bog standard silva-type compass is to get the bearing within an accuracy
of 2-4 deg. If the intrinsic error in the compass is a misalignment of the
same order then you won't be able to say which is which.

I test all my compasses using my own standard. There is a hill not too far
away (sounds like a ruddy hymn!) from which there is a very good panorama
stretching round nearly 200 deg of horizon. In that view I have five key,
easily sighted, landmarks spread reasonably evenly around the horizon: a tv
booster mast, a beacon on an off-shore island, a cathedral tower, a
lighthouse, and a tall tv mast, all at various distances (the furthest being
10 miles). I load up a compass in a little device I've made which fixes it
flat between parallel plates with an optical sighting pin located above and
perpendicular to the direction of travel arrow (as a far sight) and one
nearer to the eye (near sight). I then line up the compass on each (or
sometimes just 3 depending on visibility) landmark and take the bearing
(repeated three times and averaged). For each of the landmarks and the
hilltop (which has a trig point and a column) I have a very accurate grid
reference (8 fig at least) gained from ref. sources and from personal
measure using GPS. From the BGS I also know the magnetic variation for the
location/area. It is then a simple matter of trigonometry to work out what
the actual bearings (corrected for magnetic variation) should be. These are
then compared with those I measured. The difference being the intrinsic
error.

But of course you already knew that seeing as I sent you a compass with
those error values (anyway I think I did)!

Of course there is the extrinsic error caused by it's actual use and the
person using it to add on to find the 'overall' error. This is usually
greater than the intrinsic error (which I've rarely found to be much more
than 1-2 deg, well within the acceptable tolerances).

I thank
you :-)

You're welcome!

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

Agreed, and in open country that attack point maybe the only feature for
miles, however as you said Liner features are better as they tend to be
more
accurate for some reason.


Linear features are better simply because there is more of them and they
point in two directions! An attack point is just a point feature. A linear
feature has size and direction to help you; a linear feature is a 'signpost
in the landscape'.

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

Not in Explorers any more :-)

It's okay for you still! The advice was aimed at yp under 18 drinking
alcohol. I don't think you're under 18 (?)

:-)

GAGS


.

Relevant Pages

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