Re: Yet more Compasses

From: Lewis Jardine (jardine@rmcs.cranfield.ac.uk)
Date: Thu 08 May 1997 - 17:41:30 EEST


Thomas wrote:

> =

> Please, all who believe two compasses are not enough do this :

I'm one of them (still)!

> 1.Take a map of Glorantha (or a piece of paper) and mark two point A an=
d B on it.

Yes, I've done this in my previous post. =

> 2.Now determine what direction compass A and B point to.

Relative to what? =

This I believe is the crux of the problem. You appear to think that =

just because you have a map you know which way up it should be...
The normal way people use a map is to use a (single) compass to find =

magnetic north and then orient their map using this bearing. =

They then look for two prominant features and take bearings on =

them (relative to magnetic north) and find the intersection. =

This is why people keep coming back to the point about the angle =

between the two compasses. That is the angle of one needle relative =

to the other one which is something you can determine. =

> 3.Draw a line through point A with the same direction as the compass sh=
owed. =

> 4.Do the same with compass B.

Again how? I don't know how to orient the map. =

> 5.Your lines meet in one point, this is the point where the two
> compasses show your before determined directions. =

> =

> For those who know something about vectors :
> The two directions of the compasses can be seen as two different vector=
s. =

I believe that vectors also have a magnitude property, yours only have
direction. =

I agree that if you knew the magnitude you could find your position, but
there =

has been no discussion any valid methods to obtain this (actually, with
just 2 =

magnitudes you can narrow your position down to two possible locations =

(intersection of 2 circles). =

> With the point A and B you can make both lines. =

Hey, I thought we were talking about vectors? =

They are not tied to one location (unlike a line). =

> In a plane two different lines define one exact point !

> You can only measure the angle between the compasses ? NO! What you
> absolutely don't need is the angle, you need the directions. And they
> are absolute, =

Only if you can measure them relative to some fixed datum (like magnetic
north). =

> since they are the same in the same place. =

> Even if you turn 180=B0 they stay the same (Please do the little exerci=
se above!)

If you are hodling the map firmly, =

the needle points in the same (absolute) direction, =

however the map turns with you, now your method places you =

on the opposite side of the point! =

> Again, this works only if you have a map.

The map is completely irrelevant to this discussion as it does not help
you. =

All you are using is a plain piece of paper with two points marked a
given =

(scaled) distance appart. =

If you were using the map to sight on another fixed point I would
agree. =

However, the maths involved would still be non-trivial as it would still =

be very difficult (maths) to calculate the return-bearing from the
point. =

> I hope I have not to post that often as I had to say that Telmori are n=
o
> chaotics !
> =

Only if you persist in this line of arguement. =

Please see my previous post for a mathematical demonstration, =

which could be turned into a rigorous proof with a little effort. =

Cheers
        Lewis

PS> I'll be at the German Glorantha Con if you wish to tackle me about
this issue there.

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