**From:** Owen Jones (*oj@maths.anu.edu.au*)

**Date:** Tue 06 May 1997 - 04:13:57 EEST

**Next message:**Peter Metcalfe: "answers for bell"**Previous message:**Bill.McKinley@mailhost.dpie.gov.au: "Compasses"

Apologies to all those with no interest in geometry. Also, apologies to

Philip Hibbs for the undeservedly uncompromising tone of my response to

his initial posting: after a bit more thinking, I think I see where Philip

is coming from, and my own understanding becomes clearer.

Philip Hibbs and Trotsky are certainly correct to say that to pinpoint

your location you need more information than that given by two needles

pointing at two different locations. A third reference is needed, such as

that given by a third compass, or (for the lunars) the position of the Red

Moon in the sky, or (for the sun domers) an estimate of North based on the

sun (put a stick in the ground, when its shadow is shortest, the shadow is

pointing South, assuming you are in the Northern hemisphere, and that this

works in Glorantha). So, doing this at night in Prax would be difficult,

as it would be anywhere along the equator. The (unjustified) assumption I

made was that a Gloranthan compass would give you an angle from North

(say), rather than just pointing in whatever direction. In trigonometric

terms, to solve a triangle you need three bits of information. With just

two compasses you only get one angle and the length of the opposite side

(calculated from a map say). A third bearing gives you enough information

to work out the other two angles.

Back to the relatively straight forward investigation of stochastic

processes!

Owen

Owen Jones

Centre for Maths and its Applications, School of Math. Sciences

Australian National University, ACT 0200

Ph +61 6 249 2897 (office) 249 4552 (direct) Fax +61 6 249 4675

Web page http://wwwmaths.anu.edu.au/~oj/

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