Compass work.

From: Lorne D. Booker (booker@baynet.net)
Date: Tue 06 May 1997 - 03:43:14 EEST


Philip Hibbs and Owen Jones,

I thought I would mention a few aspects of RW maritime navigation on the
off chance that it may prove illuminating.

I'd like to say at the outset that I don't disagree with either of you
since I 'm not entirely sure about the nature of this discussion. I only
vaguely remember the contents of the original post that lead to this
discussion so I cannot comment on it intelligently. That is, I have read
your recent posts out of context so my understanding may be clouded as a
result. Therefore, I'm not really in a position to agree or disagree.

Anyway here it goes.

> So, assuming a compass works by pointing in a straight line to some
> reference, if you are on two lines given by two compasses, your
> position is uniquely determined. The only time you will have problems is
 > when your compasses give the same line.

In navigational terms this (problem, that is, two reference points laying
on the same line) is referred to as a transit. An artificial transit is
often erected on shore to mark a dredged channel. In this case a vessel
need only keep the transit aligned properly, by moving the vessel
appropriately, to be assured of safe passage through the channel.

It (the transit) is a very useful device for finding your position provided
that you have a third reference point.

In any case the more usual - though less efficient - practice is to take a
"fix" - or to fix your position - by taking a bearing on three reference
points that bear some degrees from one another. It is desireable to keep
the angle between your reference points broad, that is in the order of 120
degrees or as close to it as you can manage.

Typically, your bearings to various reference points are somewhat in error.
 (Even good compasses can't give you a perfect bearing on object far away.)
 These errors may be much less than a degree off but when that error is
translated into several thousand yards the error is projected. (This is
compounded in sea navigation by the fact that the vessel is moving,
pitching, rolling, yawing, heaving and, possibly drifting.) The result is
what is called a "cocked hat." That is, a triangle composed of the three
bearings that you have taken. Your position is presumed to be somewhere
near the middle of the hat. A smaller cocked hat indicates greater
accuracy and is desireable.

The long and the short of it all is three bearings are better.

> two will tell you that you are somewhere on a circular line between the
two > compass points.

Are you presuming that Glorantha (or the world in question) is round? I
don't recall reading the original article so I don't quite understand what
you mean by this. As I recall though Glorantha is assumed to be flat.
 Maybe the God learners or the dwarves - with their advanced technology -
know otherwise.

Also if your landmarks are (relatively) close together, or if your world is
very large, the effects of curvature upon your world could be negligible.

Booker

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