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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: On potential error introduced by rounded values
From: George Huxtable
Date: 2005 Jan 12, 12:04 +0000

Apologies to the majority of list members, who will be quite uninterested
in what follows. Peter Fogg has introduced a certain acrimony into what
should have been an entirely technical discussion, and it needs to be

Originally, he had written-

"When a fix is expressed to whole minutes of arc it doesn't mean the boat
is at that intersection (it is almost certainly not, even if the fix is
entirely accurate). It means that the position is somewhere within a
rectangle (a square at the equator) bounded by the halfway points to the
next intersection of minutes of arc of lat/long."

And later he objected to my reply, which went as follows-

"I don't follow what Peter is saying here (or if I do, then I disagree).
The true position isn't necessarily in that one rectangle. You could only
state with certainty that the true position was in that one 1' rectangle if
you knew that the fix was in itself entirely accurate, in terms of both
observation and calculation, except for some final single rounding
operation that expressed the result to the nearest whole minute."

by saying- "Wonderful stuff! Disagree first, understand later!"

That was uncalled for. There's no shame, as I see it, in accepting that
one's understanding may be wrong.

==============

"Oh dear, George, you need to read the argument carefully before rushing
into print.

" ...even if the fix is entirely accurate). It means that the position is
somewhere within a rectangle..." to quote myself.

We are assuming that this fix is accurate. Being accurate means that the
position is somewhere within the rectangle indicated by the fix."

===============

And indeed, I had misunderstood his intention, but not what he actually
WROTE. Now he says "we are assuming that the fix is accurate". If he had
written that before, his intention would have been clear. But what he wrote
was "even if the fix is entirely accurate", not "only if the fix is
entirely accurate"; so what was written applied whether or not the fix was
entirely accurate. That was the matter I was addressing. I had read his
argument carefully, and answered it carefully, but he hadn't expressed it
as he intended.

It'a a minor matter, and I'm sorry to clutter the list with such trivia.
It's a pity when technical discussion becomes so personalised.

George.

================================================================
contact George Huxtable by email at george@huxtable.u-net.com, by phone at
01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
================================================================

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