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Subject: The One In Sixty Rule (was: Re: going overboard on decimals)
From: Thomas Schmidt (schmidt@XXX.XXX)
Date: Wed Jul 28 1999 - 11:07:12 EDT
Millard Kirk wrote:
...
> After the GPS
> showed a bearing of 1 degree difference from our course I was wondering how
> much off course that represented, without plotting of course. He made a
> statement that we were only 1 nautical mile off course. I was able to
> understand from his discussion that may be true if one is at least 60 miles
> from his destination one (1) degree would calculate about that. I have
> since run across a rule "The One In Sixty Rule" although not fully
> explained, but I think it is what he was referring to.
...
Being a hopeless landlubber, I am not familiar with these rules, but it
would make sense.
If your two courses (ideal and real) differ by one degree, then the
distance d between them at a distance D from where they intersect (be
that your destination or your departure point) is
d = D * tan( 1 deg )
= D * 0.0175
= D / 57.3
which roughly translates to an error which is about one sixtieth
of the distance i.e.,
one km at 60 km distance or
one nm at 60 nm distance or
two nm at 120 nm distance etc.
Furthermore, since we generally have as an approximation for small
angles alpha (in degrees)
tan( alpha ) = alpha * 3.1415/180 = alpha * 0.017453 = alpha / 57.296
the above rule generalizes to
d = D * tan( alpha )
= D * alpha / 57.296
So for a course deviation of 2 degrees, we have an error of two
sixtieths of the distance; for three degrees, three sixtieths
of the distance etc.
This breaks down if the angles become too large since we used an
approximation which is valid for small angles only. The exact
numbers would be:
1 degree off: d = D * 1 / 57.290
2 degrees off: d = D * 2 / 57.273
3 degrees off: d = D * 3 / 57.243
4 degrees off: d = D * 4 / 57.203
5 degrees off: d = D * 5 / 57.150
10 degrees off: d = D * 10 / 56.713
20 degrees off: d = D * 20 / 54.950
30 degrees off: d = D * 30 / 51.962
40 degrees off: d = D * 40 / 47.670
etc.
So as a rule of thumb, if you are one degree off course and at a
distance of 60 nm from your destination, your lateral displacement
is 1 nm. For other distances, adjust the result proportionately.
For other course deviations, multiply with the number of degrees
you are off.
You get your result in the same length units you used for the distance
(nautical miles or kilometers or fathoms... :)
The rule starts to break down when your course deviation becomes too
large (because then '60' is no good approximation any more) and when
the distance becomes too large (because then the above formulas from
plane trigonometry cease to be applicable).
Sorry if the above lacks the appropriate lingo, but my Bowditch is
at home...
-- ------------------------------------------------------------------- Thomas Schmidt e-mail : schmidt@XXX.XXX
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