[EMAIL PROTECTED] wrote:
>
> Wayne,
> How do you figure it could convert back either way? If we agree that the
> primary source of conversions in the U.S. is the Big Green Book, then 4.70 is
> 15-5 and 4.71 is 15-5 1/2. The confusion came when it was originally (and
> incorrectly) reported that she cleared 4.71/15-5 1/2.
>
> Walt Murphy
>
> Walt Murphy
You're right. This has not been a good week for me on this list, has it.
That's not what happened in this case, but two Imperial measurements certainly
can convert to the same metric mark. For example, 15-5 1/4 converts exactly to
4.70535 and 15-5 1/2 converts exactly to 4.7117. Both would round to 4.71 if
rounded to the nearest centimeter which is what I think the Green Book does. If
it rounds to the lower whole centimeter then 15-5 3/4 converts exactly to
4.71805. In that case 15-5 1/2 and 15-5 3/4 both convert to 4.71. You must
have these double conversions because a centimeter is larger than a quarter of
an inch. If you always measure originally in metric and then always use your
Green book to convert you will never have a problem, because in the example
above 4.70 will convert to 15-5 and 4.71 to 15-5 1/2. You will never get a
converted mark of 15 5 1/4. You can in this case convert back without error.
This is not true if the measurement is made originally in Imperial, there will
be ambiguity 36.5% of the time. A mark of 15-5 1/4 converted to metric and then
back to imperial would wind up as 15 5-1/2.
You also get in trouble when you try to convert exactly to the lower whole least
unit of measurement in each system. 4.71 converts to approximately 15-5.43.
Rounded to the lower whole 1/4 inch this would be 15-5 1/4. But as we have
seen, 15-5 1/4 is exactly 4.70535 and the lower whole centimeter is 4.70.
Unfortunately, people do not always measure originally in metric and convert
only with the Green Book.
I find it much easier to keep my records in metric only; I have never owned any
of the various colored books.
By the way, we are already on the metric system in a sense in this country. The
foot is defined by Federal law as 0.3048 meters.
We have probably beat this subject to death. I think it's time to move on.
--
Wayne T. Armbrust, Ph.D.
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