# Re: [PEDA] [PROTEL EDA USERS]: Confused Newbie on Footprints

```As the guilty party who triggered this digression, I feel constrained to
contribute.  The change evidently occurred when I was out of the country
(military stuff) and I failed to notice it for over thirty years.  My 1966
Handbook of Chemistry & Physics gives US Inch = 25.40005 millimeters and
British Inch = 25.39998 millimeters.  While my 1970 CRC Standard
Mathematical Handbook gives it as US Inch = 25.4 millimeters.  Since 99SE
offers the choice of "metric or imperial", just which one do they actually
use?  Anyone taking bets???

Bye from Duane Hague (no response required)

At 11:39 AM 3/6/2001 -0800, you wrote:
>At 12:23 PM 3/6/01 -0500, Steve Smith wrote:
>
>>I would never assume any dimension to be dead accurate
>>as you must allow for rounding off and tolerances.
>
>This is true for measured or specified physical dimensions.
>
>However, we should also know that 1 inch is *exactly* 2.54 cm. because
>that is the definition of an inch. So no error is introduced by using that
>number to convert from inches to centimeters or millimeters or vice-versa.
>Any error comes from rounding off the results, or from roundoff in the
>original figures.
>
>So .1 inch is really .1 +/- .05 inch, unless it is an abstract number, and
>it is thus 2.54 mm +/- 1.27 mm.
>
>As a printed circuit designer, I am very accustomed to inch units and
>don't like changing. I can tell you immediately the inch thickness of
>copper which is 1 oz. per square foot, but I'd have to think a while to
>come up with the figure in mm, and I don't even want to think of g/cm^2.
>But if I could start from scratch, I'd definitely go metric. All my
>science training, thirty years ago, was in metric.
>
>[EMAIL PROTECTED]
>Abdulrahman Lomax
>P.O. Box 690
>El Verano, CA 95433
>
>
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