Here is how to measure a metre.

http://www.npl.co.uk/about/history_length/page09.html

"The metre was defined as "the length of the path travelled by light in a
vacuum during a time interval of 1/299 792 458 of a second" "

The definition of an inch is derived from this. 1 inch = 0.0254 metres. Note
it was only during the 2nd world war that the Amrican Inch became equal to
the British inch. This was only found out when aircraft wings made in the US
had problems fitting to bodies made in the UK. Maybe protel are still using
the pre-war inch, which could account for the rounding errors ? (NOT)

I agree the rounding errors in Protel PCB between metric and imperial are
really annoying, making things fall off grid. Many a time I have editted a
pad/part/track when I see 29.999999mm and think "Hey this should be 30mm"
and accidently go and edit it to 30.9999999mm. Grrrrrrrrrrrr.

Ian


> -----Original Message-----
> From: Ian Wilson [mailto:[EMAIL PROTECTED]]
> Sent: 02 August 2002 00:47
> To: Protel EDA Forum
> Subject: Re: [PEDA] OT Metric vs Imperial
>
>
> On 12:08 PM 1/08/2002 -0400, vincent mail said:
> >John Ross wrote:
> >
> >>Metric or Imperial does not make any real difference (except individual
> >>preference) except when having to convert.
> >yep agree
> >
> >>I think the point is that Protel has never handled Metric properly due
> >>to, what I assume is shoddy conversion due to database limitations.
> >>Metric support in the SCH for rules definition was skipped altogether
> >>until DXP (NOT).
> >>
> >>Not all imperial values do work out to round numbers due to the
> generous
> >>tolerances that manufacturers apply to their values in data sheets. Try
> >>measuring a 20 mil (0.5mm) device and see!
> >>What is missed in the argument is that we will have rounding errors
> >>either way unless the s/w and/or database supports BOTH systems equally
> >>or not at all, and that includes the rules system.
> >well there isn't a real conversion to start with . most people say that
> >25.4 millimters equals an inch , that is if you round it yes. i seem to
> >recall the ral number has lake 5 or 6 digits behind the comma. and then
> >you get into floating point problems.
>
>
> 25.4 is exact by definition - hence the advantage in the base unit being
> some small fraction of a mm.
> http://lamar.colostate.edu/~hillger/everyday.htm
>
> Ian Wilson
>
>
> ************************************************************************
> * Tracking #: FC3F289EAA968542BF100D301767B4D76671FF00
> *
> ************************************************************************
>

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