> > Depends on the format of the original.  0.1inch is far more
> > accurate than 2.54mm
> > because 0.1 = 0.100000..............
<snip>
> > Clive Broome
>
> By definition, 1 inch = 25.4mm exactly. This was standardised
> some decades ago.
>
> It's when rounded numbers are used that problems occur.
>
> John Haddy

I concur with the above definition. Unfortunately for users of Protel
however, Protel currently use an Internal Measurement Unit which is equal to
1 x 10**-7 inches. This means that imperial distances can be recorded in
*exact* form, but metric distances are (generally) recorded in *approximate*
form.

If the Internal Measurement Unit was alternatively set to some *metric*
distance, e.g. 2 x 10**-9 meters, then *both* metric distances and (most)
imperial distances could be recorded in exact form. E.g. a distance of 1 x
10**-5 inches could then be recorded as 127 Internal Measurement Units
(instead of the present 100 Internal Measurement Units).

Although the present Internal Measurement Unit is a pretty small distance, I
am sure that I am not the only user who has experienced the impact of
rounding errors. Pads originally positioned to a metric pitch can go
"off-pitch" (albiet by a tiny amount) if the measurement unit setting is
changed from metric to imperial and then back to metric again.

I have commented on this in the past, but the current thread has bought this
topic to the foreground again.

Regards,
Geoff Harland.
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