Re: [PEDA] MM's VS Mils

```On 08:41 PM 15/03/2001 +1100, Les Grant said:
>On 14 Mar 2001, at 23:14, Abd ul-Rahman Lomax wrote:
>
> > To be sure. But I think Mr. McGrath missed the point. No one is
> > arguing that one must work in metric. Rather, the issue is the native
> > database units. We, as users, do not necessarily see these units. If
> > they were in metric as has been suggested, we could have exact
> > conversions; if they are in inches, conversions will always be
> > approximate.
>
>Perhaps I have missed something here but are you implying that
>conversion from metric is exact but conversion from imperial is
>approximate? 1 inch = 1000 mil = 25.4mm exactly, both ways.
>
>Regards,
>Les.

As Geoff Harland says (and I know he is not around to say it himself):
"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)."

Thinking another way 25.4 = 254/1000 = 127/500 so as long my base unit is
500 times smaller than the desired precision in inches I can represent the
same number exactly in mm.  What would be the effect on maximum panel size
for this change - assume we are using a signed 32 bit integer to represent
the workspace we get 2e-9*2^32 = +/-4.3 metres - that will do me
thanks.  And we have perfect precision in any units to 1e-5 inches -
that'll do me as well.

Ian Wilson

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