Measurements

[Jon Nazareth]

I was measuring the length of some frames just now which are 229" as prototype but when I multiplied this by first metric and then imperial, I ended up with two lengths that don't compare.  If 229 is multiplied by 1.13mm I get 258.77mm but when I multiply by .044", I get 10.076 which is quite different from the former.  If my calculations are correct, I find it a bit worrying.  Can anyone explain why there's such a difference?  The 1.13 and the .044 are from the G3 Conversion Tables.

Regards
Jon

[John Candy]

Jon,

By my calculation (if I have understood your dimensions correctly) the scale length of 229 inches is 257.62mm in G3.

229 / 12 =19.0833ft
19.0833   x 13.5 = 257.62mm

Regards,
John.

[John Candy]

Jon,

I have just downloaded the conversion table to which you refer but the factor for 12ins. is 0.0531 not 0.44 ....again, if I have understood correctly, it appears you have misread the figure from the table.

Regards,
John.

[Jon Nazareth]

On the drawing, the length of the frames was shown as 229" so, I multiplied by the metric and imperial equivalent of 1" hence 1.13mm and .044" and I ended up with two different lengths.

Regards
Jon

[IanT]

I'm not sure where your numbers are coming from Jon (G3 conversion tables?) but I suspect it's probably 'rounding' errors in the conversion factors/tables being used.

In G3 the "Rules of Thumb" used to convert to scale are either 13.5mm/ft (which John used) or 17/32nd/ft - very much depending on your preference for the metric or imperial measurement systems. Neither are actually 'exact'.

For the benefit of anyone lurking out there - let's look at the numbers involved.

All you really need to remember when "scaling" is 22.6. This is the exact ratio between the full sized gauge (56.5") and the one we use (2.5").

So 229" would scale to 10.1327" exactly (that's 229/22.6). If you want that in metric then simply multiply by 25.4 (10.1327 = 257.37mm).

However, whatever 'rule of thumb' used - the results will be very similar. Let's look at that.

So if 229" is 19.0833ft.

Using 13.5mm/ft would give us 257.625mm (or 10.1427")
Using 17/32" that would give us 10.13802" (or 257.506mm)
Using 1:22.6 we get 10.1327 (257.37mm) - and this is the exact conversion.

So using 13.5mm gives an error of 10 thou (over +10") and using 17/32 gives an error of 5 thou (over +10") - so either would be perfectly fine to use in practice.

However, these days personally I just divide by 22.6 - it's very easy and it's also exact.

As I've said before - G3 has never had any issues or conflicts where correct 'scale' is concerned (not since we gave up 1/2" anyway).

However, I sometimes amuse myself by thinking of 13.5mm/ft as being "G3 Coarse", 17/32nd/ft as being "G3 Standard" and 1:22.6 as being "G3 Finescale" - but then with less than 10 thou between all three over a 10" model - maybe I'm just being really picky!

:-)

Regards,

Ian

[IanT]

PS John - Don't worry - I'm just having a Tea Break

I'll be back at the drawing board shortly!!

IanT



Glad to hear it ...... got to keep the "Sentinel" on track!

John.

[John Candy]

Jon,

I have looked again at the conversion table and the factors you used.
As Ian surmised, it is "rounding" of some entries to two places of decimals which has caused the problem (many entries extend to 4 places) and the smaller the measurement (in your case one inch) the larger the potential error. The table was published in the form provided by a G3S member ......... I have never used the table and nobody has previously raised the matter of accuracy.

This casts doubt on the usefulness of the table in its current form..... so it may be that the download should be removed.

John

[Jon Nazareth]

John/Ian

Thank you for the replies and yes, I'll stop using the conversion chart.

I've printed off your replies and will start scaling the proper way from now on 


Regards
Jon