Tom,
As an old "old timer" I cannot resist the temptation, with tongue firmly in
cheek, to take you up on the meaurement of the diameter of a rod. Whether
it is accurate or precise, I do not know, but to me a rod is a lineal
measurement, and does not have a diameter.
72 points = 1 inch
3 barleycorns (in length) = 1 inch
12 lines = 1 inch
12 inches = 1 foot
3 feet = 1 yard
6 feet = 1 fathom
*** 5 1/2 yards = 1 rod, (pole or perch). ***
40 poles 1 furlong
8 furlongs = 1 mile
3 miles = 1 league.
Square poles were used in my working lifetime for the measurement of
small plots of land (typically allotment gardens), but I expect they are all
in this new metric system. Very confusing. As no doubt you know :-
1 sq. pole = 30 1/4 sq. yds.
40 sq. poles = 1 rood
4 roods = 1 acre.
etc., etc..
I have never come across lines and barleycorns, but I came across a
reference to them some years ago which claimed that the last time lines had
been used as a standard measurement was was in an Air Ministry specification
for the size of the brass buttons on Royal Air Force uniforms, and that a
barleycorn, a third of an inch, still ingers on - tho' not perhaps by name
now - as the measure still used in boots and shoes. The difference
between a size ten and a size ten and a half shoe, it was said, is NOT half
the difference in size betweena a 10 and a 11, but is a size 10 plus a
barleycorn - a third of an inch!
Apologies for any momentary distraction from more essential items, such
as sundials.
Rod Eden
----- Original Message -----
From: Tom Semadeni <[EMAIL PROTECTED]>
To: Phil Pappas <[EMAIL PROTECTED]>
Cc: <sundial@rrz.uni-koeln.de>
Sent: 29 April 1999 20:12
Subject: Re: "accurate" vs. "precise"
> Yo John!
> Old Timers?!
> Elderly?.....or on old time?.... or been on the list for a while?....or?
>
> My distant history tells me this:
>
> Precision usually means the number of significant figures used to report a
> measurement.
> Accuracy usually means how close your reported measurement is to the
"truth".
>
> Try this scenario:
>
> Let's say that the diameter of a rod is measured, using accurate laser
> interferometry, to be 0. 50148 cm.
>
> You measure the diameter with a micrometer using its vernier scale and
report the
> diameter (A) as 0.5025 cm .
> You then use a coarse pair of calipers and report its diameter (B) to be
0.5 cm.
>
> Measurement A is MORE PRECISE because it implies a preciseness of
measurement
> 1000 times, or 3 orders of magnitude, more precise than Measurement B.
>
> But measurement A implies that the "true" value of the diameter is between
0.
> 50245 and 0.50255 cm while measurement B implies that the "true" value of
the
> diameter is between 0.45 cm and 0.55 cm.
> So measurement B is MORE ACCURATE since the "true" value lies within the
implied
> limits of the B measurement and outside of the A measurement limits.
> (There is a systematic error in the A measurement of about 0.001 cm,
probably due
> to a non-zeroed anvil measurement.)
>
>
> Dial accuracy is affected by the accuracy of placement and orientation of
all of
> the components.
> Dial precision is affected by the clever use of geometry, scaling, and
> interpolation lines to aid the eye to read the edge or body of a shadow
> precisely.
>
> John Shepherd's beautiful dial at:
> http://www.uwrf.edu/sundial/welcome.html
> demonstrates a clever trick to increase the precision of an accurate dial.
The
> trick is shown explicitly at:
> http://www.uwrf.edu/sundial/images.html
> where the good Professor shows three close-ups taken sequentially at 9:58,
10:00
> and 10:02 am Central Daylight Time.
> The eye/mind is very good at extrapolating to a point, much better than
finding
> and reporting the edge of a fuzzy shadow!
>
> The short and long of it, John, is that you shouldn't use "accurate",
"precise"
> and "old timer" without proper explanation!
>
> It seems that there was a discussion on this list a long time ago about
tricks to
> help the eye get increased precision of dial time. There must be some
rules of
> thumb about shadow distance, contrast, color etc. that would help us in
our quest
> for precision (assuming that our dial is free of the normal errors!). I,
for
> one, would appreciate some pointers (pun) on the topic of enhancing
precision.
>
> Cheers, and congratulations at being at the proofreading stage of your
Manual.
> Good stuff.
> t
>
> Phil Pappas wrote:
>
> > Hello Old Timers:
> >
> > I've got another knit-picky question for you all to ponder. But you're a
> > rather knit-picky group, so I don't think you'll mind.
> >
> > In proofreading the new fifth edition of my "Sundial Owner's Manual",
when
> > discussing sundials, I think that I mistakenly used the words, "precise"
and
> > "accurate", interchangeably, as if they meant the same thing.
> >
> >
> > John Carmichael
> > Tucson
> > tel: 520-696-1709
> > website: http://www.azstarnet.com/~pappas
>
> --
> Tom Semadeni O
> [EMAIL PROTECTED] o
> aka I (Ned) Ames .
> Britthome Bounty ><<((((*>
> Box 176 Britt ON P0G 1A0
> 'Phone 705 383 0195 fax 2920
> 45.768* North 80.600* West
>
>
