[Odonata-l] measurement

[Nick and Ailsa Donnelly]

Some people measure in the field for whatever purpose.  Others measure later
- in my case, with museum specimens.  If we were to use a scan to see what
people measure, it would largely defeat the purpose of the test,  My
question referred mainly to wing length.  I asked where people measure from;
i.e., "where is the base of the wing".  I did not get a clear answer to this
question, merely a repeat ("I measure from the base of the wing"). If you
take the wing base to be the line of flexure, as discussed by Tillyard, then
this line does not run at right angles to the long axis of the wing.  Where
then is the base?  By scanning wings and also a scale, I can superimpose the
scale image on the wing image and reproducibly get about 0.1 mm
reproducibility, but with my personally defined point for the base (the
costal end of the flexure).

I guess that the larger point is that we have no standards for measurement
at all, and that published measurements, whether in books or papers, must be
taken very warily.  I maintain that a single observer can measure repeatedly
and that his/her results will have internal value, say, to see whether two
groups of specimens are similar or different.  But these results probably
should not be then extended by someone else measuring his/her own specimens
and hoping to meld the results. 

There is no point agonizing over this, and I do not.  But we should realize
that all measurements have a built in operator bias.

-----Original Message-----
From: odonata-l-bounces at listhost.ups.edu
[mailto:odonata-l-bounces at listhost.ups.edu] On Behalf Of Andy Brazil
Sent: Tuesday, March 20, 2007 3:41 AM
To: odonata-l at listhost.ups.edu
Subject: Re: [Odonata-l] measurement

Perhaps we could experimentally discover individual biases in measurements?
If we had a scan of a dragonfly together with a scale rule posted on the
web, we could invite people to make the measurements as they usually do* and
post the results?  We could then determine the variation.

Andy Brazil

(*By measuring the image manually, using calipers or rule, rather than using
a digital method - then measure the scanned rule to find a scale factor for
an individual's monitor - thus reproducing field conditions)

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