Hi Jonathan and all -
> We already have standard_error as a modifier. Is that would you mean?
Hmm. I don't actually think that the ADCP's error velocity is a true
a standard_error, but I may be confused by the way the term is
used in CF.
The ADCP mfgr (RDI) has a good description of their error velocity:
"Three dimensional velocity calculations assume that the beams are all
seeing the same flow field. Error Velocity evaluates how well this
assumption is being met, providing a quantitative base for QA/QC at
each depth layer of each ping."
The CF definition of standard_error is: "The uncertainty of the data
value. The standard error includes both systematic and statistical
uncertainty."
We use this term much more narrowly, i.e. the std dev / sqrt(N).
This value would be considerably different from the ADCP's
calculated error_velocity.
It might be worthwhile to either add a standard name "error_velocity"
which could be used for data from both 3-beam and 4-beam ADCPs,
OR a new modifier, "error" which would in theory be applicable to
any error that's not a standard_error; the second option might cause
more confusion, however, if CF's standard_error is really not a stderr
in the statistics sense of the term.
Thanks for any ideas on how to proceed
- Nan
P.S. RDI also has a slightly wordier definition in their glossary:
"Error Velocity: A key quality control parameter that derives from
the four beam geometry of an ADCP. Each pair of opposing beams
provides one measurement of the vertical velocity and one component
of the horizontal velocity, so there are actually two independent
measurements of vertical velocity that can be compared. If the flow
field is homogeneous, the difference between these vertical velocities
will average to zero. To put the error velocity on a more intuitive
footing,
it is scaled to be comparable to the variance in the horizontal
velocity. In
a nutshell, the error velocity can be treated as an indication of the
standard
deviation of the horizontal velocity measurements."
--
*******************************************************
* Nan Galbraith (508) 289-2444 *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 *
*******************************************************
Received on Fri Oct 16 2009 - 14:34:55 BST
