Thank you for all the responses, it sounds like 'all of the above' is
the preferred response to my suggestions of plausible next steps. I
will pursue all of these.
Eizi's point about having no_unit in udunits is sound; I suggest we
request udunits use
'no_unit'
as a representation of
'?'
such that the behaviour is consistent; 'no_unit' should always raise
an exception when used in the udunits processing rules, exactly as
'?' does.
With regard to meaning, I have found the wikipedia entry useful:
http://en.wikipedia.org/wiki/Dimensionless_quantity
`In dimensional analysis
<http://en.wikipedia.org/wiki/Dimensional_analysis>, a *dimensionless
quantity* or *quantity of dimension one* is a quantity
<http://en.wikipedia.org/wiki/Quantity> without an associated
physical dimension
<http://en.wikipedia.org/wiki/Dimensional_analysis>. It is thus a
"pure" number, and as such always has a dimension of 1.^[1]
<http://en.wikipedia.org/wiki/Dimensionless_quantity#cite_note-1> '
which it has sourced from
"*1.8* (1.6) *quantity of dimension one* dimensionless quantity"
<http://www.iso.org/sites/JCGM/VIM/JCGM_200e_FILES/MAIN_JCGM_200e/01_e.html#L_1_8>.
/International vocabulary of metrology --- Basic and general concepts
and associated terms (VIM)/. ISO
<http://en.wikipedia.org/wiki/International_Organization_for_Standardization>.
2008. Retrieved 2011-03-22.
This is a good enough source for me.
I will wait to give space for more comments, then, if people are
content, I will raise a change request with udunits.
Assuming this is accepted and processed I will raise a change request
for CF to add some text to 3.1.
Finally I will request a change for any standard_names which appear
not to follow this approach (I have only 'area_type' so far).
I hope this seems like a reasonable response.
------------------------------------------------------------------------
*From:* Eizi TOYODA [toy...@gfd-dennou.org]
*Sent:* 31 October 2014 08:44
*To:* John Graybeal
*Cc:* Hedley, Mark; CF Metadata List
*Subject:* Re: [CF-metadata] string valued coordinates
Hi John
> I think '?' is not a definition that is helpful to most users -- it
is more like an indication that the string -- the empty string in
this case for example -- has not provided a meaningful indication of
what the units are.
I share the same impression. I was thinking it would be nicer for
maintener of udunits. We should ask modifying udunits so that it
would refuse processing "no_units" otherwise ut_multiply("no_units",
"no_units") returns "no_units 2". If I remember right the unit
string "?" causes immediate error, so we don't have to change udunits.
But I'm okay if the majority here agrees that sort of thing is not a
responsibility of udunits.
Best,
Eizi
Best Regards,
--
Eiji (aka Eizi) TOYODA
http://www.google.com/profiles/toyoda.eizi
On Fri, Oct 31, 2014 at 9:45 AM, John Graybeal
<john.grayb...@marinexplore.com
<mailto:john.grayb...@marinexplore.com>> wrote:
Thanks for summing this up so neatly Mark!
We could take the view that the conventions would benefit from
the addition of some text into 3.1 to explicitly make the point
about quantities which are not dimensioned or dimensionless.
We could alternatively defer to udunits as most unit questions
do, which already exhibits this behaviour, and just patch the
'area_type' and any similar names with erroneous canonical units.
We could also request that udunits be updated with a clearer
string for this case, given the need for it, such as including
the term 'no_units' as a valid udunits term to mean there are no
units here: this is not dimensionless, this is not dimensioned.
Why is the first option exclusive to the others? Seems useful to
improve the documentation regardless.
So I agree that '1' makes no sense for area_type. I'm wondering
if someone can crisply describe what is meant when we (or
UDUNITS) say a unit is dimensionless? I'm not entirely sure I get it.
In any case, I think '?' is not a definition that is helpful to
most users -- it is more like an indication that the string --
the empty string in this case for example -- has not provided a
meaningful indication of what the units are.
So my ideal solution has CF well aligned with UDUNITS, and a
clear concept and definition. Which I think suggests asking
UDUNITS for a term 'no_units', defined as "the values do not have
units; values are neither dimensioned nor dimensionless."
John
On Oct 30, 2014, at 11:06, Hedley, Mark
<mark.hed...@metoffice.gov.uk
<mailto:mark.hed...@metoffice.gov.uk>> wrote:
> The unit of '1' is generally used to indicate fractions and
the like. In cases where I am storing a raw binary value, I
leave off the units attribute, as the 'number' isn't something
that should be treated as a decimal quantity.
This is the same behaviour as I was looking to adopt, but CF 3.1
makes this incorrect, I believe, as a lack of a units attribute
is to be interpreted as a units of '1'.
I think a clear way to define that a quantity is not dimensioned
and is not dimensionless is required. I would have liked to use
the lack of a unit for this purpose, but this has already been
taken, so something else is needed.
>My preference is that one explicitly puts in the units. For
dimensionless, "1" or "" is ok for udunits.
udunits2 treats '1' and '' differently.
a unit of '1' has a definition of '1'
a unit of '' has a definition of '?'
The CF conventions description of units (3.1) states that an
absence of a units attribute is deemed to be equivalent to
dimensionless, a unit of '1'. This is the convention, and it
has been in force a long time.
However CF makes no statement that I can find regarding a unit
of ''. Thus I believe we defer back to udunits, which CF states
is how units are defined. Udunits states that a unit of '' is
undefined, the quantity is not dimensioned and is not
dimensionless. We could adopt this to use for the cases in
question.
>area_type is given in the standard_name table as having a unit
of 1. It is a categorical string-valued quantity.
On the basis of the discussion, I would suggest that this is an
error. If area_type is a categorical string-valued quantity, it
is not dimensionless, it is not continuous and numerical, it is
not a pure number and should not be treated as such. I think we
should fix this.
We could take the view that the conventions would benefit from
the addition of some text into 3.1 to explicitly make the point
about quantities which are not dimensioned or dimensionless.
We could alternatively defer to udunits as most unit questions
do, which already exhibits this behaviour, and just patch the
'area_type' and any similar names with erroneous canonical units.
We could also request that udunits be updated with a clearer
string for this case, given the need for it, such as including
the term 'no_units' as a valid udunits term to mean there are no
units here: this is not dimensionless, this is not dimensioned.
I don't mind which route is preferred, I'm happy to put a change
together and pursue it; whichever way seems better to people.
cheers
mark
------------------------------------------------------------------------
*From:*CF-metadata [cf-metadata-boun...@cgd.ucar.edu
<mailto:cf-metadata-boun...@cgd.ucar.edu>] on behalf of Jim
Biard [jbi...@cicsnc.org <mailto:jbi...@cicsnc.org>]
*Sent:*30 October 2014 16:12
*To:*cf-metadata@cgd.ucar.edu <mailto:cf-metadata@cgd.ucar.edu>
*Subject:*Re: [CF-metadata] string valued coordinates
CF says that if the units attribute is missing, then the
quantity has no units.
The Conventions document, section 3.1 says:
The|units|attribute is required for all variables that represent
dimensional quantities (except for boundary variables defined
inSection 7.1, "Cell
Boundaries"<http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#cell-boundaries>and
climatology variables defined inSection 7.4, "Climatological
Statistics"<http://cfconventions.org/Data/cf-conventions/cf-conventions-1.6/build/cf-conventions.html#climatological-statistics>).
and
Units are not required for dimensionless quantities. A variable
with no units attribute is assumed to be dimensionless. However,
a units attribute specifying a dimensionless unit may optionally
be included. The Udunits package defines a few dimensionless
units, such as|percent|, but is lacking commonly used units such
as ppm (parts per million). This convention does not support the
addition of new dimensionless units that are not udunits
compatible. The conforming unit for quantities that represent
fractions, or parts of a whole, is "1". The conforming unit for
parts per million is "1e-6". Descriptive information about
dimensionless quantities, such as sea-ice concentration, cloud
fraction, probability, etc., should be given in
the|long_name|or|standard_name|attributes (see below) rather
than the|units|.
The unit of '1' is generally used to indicate fractions and the
like. In cases where I am storing a raw binary value, I leave
off the units attribute, as the 'number' isn't something that
should be treated as a decimal quantity.
Grace and peace,
Jim
On 10/30/14, 11:35 AM, John Caron wrote:
My preference is that one explicitly puts in the units. For
dimensionless, "1" or "" is ok for udunits. If the units
attribute isnt there, I assume that the user forgot to specify
it, so the units are unknown.
Im not sure what CF actually says, but it would be good to clarify.
John
On Thu, Oct 30, 2014 at 2:37 AM, Hedley,
Mark<mark.hed...@metoffice.gov.uk
<mailto:mark.hed...@metoffice.gov.uk>>wrote:
Hello CF
> From: CF-metadata [cf-metadata-boun...@cgd.ucar.edu
<mailto:cf-metadata-boun...@cgd.ucar.edu>] on behalf of
Jonathan Gregory [j.m.greg...@reading.ac.uk
<mailto:j.m.greg...@reading.ac.uk>]
> Yes, there are some standard names which imply string
values, as Karl says. If the standard_name table says 1,
that means the quantity is dimensionless, so it's also fine
to omit the units, as Jim says.
I would like to raise question about this statement.
Omitting the units and stating that the units are '1' are
two very different things;
dimensionless != no_unit
is an important statement which should be clear to data
consumers and producers.
If the standard name table defines a canonical unit for a
standard_name of '1' then I expect this quantity to be
dimensionless, with a unit of '1' or some multiple there of.
If the standard name states that the canonical unit for a
standard_name is '' then I expect that quantity to have no
unit stated.
Any deviation from this behaviour is a break with the
conventions. I have code which explicitly checks this for
data sets.
Are people aware of examples of the pattern of use
described by Jonathan, such as a categorical quantities
identified by a standard_name with a canonical unit of '1'?
thank you
mark
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