[CF-metadata] positive attribute expansion of use and reserved values

Jonathan Gregory j.m.gregory at reading.ac.uk
Wed Sep 27 17:45:14 MDT 2017

Dear Ken

Thanks for your email. I sympathise with the problem, but I don't agree with
the proposal. Actually similar suggestions have been made before.

It's an important principle with the standard names that they always indicate
their sign convention. This is so that, if a standard name is provided, the
sign convention is unavoidably specified; if the sign convention were not in
the standard name, but in another attribute e.g. positive, it is certain that
it would be omitted sometimes by mistake. If the sign convention were specified
in the standard name *and* another attribute, it is certain that they would
sometimes be inconsistent by mistake. Either mistake would make the data less
usable. You mention cases where the wrong standard name is given. I agree, that
makes the data less usable too, but I'd say the solution to that is to fix the
data, when the problem has been identified; we should not have to modify the
convention in a way which would make it generally more error-prone. You also
mention cases where you don't have a standard name but you do need a direction.
Presumably you must have some other information in that case about what the
quantity is - which attribute are you using? If it's the long name, you could
put the sign convention in there, for example, as for standard names. New
standard names can also be requested for instrumental quantities, and the
direction doesn't have to be "up" or "down" in standard names. As you probably
know, there are already standard names containing away_from to indicate their
sign convention, just as you suggest.

I agree that there is sometimes a need to know how to relate quantities with
opposite sign conventions in their standard names. The standard names are
mostly systematically constructed, but for use by humans; they aren't designed
to be parsed by machines. If there is a need for some means to deal with this,
I would favour recording the sign convention as a machine-readable extra piece
of information in the standard name table. If we put it in the table, it must
be consistent with the standard name; you'd just have to look it up, instead of
trying to extract it from the standard name.

Best wishes


----- Forwarded message from "Kehoe, Kenneth E." <kkehoe at ou.edu> -----

> Date: Wed, 27 Sep 2017 22:49:20 +0000
> From: "Kehoe, Kenneth E." <kkehoe at ou.edu>
> To: "CF-metadata at cgd.ucar.edu" <CF-metadata at cgd.ucar.edu>
> Subject: [CF-metadata] positive attribute expansion of use and reserved
> 	values
> CF-metadata,
> I would like to propose an expansion of the use and reserved values for the “positive” attribute (section 4.3), specifically to include values in addition to the two reserved values of “up” and “down” to include “towards” and “away”.
> Most variables define direction in the standard_name, but with instrumentation a standard name is often not available or the correct definition of the name exists with the wrong positive direction. Also, needing to understand or extract direction from the standard_name can be difficult for a simple tool not wanting to review the standard_name definition just to see if a transformation is needed. Expanding the use of the “positive” attribute would reduce the number of standard names by not needing to include positive direction in the definition. This would also follow the recommendation of being consistent with the definition between the standard_name and positive attribute. The “positive” attribute is currently reserved for use with coordinate dimensions only, but the same logic can be used with data variables to indicate direction. For example rate of speed for vertical velocities could be described by indicating positive = “up” when vertical velocities are positive when moving away from the surface.
> Instruments are also often not installed perpendicular to the surface, and the coordinate system is better described as towards or away from the instrument. Specifically for radial instruments. Errors in misunderstanding direction with radial velocities or accelerations are comment when not specifically defined. There is no vendor standard.
> I’ll leave my suggestion at towards and away, but this could also be expanded to include cardinal direction for East-West/North-South directions.
> Thanks,
> Ken
> Kenneth E. Kehoe
>   Research Associate - University of Oklahoma
>   Cooperative Institute for Mesoscale Meteorological Studies
>   ARM Climate Research Facility - Data Quality Office
>   e-mail: kkehoe at ou.edu<mailto:kkehoe at ou.edu> | Office: 303-497-4754 | Cell: 405-826-0299

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----- End forwarded message -----

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