Hi.
I agree with Mark. In the case where the coordinate system does not correspond to 'x = longitude, y = latitude', such as a polar stereographic system, x and y are clearly identified by the definition of the coordinate system, and it makes sense to allow a standard name to identify data variables as being the x and y components of a vector quantity. Furthermore, x and y only have meaning in relation to a coordinate system definition. If that coordinate system is simply WGS84 longitude and latitude on the ellipsoid, then x = east and y = north. If the coordinate system is a local cartesian system that has no reference to fixed points on the Earth, then x and y only have meaning as orthogonal coordinates in that frame, and no connection at all can be made to east or north, yet a vector field in the coordinate system still has x and y components. (This situation arises sometimes in real-world photogrammetric applications, for example.)
Grace and peace,
Jim
Jim Biard
Research Scholar
Cooperative Institute for Climate and Satellites
Remote Sensing and Applications Division
National Climatic Data Center
151 Patton Ave, Asheville, NC 28801-5001
jim.biard at noaa.gov
828-271-4900
On Apr 19, 2012, at 7:50 AM, Hedley, Mark wrote:
>
>> The problem I have with what you are proposing is that we would then potentially have two different standard names for the *same* quantity, and until such time as we have a way of handling that properly, we ought not do it.
>
> I do not feel uncomfortable about this. I feel that this situation already exists in a number of situations.
>
> For example, there are ocean models which use a tri-polar horizontal model grid, with 2 northerly poles and a normal south pole. The consequence of this grid for vector components is as follows:
>
> - in the southern hemisphere:
> - x == eastward
> - y == northward
> - in the northern hemisphere
> - x != eastward
> - y != northward
> - the amount of discrepancy changes as either of the northerly poles is approached
>
> The Ocean models I know of write out vector components as x-<standard_name> and y-<standard_name> so in some locations x is east and in other locations it is not
>
> I suspect there are other datasets created where x may mean east.
>
> I feel it is a better compromise to allow x to mean east and encourage data consumers to be careful in interpreting vector components. I believe that https://cf-pcmdi.llnl.gov/trac/ticket/79 will help data consumers with this.
>
> mark
>
> -----Original Message-----
> From: Bryan Lawrence [mailto:bryan.lawrence at ncas.ac.uk]
>
>
> The problem I have with what you are proposing is that we would then potentially have two different standard names for the *same* quantity, and until such time as we have a way of handling that properly, we ought not do it.
>
> Cheers
> Bryan
>>
>> Hello Bryan
>>
>>> Sorry, silence doesn't mean consent.
>>
>> I didn't think it did, but prodding that notion can encourage people to pitch in.
>>
>> My reasoning is that I do not think it is the responsibility of the standard name to define what is meant by x. The initial parts of the definition in the table: '"x" indicates a vector component along the grid x-axis' ... ', positive with increasing x'; say everything there is to say.
>>
>> It is the responsibility of the coordinate and grid_mapping variables to define what 'x' means.
>>
>> Rather than this we have the case now where significant metadata inspection on coordinate system and coordinate is required to determine the correct standard_name from two mutually exclusive choices when writing CF NetCDF. This feels to me to be an unnecessary complication which delivers little benefit from a data and metadata definition perspective.
>>
>>> If you are importing something where x is used as the coordinate, and it is longitude, then why not put that in other metadata?
>>
>> I would say that I have defined this explicitly, using the approach I propose. I define that the data variable is x-wind and I define that x is longitude, therefore I can infer that the x-wind data variable is eastward wind, with respect to the defined grid_mapping. Forcing me to put it in the standard_name adds complexity to software which writes data and increases the opportunity for data to be written incorrectly.
>>
>> For example, does the cf_checker cross reference the 'x' coordinate and any standard names to ensure that datasets defined with respect to a true longitude coordinate variable do not use standard names with the 'x' modifier?
>>
>>> The you say x, I say x, and we both mean different things, is what we need to avoid
>>
>> This cannot be avoided, in almost all cases x means different things in different datasets. It can even mean different things in the same file.
>>
>>> in particular we must not change definitions of existing quantitities.
>>
>> I don't think that it is safe to make that strong a statement on definition changes over time. I can understand the desire to avoid invalidating datasets by narrowing definitions after they are defined; but I don't think that a constrained broadening of the definition of a modifier should be refused on principle. Such changes sometimes need to take place to keep the standard as applicable to its community as possible.
>>
>>
>> That's not to say 'eastward' isn't a useful standard name: there is a good case for model intercomparison, as there is no guarantee that my 'x' is anything like your 'x' for a given dataset: we can agree to publish data as 'eastward' to allow quick and easy intercomparison.
>>
>> even this becomes slightly problematic at small scales, as eastward is with respect to a coordinate reference system, so my east may be subtly different from yours.
>>
>> many thanks
>> mark
>>
>> -----Original Message-----
>> From: Bryan Lawrence [mailto:bryan.lawrence at ncas.ac.uk]
>> Sent: Wed 18/04/2012 11:34
>> To: cf-metadata at cgd.ucar.edu
>> Cc: Hedley, Mark
>> Subject: Re: [CF-metadata] identification of vector components
>>
>> Hi Mark
>>
>> Sorry, silence doesn't mean consent.
>>
>> I think it is exactly the place of standard names to be completely proscriptive about what terms mean.
>>
>> The you say x, I say x, and we both mean different things, is what we need to avoid, and in particular we must not change definitions of existing quantitities.
>>
>> Admittedly, your change wouldn't strictly change anything retrospectively, since it's an inclusive change, but it's probably a dangerous thing to do. (My sense of deja vu tells me we've been here before, and I may even have been on the other side of the argument :-).
>>
>> If you are importing something where x is used as the coordinate, and it is longitude, then why not put that in other metadata? The point of the CF standard is that it just that ....
>>
>> Bryan
>>
>>>
>>> There have not been any responses to this post in the last 10 days.
>>>
>>> I know that this is a dangerous philosophy, but can I suggest that, in this case, silence equals consent?
>>>
>>> If it is, I would like to see these amendments in the standard_name publications as soon as possible. Would this cause concern?
>>>
>>> many thanks
>>> mark
>>>
>>> -----Original Message-----
>>> From: cf-metadata-bounces at cgd.ucar.edu on behalf of Hedley, Mark
>>> Sent: Thu 05/04/2012 17:35
>>> To: cf-metadata at cgd.ucar.edu
>>> Subject: [CF-metadata] identification of vector components
>>>
>>>
>>> There is a statement in the definition of many standard names which are used for vector component definitions, e.g.:
>>>
>>> x_wind
>>> alias: grid_eastward_wind
>>> "x" indicates a vector component along the grid x-axis, when this is not true longitude, positive with increasing x. Wind is defined as a two-dimensional (horizontal) air velocity vector, with no vertical component. (Vertical motion in the atmosphere has the standard name upward_air_velocity.)
>>>
>>> I think that the statement 'when this is not true longitude' is problematic, particularly for software converting from other formats, where x indicates the grid i direction, independent of rotation or projection. I do not think it is the place for standard_name to limit the use of the term 'x' to cases where the horizontal coordinate reference system is not 'true latitude longitude'
>>>
>>> I propose that these terms be removed from all standard names which have 'x' or 'y' as a modifier.
>>>
>>> This would enable all x-ward and y-ward definitions to be used, independent of the grid_mapping, as standard names.
>>>
>>> eastward and northward remain useful modifiers as many models may choose to output eastward vector components where east is not the x direction for the model grid.
>>>
>>> The work on vector containers in:
>>> https://cf-pcmdi.llnl.gov/trac/ticket/79
>>> has indicated a good way forward for identifying vector components, and identifying that vectors are with respect to a grid_mapping. I think this proposed change would interface nicely to the proposal in ticket 79
>>>
>>> How would this proposal be viewed by the community?
>>>
>>> mark
>>> _______________________________________________
>>> CF-metadata mailing list
>>> CF-metadata at cgd.ucar.edu
>>> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>>>
>>>
>>> _______________________________________________
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>>> CF-metadata at cgd.ucar.edu
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>>>
>>
>> --
>> Bryan Lawrence
>> University of Reading: Professor of Weather and Climate Computing.
>> National Centre for Atmospheric Science: Director of Models and Data.
>> STFC: Director of the Centre for Environmental Data Archival.
>> Ph: +44 118 3786507 or 1235 445012; Web:home.badc.rl.ac.uk/lawrence
>>
>>
>
> --
> Bryan Lawrence
> University of Reading: Professor of Weather and Climate Computing.
> National Centre for Atmospheric Science: Director of Models and Data.
> STFC: Director of the Centre for Environmental Data Archival.
> Ph: +44 118 3786507 or 1235 445012; Web:home.badc.rl.ac.uk/lawrence
>
>
> _______________________________________________
> CF-metadata mailing list
> CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
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