For the sake of the IDV community, I'm reposting my response to the
netcdf-java and thredds lists:
<repost>
Hi John and Ethan-
As I have discussed with you at length privately, I am not in favor of
this change. This will break every IDV bundle that points to GRIB data
in a local file or on a TDS server. This will also affect users of the
TDS on the NCDC NOMADS servers who access data either through scripts or
the IDV. It's not a simple matter of users just picking new names and
resaving the bundles when the bundles are stored on remote servers or
used in a classroom setting.
Below, for the benefit of the list, are my arguments for using the human
readable variable names in the previous netCDF-Java 4.3 beta release:
<quote>
I believe keeping the human readable variable names (as in the previous
4.3 release - with slight modifications) is much preferable and backward
compatible. I understand your reasons for wanting to change, but while
that makes the programmer's life easier, it makes the user's (and other
programmers') life harder.
For example, from a user perspective, with your changes, I'm going to
have to modify 50 or more bundles that are on my local machines
(including the NOAA viz wall) or stored on RAMADDA servers which will
take several days. I'm also going to have to modify the customizations
to my IDV parameter tables that I've made over the past 7 years.
From a programmer's perspective, here are the impacts of your changes
to the IDV:
- bundles which use the variable name for lookup
- data aliases used for derived quantities
- parameter aliases used for automatically assigning color tables,
contour intervals and units
- User guide and workshop documentation and examples will need to be
updated
For the past 7 or so years, IDV users have been able to access realtime
GRIB datasets and have had stability in using and interchanging those
datasets. For example, I have a bundle:
http://motherlode.ucar.edu/repository/entry/get/GFS%2080%20km.xidv?entryid=9f77ca66-2264-4f8b-a460-e02fb42606ea
which has displays of 500 hPa geopotential heights, sea level pressure
and precipition from the GFS 80km data. These are simple, commonly used
parameters. The IDV has a DataAlias table that equates the variable
name Geopotential_height with a canonical name of HGT which is used to
present derived quantities to the user of thickness and geostrophic
wind. It also uses this name to assign a color table, unit and contour
levels for any display created for the variable Geopotential height.
Same idea goes for Pressure_reduced_to_MSL and Total_precipitation. It
doesn't matter whether I go to the GFS 80 km (grib1) or the GFS .5
degree global (grib2), or even a NAM 80km dataset. I can apply the
bundle and use the same information to get the same type of display.
Under the scheme in the previous version of 4.3beta, Geopotential_height
will change to Geopotential_height_Pressure, Pressure_reduced_to_MSL
will change to Pressure_reduced_to_MSL_Msl and Total_precipitation will
change to one of:
Total_precipitation_Surface_12_Hour_Accumulation
Total_precipitation_Surface_1_Hour_Accumulation
Total_precipitation_Surface_3_Hour_Accumulation
Total_precipitation_Surface_6_Hour_Accumulation
Total_precipitation_Surface_Mixed_intervals_Accumulation
From the IDV perspective, the DataAlias and ParameterDefaults use
patterns and case insensitive, so this should not be a problem because
the old names would match into the new names. For the bundles, this
will be problem, but one that can be dealt with on the IDV or
netCDF-Java side with a paramater lookup as discussed at the recent IDV
Developers teleconference and which is outlined from the IDV perspective
here:
https://mcidasv.ssec.wisc.edu/issues/11
With the new naming:
VAR_%d-%d-%d[_error][_L%d][_layer][_I%s_S%d][_D%d][_Prob_%s]
The three variables would have different names depending on whether they
came from a grib1 or grib2 dataset. This would require the Unidata IDV
programmers to redo all the alias and parameter default tables and
require a more complicated lookup just to find the 500 hPa geopotential
height, sea level pressure and total_precipitation field depending on
the dataset used. I think providing consistency between grib1 and grib2
datasets at the very least is an important consideration - in the end,
it's all GRIB. GEMPAK and McIDAS (as well as wgrib2 and NCL) create the
same names for their variables independent of whether they came from
Grib1 or 2.
I fully support the notion of adding in the level information to the
variable name as is the case for Geopotential_height. I know for
variables like Temperature in the 4.2 scheme can provide different
results depending on whether your grib files had a mixture of 2D and 3D
varaibles (Temperature = the one on pressure levels) or just 2D
variables (Temperature = the one on height above ground level). I
understand the problems it creates on both the netCDF-Java/TDS side and
sometimes the IDV side (e.g. creating derived quantities) and think that
this change can be handled pretty well on the IDV side.
I support adding the accumulation interval for parameters like
Total_precipitation above because now some variables have a mixture of
the different types of intervals.
One of your arguments is that over time, names change and it's difficult
to maintain tables. While that may be true for lesser variables, I
would suggest that the most commonly used variable names rarely change
(Temperature, geopotential height, relative humidity, u and v wind
components, etc). Unidata has always been in the business of
maintaining tables and that's part of the job it does to support the
user community. While it's not easy, it is a necessary function of the
services that Unidata provides. And, changing the names just pushes the
work off to others at Unidata. Perhaps Unidata could look at having
common tables used by all it's software for consistency. Or perhaps
Unidata could work with the NCL group and use their lookup tables?
In the end, I would like to see the netCDF-Java library evolve to suit
the needs of the data providers, while also maintaining as much backward
compatibility for the end users and software developers who rely on it.
I think a lot of the ancillary information can be provided through
variable attributes as it is in 4.2 (description, table number,
Discipline/Category/Parmeter, GRIB GDS/PDS information) as NCL does, but
leave human readable variable names.
</quote>
Outside the IDV, I have been using the netCDF-Java library in
conjunction with PyNIO to convert grib2 data to netCDF. I use the
human-readable netCDF-Java 4.2 variable names on my output files instead
of the PyNIO names because I believe that the users of my output would
prefer to see those than something like VAR_0-0-0_L6_I6_Hour_S194.
</repost>
And I agree with Jeff McWhirter's sentiment that it is not an onerous
task to maintain the GRIB tables - Robb Kambic did it for years, Chiz
did it for GEMPAK. If it is a priority of the community, then Unidata
should undertake that task.
Don
On 2/27/12 5:24 PM, Yuan Ho wrote:
Hi All:
Please read the following message from thredds email list. As you know,
the IDV relies on the netCDF-Java library, and the coming changes on
netCDF-Java 4.3 would have huge impacts (all bundles with grib data need
to be redone; all preference tables need to be updated; etc.) on the
IDV. We need to have your feedback here.
Thanks,
IDV developer team
-------- Original Message --------
Subject: [thredds] GRIB variable name changes in 4.3
Date: Mon, 27 Feb 2012 11:17:23 -0700
From: John Caron <caron@xxxxxxxxxxxxxxxx>
To: Java NetCDF <netcdf-java@xxxxxxxxxxxxxxxx>, THREDDS community
<thredds@xxxxxxxxxxxxxxxx>
To all:
The CDM / netCDF-Java library version 4.3 (and also TDS version 4.3) is
considering a radical change in the way that GRIB variables are named.
Instead of nice human readable names like
float Temperature(time=1, lat=361, lon=720);
they are now like
float VAR_0-0-0_L6_I6_Hour_S194(time=1, lat=361, lon=720);
with "human readable names" in the long_name:
:long_name = "Temperature (6_Hour Average) @ Maximum wind level";
The reasons for this change are that the "nice human readable names"
come from external GRIB tables, that is, the names are not in the files
themselves. GRIB table parameter names have no requirement to be unique
nor simple nor unchanging, i.e. they have no requirement to be suitable
as netCDF variable names. Maintainers of GRIB tables often make minor
changes to GRIB names, correcting typos or otherwise improving the
readability of the name. In some cases, the GRIB names are completely
changed. When the CDM starts to use new versions of the tables, the
variable names can and do change. Since calls to access data use the
name of the variable, many things break if the name changes.
Any GRIB to netCDF translation software is in the position of either
hand-maintaining the tables to prevent names from changing (and fixing
duplicates or unsuitable names), or doing something else. Hand
maintaining GRIB tables is not a viable option due to resource
constraints. The something else is to give variables unique names based
only on the information actually in the file. The NCL package has
adopted a similar scheme:
http://www.ncl.ucar.edu/Document/Manuals/Ref_Manual/NclFormatSupport.shtml#GRIB
More background on this problem is here:
http://www.unidata.ucar.edu/staff/caron/papers/GRIBarchivals.pdf
Another aspect of this problem is that errors were found in version 4.2
with GRIB tables, with handling GRIB time intervals and ensemble data,
as well as with the algorithm for generating names when multiple
variables from the same parameter are in the same file. About 1 in 5
variable names (in the NCEP IDD data) need to change to fix these
problems. In reviewing how variable names are created, and how GRIB
tables are handled, these other problems became clear. Rather than
fixing the problem piecemeal, we are trying to make one big change all
at once, then do our best to not let this happen again.
The main impact this will have is probably on:
1) scripts or IDV bundles that have a GRIB variable name in them;
hopefully a one-time change will fix this.
2) interactive applications that are built on top of the CDM. For
GRIB, users will need to see the long_name, not the variable name, to
know what they want. However, the CDM presents a uniform interface for
all files, not just GRIB, so the application can't assume that the
long_name is even present. So the application should present both the
variable name and the long_name (if it exists) to the user when
selecting variables.
We think that this change, though painful, is a necessary way forward,
but we want to get input from users, and especially application
developers. The latest 4.3 snapshot has these changes, please try it out
and let us know what you think, and how it will affect you. Post your
comments to these email lists so the entire discussion can be public.
thanks
John, Ethan
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--
Don Murray
NOAA/ESRL/PSD and CIRES
303-497-3596
http://www.esrl.noaa.gov/psd/people/don.murray/
