Re: [netcdf-java] GRIB variable name changes in 4.3

Hi Don:

On 2/27/2012 3:43 PM, Don Murray wrote:
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.

I realize its a deep problem for the IDV, but its also an opportunity to figure out how to gracefully evolve bundles when things change, which they do.

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:

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.
In the long-term, if we get the fundamentals right, everyone's life gets easier.

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:

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:


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:

With the new naming:


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.
There is simply no way to maintain grib1 and grib2 name compatibility, because 
of the table-driven nature of GRIB, and the fact that they use different tables.

Again, along with the problem, its also an opportunity to rethink how the 
aliases and color tables etc are done. Its possible I can add other attributes 
that will make this easier.

I do apologize for this fiasco. Ive just spent most of the last 4-6 months 
trying to dig our way out of this hole.

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?

We cant maintain tables for all centers. We could try to do so for
just NCEP, but its probably not the right thing to do. It sucks
resources that we dont have. It makes NCEP GRIB files different from
non-NCEP GRIB files. Really, we have to rethink this, not hack in
lookup tables that will never be 100% right.

NCL has adopted a similar variable naming scheme for similar reasons.

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.

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.

A very nice (but not unchanging) human readable string is in the long_name. I understand its a pain to change to using that, but once you make that change, I think your objections above should be resolved. The trick will be to have both the long_name and the (unchanging) variable name.

I'll be glad to work with the IDV team to help wherever I can.

Once again, I apologize for this trouble.


  • 2012 messages navigation, sorted by:
    1. Thread
    2. Subject
    3. Author
    4. Date
    5. ↑ Table Of Contents
  • Search the netcdf-java archives: