NUWG NetCDF Conventions Information
What is the NUWG?
The netCDF Users Working Group (NUWG) was formed in September of 1992 by interested
parties from COMET, Unidata, and FSL (NOAA Forecast Systems Lab). Its purpose
is to develop conventions for representing atmospheric science data and members
are drawn from various organizations based on their expertise with specific data
sets. NUWG conventions identify the minimum set of variables and attributes required
to exchange and access common data sets. The group meets irregularly and works
to define conventions for major categories of data such as surface observations
and grids and also standard representations for information such as station identification
and time. Results of the meetings are distributed on the email@example.com
mail list and conventions are maintained here and in the Unidata pub/netcdf/Conventions/NUWG
directory. Interested parties may join the mailing list by sending a request to
with an empty subject and the message "subscribe
When is the next meeting?
The next meeting will be at (time and date yet to be determined) in the conference
room at the Unidata Program Center. (3300 Mitchell Lane, Boulder CO, click here
Topics for discussion:
- We decided to revisit the conventions for surface and upper-air data, now
that both Unidata and FSL have written decoders to produce netCDF files and
have adapted and modified the conventions to work better with these decoders.
This topic may be appropriate for email discussions. Dave Osborn of FSL has
supplied the following FSL CDL files for NUWG consideration:
- In light of our discussions with John Caron of NCAR, we may consider a
"simple" generalized parameterization for standard navigations/projections.
- (Tabled from last meeting) Russ Rew will present a discussion of an alternative
to the NUWG dimension attribute convention involving multidimensional coordinate
variables instead. The multi-dimensional coordinate variable solution has
some advantages and limitations when compared with the dimension attribute
convention. We may also discuss a proposal for keyed access to netCDF data
that can be implemented over either of these alternative conventions.
When was the last meeting?
of some of our recentt meetings are available.)
The most recent meeting was 30 August 1995. Topics discussed included:
- We considered the re-mapped satellite case, most similar to the gridded
data case, and the navigation variables conform to the gridded data conventions.
Andrew Sundelin presented an example CDL for
the remapped AWIPS/ISPAN satellite data. The NUWG group approved this
example, with the addition of a nav_model="GRIB1" variable to specify the
navigation model, deletion of redundant "missing_value" attributes, and addition
of valid_min and valid_max attributes to the image variable.
- We discussed whether the default for byte data should be signed or unsigned.
Current documentation of the horribly-named signedness attribute says the
default is unsigned, but Harvey Davies has pointed out that this conflicts
with the current default _FillValue for netCDF byte data, which is -127. For
now, data providers may use valid_min and valid_max attributes to indicate
the range of values for byte data.
- We discussed possible values for the nav_model navigation variable. for
parameterizations of georeferencing information. Previously we had considered
the WMO GRIB
edition 1 GDS specification and the U.S. Federal Geographic Data Committee (FGDC)
Content Standards for Digital Geospatial Metadata available in text, Postscript, or
Russ Rew promised to send information on the new GeoTIFF
GeoTIFF represents an effort by over 140 different remote sensing,
GIS, cartographic, and surveying related companies and organizations to
establish a TIFF based interchange format for georeferenced raster imagery.
build libgeotiff is freely available, including Niles Ritter's public-domain
GeoTIFF key parsing library, example programs, and makefiles.
Someone should look at this and evaluate whether GeoTIFF is useful for
a navigation model of our georeferencing conventions. We discussed whether
it would be better if we recommended a single navigation model in the NUWG
conventions, rather than providing a list of usable navigation models, thus
requiring applications or the udnav library to be able to handle all of
- We decided to table until the next meeting a discussion of a new proposed
convention for coordinate reference variables (John Caron's terminology) that
has some advabntages over the current dimension attribute convention. Details
will be made available on a web page soon.
- Current NUWG Email Threads
- Current draft of "Report on the NetCDF Users Working
- General Conventions approved by NUWG
See next four bullets for specific conventions by data type
- Conventions for Surface Observations
- Conventions for Upper Air Observations
- Conventions for Georeferenced Gridded Data
- Conventions for Satellite Data -- The NUWG has been working on the un-remapped
satellite conventions. Below you will find an example CDL and the first draft
of a summary document. The NUWG encourages comments and questions about these
These sample CDLs come from around the community. The NUWG has not explicitly
endorsed any of of these samples. They are referenced here for illustrative
- Summaries of Past Meetings
Who are Susan Jesuroga and Peggy Bruehl?
Susan Jesuroga (firstname.lastname@example.org
) of UCAR/COMET is our NUWG scribe.
She summarizes and records what occurs at each meeting. She is the author of most
of the conventions documents on this page. Peggy Bruehl (email@example.com
of the National Weather Service, prepared this page.
Prepared by Peggy Bruehl
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