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Data File Formats: netCDF

by Tom Whittaker University of Wisconsin-Madison SSEC/CIMSS 2009 MUG Meeting June, 2009. Data File Formats: netCDF. What it is...and is not. netCDF is a model for scientific data Variables, dimensions, attributes, coordinates netCDF is not a DBMS

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Data File Formats: netCDF

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  1. by Tom Whittaker University of Wisconsin-Madison SSEC/CIMSS 2009 MUG Meeting June, 2009 Data File Formats: netCDF

  2. What it is...and is not... • netCDF is a model for scientific data • Variables, dimensions, attributes, coordinates • netCDF is not a DBMS • netCDF provides libraries for data access: • C, Fortran, C++, Java, Python, Ruby, IDL, Matlab • netCDF is not a data transmission format • netCDF is a format for portable binary data • Direct access, metadata, appending new data

  3. Brief History • Developed at Unidata beginning in 1987 • Concepts grew out of NASA Common Data Format • New Mexico Institute of Mining and Technology • SeaSpace Inc. • Version 2 released in 1999; Version 4 in 2009 • Why not HDF? • Too easy to abuse; too complex; no conventions • NASA sees the light; funds netCDF4

  4. What is important about netCDF? • It is a self-contained, self-describing file format • It is a community-driven project • Updates are always backward-compatible • It is extensible • The Community is active • Creating conventions for structure • Testing and making suggestions

  5. netCDF Data Model • Variables: name, shape (list of Dimensions), type, Attributes, values • Dimensions: name, length • Attributes: name, type, value(s) • Coordinate Variables: values associated with Dimensions

  6. Supported File Formats • One API for all these: • NetCDF3 (“classic”), netCDF4 • HDF5, HDF4, HDF5-EOS, HDF-EOS • OpenDAP, GRIB1 and GRIB2, GINI • GEMPAK, DMSP, NEXRAD (level 2 & 3) • ADDE, BURF, GTOPO, McIDAS AREA • TDWR, URF, USPLN and NLDN

  7. Conventions.....not “standards” • Many, for our varied data • NUWG (1992) – observations • RAF – aircraft • CDC – gridded data • COARDS (1995) – grids • CF (now....) - grids, etc. • Why? • Sharing data, writing applications • Software is NOT omnipotent!

  8. ....a word about Units • International “standards” for Units • I say “deg_C” • You say “C” • I think you mean Coulombs.......... • UDUNITS package provides definitions and ways to combine units • McIDAS-V (via the VisAD library) enforces legitimate combinations of parameters based on their units • e.g., cannot add a temperature to a pressure....

  9. CF Conventions • CF literally means “Climate and Forecast” • Mostly designed for gridded data • Good documentation available on-line • Defines structure • e.g., use of Coordinate variables to describe data variables • McIDAS-V has a few “requirements” • e.g., Time • What about satellite data? • Conventions in the CF-style being developed

  10. What if..... • ....I cannot read my data? • Check for CF compliance • Use netCDF Tools to check variables, etc. • Use NCML to fix things up • ....I have one file per time, but want to animate? • For some, use the formula • For others, use NCML aggregation • ....I do not know what to do... • Write the Help Desk

  11. Who is responsible? • The MUG Help Desk needs to know • Don't be afraid to ask for assistance! Unidata maintains netCDF:http://www.unidata.ucar.edu/software/netcdf/ CF Conventions Group specifies CFhttp://cf-pcmdi.llnl.gov/

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